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COVID-19 vaccine information

Learn about the efficacy and effectiveness of COVID-19 vaccines available in Australia.

Vaccine efficacy in clinical trials

Comirnaty (Pfizer COVID-19 vaccine)

A phase II/III trial of the Pfizer COVID-19 vaccine enrolled more than 43,000 people aged 12 years or older.

An interim analysis at 2 months after dose 2 reported vaccine efficacy of 95.0% (95% CI: 90.3–97.6) in preventing symptomatic laboratory-confirmed COVID-19 in people aged 16 or older years who did not have evidence of previous infection with SARS-CoV-2.1

Short-term vaccine efficacy after a single dose was 52.4% (95% CI: 29.5–68.4). A protective effect was observed starting 12 days after dose 1.

Efficacy against severe illness was estimated at 88.9% after the first dose (95% CI: 20.1–99.7). This estimate is imprecise because of the small number of people who developed severe disease and there was only a very short period of follow-up as most people then received the second dose.1

In follow-up analysis of the phase III clinical trial for the Pfizer COVID-19 vaccine, efficacy against severe COVID-19 remained high for up to 6 months after dose 2 (95.7%, 95% CI: 73.9–99.9). Efficacy against laboratory-confirmed symptomatic COVID-19 decreased from 96.2% (95% CI: 93.3–98.1) between 7 days and under 2 months after dose 2, to 83.7% (95% CI: 74.7– 89.9) between 4 or more months and 6 months.2

No data are currently available on efficacy for preventing asymptomatic infection.

People aged 65 years or older

Subgroup analyses showed high efficacy in:

  • adults aged 65 years or older (vaccine efficacy 94.7%, 95% CI: 66.7–99.9)
  • adults with at least one medical condition including obesity (vaccine efficacy 95.3%, 95% CI: 87.7–98.8).3

Children aged 12 to 15 years

A study is ongoing that involves more than 2,000 adolescents aged 12 to 15 years. Early results showed that efficacy was 100% (95% CI: 78.1–100) against COVID-19 occurrence at least 7 days after dose 2 in participants with or without evidence of previous infection. There were no cases in the vaccine arm.

After dose 1 and before dose 2, there were 3 COVID-19 cases (within 11 days after dose 1) among vaccine recipients, and 12 cases among placebo recipients. Efficacy was 75% (95% CI: 7.6–95.5).

No severe cases of COVID-19 were observed in this age cohort.

The neutralising antibody response after 2 doses was higher among people aged 12 to 15 years than people aged 16 to 25 years.4 The safety profile was acceptable.4

Children aged 5 to 11 years

A study is ongoing that involves children aged 6 months to under 12 years, with results currently available for the 5–11-year age group.5 These children received the paediatric formulation of the Pfizer vaccine (10 µg dose).

Among 2,186 trial participants aged 5 to 11 years without evidence of prior COVID-19 infection, the 10 µg dose of paediatric Pfizer COVID-19 vaccine was 90.7% effective (95% CI: 67.7–98.3) at preventing laboratory-confirmed symptomatic COVID-19 from day 7 after dose 2 (with an interval of 3 weeks between doses). This was based on 3 observed cases among 1,305 paediatric Pfizer COVID-19 vaccine recipients compared with 16 cases among 663 placebo recipients. All cases were reported between July and September 2021. The three cases in the paediatric Pfizer COVID-19 vaccine group were mild and without fever, whereas most cases in the placebo group had documented fever. Multiple concurrent symptoms were also observed more frequently among cases in the placebo group. There were no cases of severe COVID-19 in either arm.

Neutralising antibody titres after two doses among 264 participants aged 5 to 11 years who received the 10 µg dose of the paediatric Pfizer COVID-19 vaccine were comparable to those observed in 253 trial participants aged 16 to 25 years who received two 30 µg doses of the formulation for people aged 12 years or older, with a ratio of 1.04 (95% CI: 0.93–1.18).5 The proportion achieving seroconversion was also similar (99.2%). Additionally, in a small subset of 34 children who received the paediatric Pfizer COVID-19 vaccine, the increase in neutralisation titre against the Delta variant strain from pre-vaccination to after dose 2 (29.5-fold increase) was similar to the fold-increase observed for the reference strain (36.5-fold increase).

People with specified medical conditions

The ongoing phase II/III trial includes participants with well-controlled chronic medical conditions. An interim sub-analysis in this group showed an efficacy of 95.3% (95% CI: 87.7–98.8). This was similar to the efficacy in people without these conditions (94.7%, 95% CI: 85.9–98.6).1

Data on safety, immunogenicity or efficacy of the Pfizer COVID-19 vaccine in people living with stable HIV from the trial have not yet been published, but were stated to be similar to that in other participants.

Booster doses

Pfizer conducted a randomised blinded placebo-controlled trial of approximately 10,000 participants aged ≥16 years, including 78 participants aged 16–17 years. The study included people who had completed a 2-dose primary schedule of Pfizer vaccine at least 6 months previously. The relative vaccine efficacy against infection across all ages was 95.3% (95% CI: 89.5–98.3) for boosted compared to non-boosted participants during a period of Delta variant circulation. Only two COVID cases occurred in the 16–17-year age cohort, both in the placebo non-booster group.6

Co-administration with influenza vaccine

The UK ComFluCOV study was a phase IV randomised controlled trial of co-administration of dose 2 COVID-19 vaccine (AstraZeneca or Pfizer) with one of three seasonal inactivated influenza vaccines (adjuvanted trivalent vaccine for participants aged 65 years and older, and either cellular or recombinant quadrivalent vaccine for participants aged under 65 years). It found no significant safety concerns, and the immune response to both vaccines was preserved.7

Spikevax (Moderna)

An ongoing phase III trial of the Moderna COVID-19 vaccine involves more than 30,000 people aged 18 years or older (mean age 51.4 years, range 18 to 95 years).8 About one-quarter were aged 65 years or older, and about one-fifth of adults in the study aged 18–64 years had a medical condition with increased risk of severe COVID-19.

Preliminary results of this trial at 2 months after dose 2 reported an efficacy of 94.1% (95% CI: 89.3–96.8) in preventing symptomatic laboratory-confirmed COVID-19 in participants who had not previously been infected with SARS-CoV-2. All 30 severe COVID-19 cases occurred in the placebo group, resulting in a vaccine efficacy estimate of 100% (95% CI unable to be estimated). One death due to SARS-CoV-2 infection occurred in the placebo group. 

Short-term efficacy against symptomatic laboratory-confirmed COVID-19, from 14 days after the first dose and before the second dose, was 92.1% (95% CI: 68.8–99.1).  

The phase I trial indicated that vaccine-induced antibodies lasted for at least 6 months after dose 2.9

In follow-up analysis of the phase III clinical trial of the Moderna COVID-19 vaccine,10 efficacy against severe COVID-19 remained high up to 6 months after dose 2 (97.6%, 95% CI: 92.4–99.2). Efficacy against laboratory-confirmed symptomatic COVID-19 was the same at 4 months or more to 6 months (92.4%, 95% CI: 84.3–96.8) as at 14 days to less than 2 months.10

People aged 65 years or older

In the ongoing phase III trial, 24.8% of participants were aged 65 years or over.11 In this subgroup, efficacy against symptomatic disease was estimated to be 86.4% (95% CI: 61.4–95.2), compared with 95.6% (95% CI: 90.6–97.9) among participants aged 18 to 64 years. 

Children aged under 18 years

A phase II/III trial evaluating the safety and efficacy of the Moderna COVID-19 vaccine in 3,732 adolescents aged 12 to 17 years is ongoing.12,13

The interim results of the phase II/III trial14 showed a vaccine efficacy of 92.7% (95% CI: 67.8–99.2) in preventing symptomatic PCR-confirmed SARS-CoV-2 infection from day 14 after dose 1. No cases of COVID-19 with an onset of 14 days after dose 2 were reported in the vaccine group, and 4 cases occurred in the placebo group. Antibody responses (titre and seroconversion) to the Moderna COVID-19 vaccine (measured by pseudotyped virus neutralisation assay) in this age group were similar to the response in people aged 18 to 25 years for both antibody titre and seroresponse rate.

A phase II/III trial evaluating the efficacy of the Moderna vaccine among children aged 6 months to 11 years of age is also ongoing.13 Limited preliminary data are available from this trial for children aged 6 to 11 years. Vaccine efficacy against symptomatic COVID-19 was not able to be determined due to the small number of COVID-19 vases that occurred from 14 days after dose 2, and the relatively short observation period for this analysis (20 days).15 However, the high short-term efficacy of the Moderna vaccine seen in young adults suggests that the vaccine will have similar efficacy for children aged 6 to 11 years.

Good efficacy is also expected as the immunogenicity is similar between children aged 6 to 11 years and young adults 18 to 26 years. Neutralising antibody titres 28 days after the second dose were higher among 134 children aged 6 to 11 years who received 50 µg per dose (with a 28-day dosing interval) than among 295 young adults aged 18 to 25 years who received 100 µg per dose, with a geometric mean ration of 1.2 (95%CI: 1.1-1.4). The seroresponse rate was also similar between these age groups, with a difference of 0.1% (95% CI: -1.9 -2.1).16

People with specified medical conditions 

The ongoing phase III trial enrolled people with stable medical conditions that put them at increased risk of severe COVID-19. An analysis of this subgroup demonstrated efficacy similar to the efficacy estimated in those without risk factors for severe disease (90.9%, 95% CI: 74.7–96.7, compared with 95.1%, 95% CI: 89.6–97.7).10

Co-administration with influenza vaccine

Preliminary findings from a phase II descriptive randomised open-label study of a Moderna booster administered at the same time as Fluzone high-dose quadrivalent vaccine showed acceptable reactogenicity and immunogenicity to both vaccines, with no safety signals.17

Vaxzevria (AstraZeneca)

Phase II/III trials of the AstraZeneca COVID-19 vaccine are ongoing. More than 57,000 people aged 18 years or older are enrolled.

An interim analysis of pooled data was conducted as of 4 November 2020 from 2 ongoing randomised, blinded, controlled trials:

  • a phase II/III study, COV002, in adults aged 18 years or older in the United Kingdom
  • a phase III study, COV003, in adults aged 18 years or older in Brazil.

This analysis showed the overall vaccine efficacy was 70.4% (95% CI: 54.8–80.6) in preventing symptomatic laboratory-confirmed COVID-19 in people aged 18 years or older 15 or more days after the second dose in the primary efficacy study population.18 The median duration of follow-up from 15 or more days after the second dose was 48 days.18

The AstraZeneca COVID-19 vaccine was demonstrated to have reduced neutralisation activity against the Alpha variant than against an earlier lineage of SARS-CoV-2. But vaccine efficacy was similar, with an efficacy of 70.4% (95% CI: 43.6–84.5) against the Alpha variant, compared with an efficacy of 81.5% (95% CI: 67.9–89.4) against the Victoria lineage.19

Updated analysis of pooled data as of 7 December 2020 reported a vaccine efficacy of 63.09% (95% CI: 51.81–71.73) in preventing symptomatic laboratory-confirmed COVID-19 in people aged 18 years or older who received 2 standard doses.18,20 Efficacy from day 22 after the first dose until up to 12 weeks after vaccination was 76.0% (95% CI: 59.3–85.9).18

Efficacy by interval between doses

In clinical trials, the interval between the 2 doses of the AstraZeneca COVID-19 vaccine ranged from about 4 weeks up to 26 weeks. Among participants who received 2 standard doses at an interval of 4 to 12 weeks, the overall vaccine efficacy was 59.5% (95% CI: 45.8–69.7) for preventing symptomatic laboratory-confirmed COVID-19. This was based on 218 cases.21

In a post-hoc analysis there was a trend toward a greater vaccine efficacy (after dose 2) with increasing dose interval between doses. The efficacy for preventing symptomatic laboratory-confirmed COVID-19 more than 14 days after the second dose was:22

  • 55.1% (95% CI: 33.0–69.9) with an interval of less than 6 weeks between doses
  • 59.9% (95% CI: 32.0–76.4) with an interval of 6 to 8 weeks between doses
  • 63.7% (95% CI: 28.0–81.7) with an interval of 9 to 11 weeks between doses
  • 81.3% (95% CI: 60.3–91.2) with an interval of 12 weeks or more between doses.

There were very few people with severe disease and hospitalisation in the interim analysis of clinical trials to assess efficacy against these outcomes. In the population who received 2 standard doses, zero out of 4,440 participants who received the AstraZeneca COVID-19 vaccine were hospitalised, and 4 out of 4,455 in the control group were hospitalised.23

People aged 65 years or older

Less than 6% of participants included in the interim analysis were aged 65 years or older.18 In this age group, there were 4 cases of COVID-19 in people who received the AstraZeneca COVID-19 vaccine, compared with 8 cases of COVID-19 in people who received the placebo. Overall, there were no cases of COVID-19 hospitalisation, severe disease or COVID-19 deaths among trial participants aged 65 years or older.20

These small numbers mean that efficacy of the AstraZeneca COVID-19 vaccine cannot be assessed in this age group yet. Participants aged 65 years or older who received 2 doses showed SARS-CoV-2-specific neutralising antibody levels that were comparable with those in serum samples from people who had recovered from COVID-19 (convalescent sera).

Additional information on the efficacy of the AstraZeneca COVID-19 vaccine in adults aged 65 years or older is anticipated from a phase III clinical trial underway in the United States and South America (NCT04516746). This trial has more than 30,000 participants, including at least 25% of participants aged 65 years or older.24,25

People with specified medical conditions

An interim sub-analysis of data on people with specified medical conditions in the phase II/III trials showed vaccine efficacy in this group was similar to that in people without such conditions.

A total of 2,068 (39.3%) participants had at least one pre-existing medical condition (defined as a BMI ≥30 kg/m2, cardiovascular disorder, respiratory disease or diabetes). Pooled data analysis (as of 7 December 2020) showed efficacy was 58.3% (95% CI: 33.6–73.9) in participants who had one or more medical conditions.21

Co-administration with influenza vaccine 

The UK ComFluCOV study was a phase IV randomised controlled trial of co-administration of dose 2 COVID-19 vaccine (AstraZeneca or Pfizer) with one of three seasonal inactivated influenza vaccines (adjuvanted trivalent vaccine for participants aged 65 years and older, and either cellular or recombinant quadrivalent vaccine for participants aged under 65 years). It found no significant safety concerns, and the immune response to both vaccines was preserved.7

Nuvaxovid (Novavax)

Two phase III trials were conducted in the United States, Mexico and the United Kingdom. Around 24,000 people received two doses of Novavax vaccine during the trial, and around 17,500 people received placebo.26,27 

Vaccine efficacy against PCR-confirmed symptomatic mild, moderate or severe COVID-19 in serologically negative adults, with onset at least 7 days after dose 2 was: 

  • 90.4% (95% CI: 82.9–94.6) in the United States/Mexico trial27
  • 89.7% (95% CI: 80.2–94.6) in the United Kingdom trial.26

Estimated efficacy against moderate or severe COVID-19 was: 

  • 100% (95% CI: 80.9–100) in the United States/Mexico trial27
  • 86.9% (95% CI: 73.7–93.5) in the United Kingdom trial.26

A phase II trial conducted in South Africa included more than 4,000 participants and provided data on vaccine efficacy against the Beta variant of SARS-CoV-2.28 Vaccine efficacy among HIV-negative adults was 60.1% (95% CI: 19.9–80.1) overall, and specifically against the Beta variant was estimated at 51% (95% CI: 0.6–76.2). 

The significant difference in vaccine efficacy estimates between the United States/Mexico and United Kingdom trials and the South African trial has been attributed to the prevalence of the Beta variant in South Africa during the study period. However, other factors cannot be excluded

People aged 65 years or older

Vaccine efficacy was similar in younger and older age groups:

  • age 18–64 years – 89.8–91.5% (95% CI: 79.7–95.5)26,27
  • age 65 years or older – 88.9% (95% CI: 20.2–99.7).26

People with specified medical conditions

Vaccine efficacy was similar in people with stable chronic conditions and in those without these conditions:26,27

  • with stable chronic conditions – 90.8–90.9% (95% CI: 70.4–95.9)
  • without stable chronic conditions – 89.1–89.9% (95% CI: 76.2–95.6).

There are limited data on the safety and immunogenicity of Novavax COVID-19 vaccine in people with immunocompromise. In the South African phase II trial, among 2,684 participants, 6% were HIV positive. When including all participants, vaccine efficacy was 49.4% (95% CI: 6.1–72.8). When HIV-positive participants were excluded, vaccine efficacy was 60.1% (95% CI: 19.9–80.1).28 Neutralising antibody geometric mean titres were comparable in HIV-positive and HIV-negative participants.

Co-administration with influenza vaccine

A small sub-study (n = 431) of the United Kingdom phase III trial assessed the impact of co-administering the first dose of Novavax COVID-19 vaccine or placebo with influenza vaccine.29 For people aged 18–64 years, vaccine efficacy against laboratory-confirmed symptomatic COVID-19 was not significantly lower in the co-administration group, estimated at 87.5% (95% CI: –0.2 to 98.4) in the co-administration group, compared with 89.8% (95% CI: 79.7–95.5) in the main study who received Novavax COVID-19 vaccine alone. SARS-CoV-2 binding antibody responses were approximately 0.6-fold lower in participants who received the co-administered vaccines compared with those who received Novavax COVID-19 vaccine alone. There were no significant differences in the immune responses to influenza vaccines between the groups. It is not yet known whether the impact of co-administration on immunogenicity translates to any difference in clinical protection or duration of protection against COVID-19.

Vaccine effectiveness in post-licensure studies

The effectiveness of COVID-19 vaccines has been studied in national vaccination programs since late 2020. Over this time, the distribution of variant strains in the country of study and during the period of study may have changed. This is a key consideration when interpreting results of vaccine effectiveness studies.

Some key findings from selected major studies on effectiveness of the vaccines currently available in Australia are summarised below. Results of other studies were generally consistent with these findings.

Vaccine effectiveness data for children under 12 years of age are not yet available. Data are expected in the coming months.

Vaccine effectiveness data for the Novavax vaccine are not yet available.

Vaccine effectiveness in the period before the Delta variant became dominant

Pfizer COVID-19 vaccine

A study was conducted in Israel that included more than 1.1 million people aged 16 years or older, from 20 December 2020 to 1 February 2021. The Alpha variant was dominant in Israel during this time. The effectiveness of the Pfizer COVID-19 vaccine from 7 days after dose 2 was 87% (95% CI: 55–100) against COVID-19 hospitalisations and 92% (95% CI: 75–100) against severe disease.30 Detailed data on the duration of protection from the vaccine are not available yet.

Moderna COVID-19 vaccine

A US Mayo Clinic Health System study included more than 16,000 people aged 18 years or older, conducted from 15 February 2020 to 20 April 2021. The effectiveness of the Moderna COVID-19 vaccine from 14 days after dose 2 was 92% (95% CI: 82–97) against PCR-positive SARS-CoV-2 infection.31 From 7 days after dose 2, the effectiveness of the Moderna COVID-19 vaccine was 86% (95% CI: 72–94) against hospitalisation and 100% (95% CI: 43–100) against ICU admission. 

AstraZeneca COVID-19 vaccine

A population-based cohort study was conducted in the United Kingdom with more than 300,000 people aged 16 years or older, from 1 December 2020 to 8 May 2021. The Alpha variant was dominant in the United Kingdom during this time. The effectiveness of the AstraZeneca COVID-19 vaccine 21 days after dose 1 was 64% (95% CI: 55–70) against PCR-positive SARS-CoV-2 infection.32 Effectiveness against symptomatic infection was marginally higher than against asymptomatic infection.

A prospective population cohort study was conducted in Scotland with 5.4 million people aged 18 years or older, from 8 December 2020 to 15 February 2021. The Alpha variant was dominant in the United Kingdom at that time. The effectiveness of the AstraZeneca COVID-19 vaccine in the 28 to 34 days after 1 dose was 94% (95% CI: 73–99) against COVID-19 hospitalisations.33

Vaccine effectiveness against the Delta variant

Vaccine effectiveness against symptomatic SARS-COV-2 infection with the Delta variant was lower compared with that due to the Alpha variant, but effectiveness was maintained against hospitalisation.34,35 This lower effectiveness against symptomatic COVID-19 was more marked for dose 1. This was shown in studies from the United Kingdom, Qatar, United States and Canada using the Pfizer, Moderna and AstraZeneca COVID-19 vaccines.

Pfizer COVID-19 vaccine

A cohort study was conducted in Scotland from 1 April 2021 to 6 June 2021. It included more than 19,000 sequenced cases, of which 7,723 (39.5%) were the Delta variant. Effectiveness of the Pfizer COVID-19 vaccine against PCR-positive SARS-CoV-2 infection (with or without symptoms at the time of testing) was 30% (95% CI: 17–41) ≥28 days after dose 1 and 79% (95% CI: 75–82) ≥14 days after dose 2. When assessed against symptomatic infection, effectiveness estimates were 33% (95% CI: 15–47) ≥28 days after dose 1 and 83% (95% CI: 78–87) ≥14 days after dose 2.36

A test-negative case–control study was conducted in the United Kingdom from 26 October 2020 to 16 May 2021. It included more than 19,000 sequenced cases, of which greater than 4,000 were the Delta variant. Effectiveness of the Pfizer COVID-19 vaccine against PCR-positive symptomatic disease after 2 doses was 88.0% (95% CI: 85.3–90.1) for Delta variant cases, compared with 93.7% (95% CI: 91.6–95.3) for Alpha variant cases.37 After 1 dose, effectiveness was 35.6% (95% CI: 22.7–46.4) for Delta variant cases and 47.5% (95% CI: 41.6–528) for Alpha variant cases. 

A study conducted in the United Kingdom from 12 April 2021 to 4 June 2021 included 14,019 symptomatic cases with the Delta variant. Effectiveness of the Pfizer COVID-19 vaccine against hospitalisation after 2 doses was 96% (95% CI: 86–99) for Delta variant cases, compared with 95% (95% CI: 78–99) for Alpha variant cases.34 After 1 dose, effectiveness was 94% (95% CI: 46–99) for Delta variant cases and 83% (95% CI: 62–93) for Alpha variant cases.

Studies in the United Kingdom, United States, Qatar and Canada reinforce these results. Effectiveness of the Pfizer COVID-19 vaccine after 2 doses ranges from 79% to 93% against PCR-confirmed infection, 83% to 93% against symptomatic disease, and 75% to 97% against severe disease.35

Moderna COVID-19 vaccine

A test-negative case–control study in Canada was conducted from 14 December 2020 to 30 May 2021. It included more than 400,000 symptomatic cases. The Delta variant became dominant in May 2021. Effectiveness of the Moderna COVID-19 vaccine against PCR-positive symptomatic disease ≥21 days after dose 1 was 70% (95% CI: 52–81) for Delta variant cases compared with 84% (95% CI: 80–86) for Alpha variant cases. Effectiveness against hospitalisation or death ≥21 days after dose 1 was 95% (95% CI: 67–99) for Delta variant cases compared with 80% (95% CI: 74–85) for Alpha variant cases.35

Studies in the United States and Qatar reinforce these results. Effectiveness of the Moderna COVID-19 vaccine after 2 doses ranges from 76% to 86% against PCR-confirmed infection, and 81% to 100% against severe disease.38,39

AstraZeneca COVID-19 vaccine

A cohort study in Scotland was conducted from 1 April 2021 to 6 June 2021. It included more than 19,000 sequenced cases, of which 7,723 (39.5%) were Delta variant cases. Effectiveness of the AstraZeneca COVID-19 vaccine against PCR-positive SARS-CoV-2 infection (with or without symptoms at the time of testing) was 18% (95% CI: 9–25) ≥28 days after dose 1, and 60% (95% CI: 53–66) ≥14 days after dose 2. When assessed against symptomatic infection, effectiveness estimates were 33% (95% CI: 23–41) ≥28 days after dose 1, and 61% (95% CI: 51–70) ≥14 days after dose 2.36 

A test-negative case–control study in the United Kingdom was conducted from 26 October 2021 to 16 May 2021. It included more than 19,000 sequenced cases, of which more than 4,000 were Delta variant cases. Effectiveness of the AstraZeneca COVID-19 vaccine against PCR-positive symptomatic disease after 2 doses was 67.0% (95% CI: 61.3–71.8) for Delta variant cases, compared with 74.5% (95% CI: 68.4–79.4) for Alpha variant cases.37 After 1 dose, effectiveness was 30.0% (95% CI: 24.3–35.3) for Delta variant cases and 48.7% (95% CI: 45.2–51.9) for Alpha variant cases. 

Another study conducted in the United Kingdom from 12 April 2021 to 4 June 2021 included 14,019 symptomatic cases with the Delta variant. Effectiveness of the AstraZeneca COVID-19 vaccine against hospitalisation after 2 doses was 92% (95% CI: 75–97) for Delta variant cases, compared with 86% (95% CI: 53–96) for Alpha variant cases.34 After 1 dose, effectiveness was 71% (95% CI: 51–83) for Delta variant cases and 76% (95% CI: 61–85) for Alpha variant cases.

Studies in the United Kingdom, India and Canada reinforce these results. Effectiveness of the AstraZeneca COVID-19 vaccine after 2 doses ranged from 60% to 67% against PCR-confirmed infection, 61% to 71% against symptomatic disease, and 77% to 92% against severe disease.40,41

Regional differences in the vaccine rollout may have confounded the estimates of vaccine effectiveness. For example, different vaccine intervals may have been used over time and/or across different countries. Emerging data are constantly being monitored to inform future recommendations.

Vaccine effectiveness over time

Several observational studies examine the post-licensure effectiveness of the 3 COVID-19 vaccines registered in Australia in different populations in different countries. Overall, these studies showed that vaccine effectiveness waned over about 4 to 6 months after completing the 2-dose primary schedule. These studies used varying study designs, and some stratified analyses by age group, vaccine brand, the presence of underlying risk conditions and infection due to different virus variants (e.g. Delta compared to Alpha). Data on effectiveness beyond 6 months after dose 2 are very sparse.

Across these studies, vaccine effectiveness against more severe outcomes of COVID-19 (such as hospitalisation, ICU admission and death) appeared to be maintained with little or no decline over time up to 6 months.36,42-46 However, vaccine effectiveness against any PCR-confirmed infection (asymptomatic and symptomatic) and symptomatic infection (any COVID-19) decreased over time.45,47 The estimated decline in effectiveness against any infection due to the Delta variant in 4–6 months after 2 doses of Pfizer was 20–40%.45,47 The pattern of waning varied between individual vaccines. The decline in effectiveness over time against any PCR-confirmed SARS-CoV-2 infection has been associated with a reduced protective effect of vaccine in preventing virus transmission from vaccinated people.40,43,48

Evaluation of vaccine effectiveness may be confounded by changes over time in public health and social measures, such as mask wearing, social distancing and travel. There may also be differences between vaccinated and unvaccinated people in how they adhere to these measures. This emphasises the importance of evaluating the entire body of evidence, and not just relying on single study outputs.

Vaccine effectiveness against SARS-CoV-2 transmission

Data from studies in the United Kingdom showed that both the Pfizer and AstraZeneca COVID-19 vaccines were effective in preventing onward transmission of the virus to close contacts in case of breakthrough infections. In one study among the general population in the United Kingdom, the effectiveness against transmission from breakthrough infections to household contacts from 21 days after the first dose was 47% (95% CI: 37–57) for the AstraZeneca COVID-19 vaccine and 49% (95% CI: 41–56) for the Pfizer COVID-19 vaccine.49 Another study from the United Kingdom reported that, among healthcare workers, who predominantly received the Pfizer COVID-19 vaccine, vaccination was associated with a 30% (95% CI: 22–37) reduction in transmission of SARS-CoV-2 to household contacts.50

Preliminary data from Finland also suggest that there was an indirect effect of both the Pfizer and Moderna COVID-19 vaccines on close contacts of vaccine recipients.51 Among healthcare workers, vaccine effectiveness against PCR-confirmed infection in their unvaccinated household contacts was 43% (95% CI: 22–58) 10 weeks after the first vaccine dose. The Alpha variant was the dominant strain in Finland at time of this study.

The National Institute of Allergy and Infectious Diseases (NIAID) in the United States is currently undertaking a clinical trial evaluating the effect of the Moderna COVID-19 vaccine on preventing transmission.52

A study in the general population in the United Kingdom examined the effectiveness of the Pfizer and AstraZeneca COVID-19 vaccines in preventing onward transmission after breakthrough cases of Delta variant infection in vaccinated people.53

Vaccine effectiveness in older adults

In a single-centre case–control study in Bristol in the United Kingdom,54 vaccine effectiveness against hospitalisation among adults aged ≥80 years from 14 days after dose 1 was 71% (95% CI: 36–95) for the AstraZeneca COVID-19 vaccine and 79% (95% CI: 47–93) for the Pfizer COVID-19 vaccine.54

Other studies in the United Kingdom have reported the effectiveness of a first dose of either the Pfizer or AstraZeneca COVID-19 vaccines at 76% (95% CI: 68–82) against overall SARS-CoV-2 infection in people aged 75 years or older and 81% (95% CI: 65–90) against hospitalisation in people aged 80 years or older.32,33,55 Among long-term care facility residents aged 65 years or older, vaccine effectiveness against PCR-confirmed SARS-CoV-2 infection (with or without symptoms) was estimated to be 62% (95% CI: 23–81), with no difference between the AstraZeneca and Pfizer COVID-19 vaccines.55

A test-negative study in Ontario, Canada, included more than 300,000 participants. It was conducted from 14 December 2020 to 19 April 2021, when the Alpha variant was predominant in Ontario. Effectiveness for the mRNA vaccines (Pfizer and Moderna) against PCR-confirmed symptomatic disease in adults aged 70 years or older was 40% (95% CI: 29–49) ≥14 days after dose 1 and 94% (95% CI: 87–97) ≥7 days after dose 2.56

Vaccine effectiveness data for 2 doses of the AstraZeneca COVID-19 vaccine are still limited because of the 12-week interval between doses.32

Waning of vaccine effectiveness has also been observed in older adults. For older adults (65 years or older), the peak effectiveness after dose 2 was somewhat lower than younger adults.42,57 In data from the United Kingdom, the peak vaccine effectiveness against symptomatic Delta infection among adults aged 65 years or older was significantly lower for the AstraZeneca COVID-19 vaccine than for the Pfizer COVID-19 vaccine. These data also showed greater waning of vaccine effectiveness in older adults.40,43

Vaccine effectiveness in people who are immunocompromised

Vaccine effectiveness studies suggest that immunocompromising conditions may be associated with a reduction in protection against COVID-19 compared with immunocompetent individuals. However, this finding has not been consistently demonstrated and these studies have some limitations. Overall, vaccine effectiveness against COVID-19 was around 70% to 90% in people who are immunocompromised,30,58-60 compared with 84% to 94% in the general population.

One study by Whitaker et al.60 examined both Pfizer and AstraZeneca COVID-19 vaccines in the United Kingdom to 13 June 2021. In a general immunocompromised population at least 4 weeks after 2 doses of vaccine, vaccine effectiveness against medically attended PCR-confirmed COVID-19 was estimated as 73.0% (33.9% to 89.0%) for Pfizer, and 74.6% (18.7 to 92.1%) for AstraZeneca. 

In people with higher degrees of immunosuppression, geometric mean titres of antibodies after vaccination are generally lower than in the general population. This includes people with solid organ transplant and haematological malignancies61-66 and people undergoing B cell depleting therapies (anti-CD20 monoclonal antibodies).67-69

Vaccine effectiveness in pregnancy

Evidence of good vaccine effectiveness of mRNA COVID-19 vaccines in pregnant women is also emerging. A retrospective cohort study that included 15,060 pregnant women in Israel, including 7,530 who received Pfizer, estimated effectiveness against PCR-confirmed SARS-CoV-2 infection from ≥28 days after vaccination to be 78% (95% CI: 57–89).70  

Booster vaccine effectiveness

First booster dose

Israel has progressively implemented a booster vaccine program using the Pfizer COVID-19 vaccine in the general population aged 16 years or older since July 2021. Data from Israel suggest that a booster dose was effective at reducing infection (in eligible people of any age), severe disease (in people 40 years or older) and death (in people 60 years or older) compared with non-boosted people at least 5 months after their second dose.71-73 Limitations are that the data are from only a short observation period after the booster and are preliminary.

One study demonstrated an incremental protective effect with a booster dose of the Pfizer COVID-19 vaccine at least 5 months since the previous dose among adults aged 60 years or older in the very short term (within 12–25 days after the booster dose). The study reported a greater than 10 times lower rate of COVID-19 and severe COVID-19 among people who had a booster than in people who did not have a booster.72 A recent extension of this study showed a further protective effect of a Pfizer COVID-19 booster dose by age, given at least 5 months since the previous dose, among people aged 16 years or older.72 Compared with the non-booster group, the rate of PCR-confirmed infection in the booster group was:

  • 12.4 times lower for people 60 years or older
  • 12.2 times lower for people aged 50–59 years
  • 9.7 times lower for people aged 40–49 years
  • 8.8 times lower for people aged 30–39 years
  • 17.6 times lower for people aged 16–29 years.

The rate of severe COVID-19 was 18.7 times lower for people aged 60 years or older, and 22.0 times lower for people aged 40–59 years. The rate of death due to COVID-19 was 14.7 times lower for people aged 60 years or older in the booster group compared to the non-booster group.72

Another study from Israel (using 2 different methods) found a relative vaccine effectiveness within 14–20 days after a booster dose of the Pfizer COVID-19 vaccine of 70–79% against PCR-confirmed SARS-CoV-2 infection among people aged 40 years or older compared with people who had a 2-dose primary vaccination and no booster.74

Waning after first booster dose 

Evolving evidence based on early vaccine effectiveness data and analysis of antibody levels after the first booster dose suggest there is gradual waning of immunity against the Omicron variant.75-78 This is most prominent for vaccine effectiveness against symptomatic infection, which declines from 60–75% at 2–4 weeks after a booster dose of either the Pfizer or Moderna vaccine to 25–40% from 15 or more weeks after the booster.79 Vaccine effectiveness against COVID-19 hospitalisation wanes more slowly, and was 75% by 10–14 weeks for Pfizer vaccine79 and 78% at least 4 months after mRNA vaccine.76 

Data from Qatar show that effectiveness against severe disease remained at >90% after 7 weeks or more after the first booster, although this was in a relatively younger population and may not be directly comparable.75 

Early pre-print data suggest that the benefit of first booster doses in preventing onward transmission in breakthrough cases of Omicron may be substantially less than Delta and may be short-lived.80,81 

Additional booster doses 

Limited pre-print data from Israel suggest that, in people aged ≥60 years, an additional booster dose of Pfizer vaccine at 4 months after the first booster resulted in a 2-fold lower rate of confirmed infection and 4.3-fold lower rate of severe illness.82 

Another study in younger people aged ≥18 years showed the additional protection from an additional booster dose to be modest and uncertain. Those who received an additional booster dose were 11–30% less likely to be infected and 31–43% less likely to have symptomatic disease than those who had received only one booster. However, estimates were imprecise due to small numbers of infected people.83 Up to 30% of second booster recipients who had breakthrough infection were asymptomatic and their virus levels were no different from people who received only one booster, suggesting that the additional booster dose may not significantly reduce onward virus transmission in infected people.83 

Further reading

 

  1. Polack FP, Thomas SJ, Kitchin N, et al. Safety and efficacy of the BNT162b2 mRNA COVID-19 vaccine. New England Journal of Medicine 2020;383:2603-15. 
  2. Thomas SJ, Moreira ED, Jr., Kitchin N, et al. Safety and efficacy of the BNT162b2 mRNA Covid-19 vaccine through 6 months. New England Journal of Medicine 2021. 
  3. Kadali RAK, Janagama R, Peruru SR, et al. Adverse effects of COVID-19 messenger RNA vaccines among pregnant women: a cross-sectional study on healthcare workers with detailed self-reported symptoms. Am J Obstet Gynecol 2021. 
  4. Frenck RW, Jr., Klein NP, Kitchin N, et al. Safety, immunogenicity, and efficacy of the BNT162b2 Covid-19 vaccine in adolescents. New England Journal of Medicine 2021;385:239-50. 
  5. Walter E, Talaat K, Sabharwal C, et al. Evaluation of the BNT162b2 Covid-19 Vaccine in Children 5 to 11 Years of Age. New England Journal of Medicine 2021;2021:10.1056/NEJMoa2116298. 
  6. United States Food and Drug Administration. Pfizer-Biontech COVID-19 Vaccine Review Memorandum, December 8, 2021. 2021. (Accessed 11 January 2022). https://www.fda.gov/media/154869/download 
  7. Lazarus R, Baos S, Cappel-Porter H, et al. The safety and immunogenicity of concomitant administration of COVID-19 vaccines (ChAdOx1 or BNT162b2) with seasonal influenza vaccines in adults: a phase IV, multicentre randomised controlled trial with blinding (ComFluCOV). 2021. (Accessed 28 October 2021). https://ssrn.com/abstract=3931758  
  8. US National Library of Medicine. A study to evaluate efficacy, safety, and immunogenicity of mRNA-1273 vaccine in adults aged 18 years and older to prevent COVID-19. Identifier: NCT04470427. 2021. (Accessed 12 August 2021). https://clinicaltrials.gov/ct2/show/NCT04470427  
  9. Doria-Rose N, Suthar MS, Makowski M, et al. Antibody Persistence through 6 Months after the Second Dose of mRNA-1273 Vaccine for Covid-19. New England Journal of Medicine 2021;384:2259-61. 
  10. El Sahly HM, Baden LR, Essink B, et al. Efficacy of the mRNA-1273 SARS-CoV-2 vaccine at completion of blinded phase. New England Journal of Medicine 2021. 
  11. Baden LR, El Sahly HM, Essink B, et al. Efficacy and Safety of the mRNA-1273 SARS-CoV-2 Vaccine. New England Journal of Medicine 2021;384:403-16. 
  12. US National Library of Medicine. A study to evaluate the safety, reactogenicity, and effectiveness of mRNA-1273 vaccine in adolescents 12 to <18 years old to prevent COVID-19 (TeenCove). Identifier: NCT04649151. 2021. (Accessed 12 August 2021). https://clinicaltrials.gov/ct2/show/NCT04649151  
  13. US National Library of Medicine. A study to evaluate safety and effectiveness of mRNA-1273 COVID-19 vaccine in healthy children between 6 months of age and less than 12 years of age. 2021. (Accessed 12 August 2021). https://clinicaltrials.gov/ct2/show/NCT04796896 
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Last updated: 
25 March 2022

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