Vaccine Preventable Diseases and Vaccination Coverage in Australia, 2003 to 2005

Measles

Disclaimer: This is the fourth report on vaccine preventable disease and vaccination coverage in Australia, and is produced by the National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases and the Australian Institute of Health and Welfare on behalf of the Australian Government Department of Health and Ageing published as a supplement to the Communicable Diseases Intelligence journal Volume 31, June 2007.

Page last updated: 20 July 2007

Measles is an acute and highly communicable disease caused by a morbillivirus. The clinical picture primarily includes prodromal fever and Koplik spots on the buccal mucosa, rash, and often conjunctivitis, coryza, and cough. Complications include otitis media, pneumonia and encephalitis. Subacute sclerosing panencephalitis (SSPE) occurs very rarely as a late sequel of wild infection but not vaccination.44

Case definitions

See Appendix 6 for pre 2004 definition

National definition from January 2004:11

  1. Isolation of measles virus (confirmed case); or
  2. Detection of measles virus by nucleic acid testing (confirmed case); or
  3. Detection of measles virus antigen (confirmed case); or
  4. Measles virus-specific IgG seroconversion or significant increase in IgG antibody level or a fourfold or greater rise in antibody titre to measles virus, with paired sera tested in parallel and in the absence of receipt of measles containing vaccine eight days to eight weeks prior to testing (confirmed case); or
  5. Detection of measles virus-specific IgM antibody confirmed in an approved reference laboratory, in the absence of recent measles containing vaccination (confirmed case); or
  6. A clinical illness characterised by a generalised maculopapular rash lasting at least three days, fever of at least 38°C at the time of rash onset and either cough, coryza, conjunctivitis or Koplik spots, together with an epidemiological link to a confirmed case (confirmed case); or
  7. A clinical illness as in point (f) above, together with detection of measles-specific IgM antibody other than by an approved reference laboratory (in the absence of recent measles-containing vaccination) (probable case).

Hospitalisations and deaths

The ICD-10-AM/ICD-10 code B05 (measles) was used to identify hospitalisations and deaths. SSPE was not included in these analyses.

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Secular trends

In the three year review period, there were 148 notified cases of measles, an average annual notification rate of 0.25 per 100,000 (Table 8). The number of measles notifications progressively decreased from 2003 (n=93) to 2004 (n=45), and in 2005 to the lowest on record (n=10). The median number of notifications per month was two (range 0–15).

In the period 2002/2003 to 2004/2005, there were 94 hospitalisations with the ICD-10-AM code B05 (measles). This equates to an average annual hospitalisation rate of 0.16 per 100,000. Since a decline in the mid-1990s, annual hospitalisation rates have been fluctuating at a low level, and in 2004/2005 were the lowest on record with only 23 separations, a rate of 0.11 per 100,000. The highest rates of hospitalisations were in the 0–4 and 20–35 year age groups. There were two peaks in the number of monthly hospitalisations during the review period, one in May 2003 (n=7), the other in October of 2003 (n=7) (Figure 12). The median number of hospitalisations per month was two (range 0–7).

Figure 12. Measles notifications and hospitalisations, Australia, 1993 to 2005,* by month of diagnosis or admission

Figure 12. Measles  notifications and hospitalisations, Australia, 1993 to 2005, by month  of diagnosis or admission

* Notifications where the month of diagnosis was between January 1993 and December 2005;
hospitalisations where the month of admission was between 1 July 1993 and 30 June 2005.

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Severe morbidity and mortality

In the three year review period, hospital separations for measles accounted for 375 hospital bed days. The median length of stay (LOS) was two days, with the highest median LOS (4.5) in the 60 years and over age group (Table 8). Of the 94 hospitalisations, 72 (77%) had measles recorded as the principal diagnosis. Complications arising from measles infection were recorded for 20 (21%) separations. There was one hospitalisation coded as having otitis media, one with neurological (encephalitis or meningitis) complications, six (6%) as having pneumonia, none with intestinal complications, and 12 (13%) as having other complications (Table 9). Adults aged 15 years and over accounted for nine of the 12 (75%) hospitalisations coded with other complications.

In 2004, there was one death recorded in the National Mortality database where the underlying cause was measles; it was coded as measles complicated by encephalitis. However, further investigation demonstrated that the death was miscoded and was actually a result of subacute sclerosing panencephalitis rather than acute measles encephalitis (Peter Markey, Northern Territory Department of Health and Community Services, personal communication).

Table 8. Measles notifications, hospitalisations and deaths, Australia, 2002 to 2005,* by age group

Age group
(years)
Notifications
3 years
(2003–2005)

Hospitalisations
3 years
(July 2002–June 2005)

LOS per admission
(days)
Deaths
2 years
(2003–2004)
n Rate n (§) Rate (§) Median (§) n Rate
0–4
23
0.61
37
(29)
0.97
(0.76)
2.0 (1.0)
0
0.0
5–14
13
0.16
2
(2)
0.02
(0.02)
n.p.
0
0.0
15–24
42
0.51
19
(15)
0.23
(0.18)
2.0 (2.0)
0
0.0
25–59
70
0.24
32
(24)
0.11
(0.08)
3.0 (3.0)
0
0.0
60+
0
0.00
4
(2)
0.04
(0.02)
4.5 (2.0)
0
0.0
All ages
148
0.25
94
(72)
0.16
(0.12)
2.0 (2.0)
0
0.0

* Notifications where the month of diagnosis was between January 2003 and December 2005; hospitalisations where the month of separation was between 1 July 2002 and 30 June 2005; deaths where the death was recorded in 2003 or 2004.

† LOS = length of stay in hospital.

‡ Average annual age-specific rate per 100,000 population.

§ Principal diagnosis (hospitalisations).

n.p. Not published due to small cell size.

Table 9. Indicators of severe morbidity* for hospitalised cases of measles, Australia, 2002 to 2005*, by age group

Age group
(years)
Measles encephalitis Measles pneumonia
n % total n % total
0–4
0
0.0
2
2.1
5–14
0
0.0
0
0.0
15–24
1
1.1
1
1.1
25–59
0
0.0
2
2.1
60+
0
0.0
1
1.1
All ages
1
1.1
6
6.4

* Measured using National Hospital Morbidity data where the month of hospital separation was between
1 July 2002 and 30 June 2005.

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Age and sex distribution

By the end of the three year review period, notification rates for all age groups declined to their lowest recorded levels (Figure 13). Since 1999, there has been a decrease in measles notification rates in 0–12 month olds. Over the review period, notifications in this age group decreased from seven in 2003 to two in 2004 and zero in 2005. The notification rate for 20–34 year olds also decreased each year from 1.30 per 100,000 in 2003, to 0.70 per 100,000 in 2004 and was the lowest on record at 0.12 per 100,000 in 2005. For each year of the review period, the notification rate for the 20–34 year old age group was highest of all groups for the first time since the Measles Control Campaign (MCC) in 1998, followed by the 0–4 year age group which had the highest rates in previous reporting periods.

In 2005, hospitalisation rates were the lowest on record for the 0–4 (0.40 per 100,000), 5–9 (0.00 per 100,000) and 20–34 (0.21 per 100,000) year age groups. Hospitalisation rates for the 10–19 year and over 35 year age groups were higher at the end of the review period than at the beginning (Figure 14). Hospitalisation rates for the 0–4 year age group were consistently the highest of all age groups since reporting began. However, the rates of hospitalisation of 0–12 month olds decreased over the review period, from six in 2003 to zero in 2005 (data not shown).

Over the three year review period, there were more notifications for males than females (male:female ratio 1.5:1). There were also more hospitalisations of males than females (male:female ratio 1.4:1).

Figure 13. Measles notification rates, Australia, 1999 to 2005,* by age group and year of diagnosis

Figure 13. Measles notification rates, Australia, 1999 to 2005, by age  group and year of diagnosis

* Notifications where the date of diagnosis was between January 1999 and December 2005.

Figure 14. Measles hospitalisation rates, Australia, 1998 to 2005,* by age group and year of separation

Figure  14. Measles hospitalisation  rates, Australia,  1998 to 2005, by age group and year of separation

* Hospitalisations where the date of separation was between 1 July 1998 and 30 June 2005.

Geographical distribution

There were 93 measles notifications in 2003, of which 75 (81%) occurred in seven outbreaks.13 Five of the seven outbreaks are known to have begun with an imported measles case. There were 45 cases of measles notified in 2004, of which 21 (47%) occurred in six outbreaks.12 Four of the six outbreaks are known to have begun with imported measles cases. One of the 2004 outbreaks was an outbreak of measles in six Indigenous people in Western Australia, with no known link to a confirmed imported source case.12

The rate of notification over the reporting period was highest in the Northern Territory with 0.67 per 100,000 (n=4), followed by South Australia with 0.65 per 100,000 (n=30), more than twice the national rate (0.25 per 100,000). The increased notification rates in South Australia during the review period were largely due to one outbreak involving a returned traveller which began in August 2003 (n=21). The high notification rate in the Northern Territory was largely due to unrelated notifications in an overseas traveller and foreign tourists. All jurisdictions reported cases during the reporting period except the Australian Capital Territory (Appendix 2).

There was little variation between the rates of hospitalisation across the states and territories during the review period (0.1–0.2 per 100,000) (Appendix 3).

Victoria reported the highest percentage of notifications during the reporting period with 38% of cases (n=56). New South Wales reported the highest percentage of hospitalisations with 35% (n=33).

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Measles typing and vaccination status

In 2003, seven measles virus genotypes were identified by molecular analysis. The majority of these genotypes were associated with virus importations from countries in Southeast Asia and Japan (H1, D5, D8 and D9). Genotype D2 was also identified after importation from South Africa. The 2003 outbreak of measles in Central Northern Victoria was due to an imported measles strain, confirmed by the Victorian Infectious Diseases Reference Laboratory (VIDRL) as genotype H1. Molecular analysis enabled a second, overlapping outbreak in Melbourne, in which the primary case was a German traveller, to be distinguished from the rural outbreak by the identification of measles virus genotype D8 in clinical samples.126 In 2004, genotyping identified importations of D4 and D5 (from Thailand), D9 (Malaysia and Indonesia), G2 (Singapore) and G3 (Indonesia). Genotypes D4, H1 and G3 were also detected but the index cases were not identified, thus no source country could be identified. In 2005, genotypes D4 (India) and D5 (Thailand) were detected, as were genotypes D8, D9, G3 and H1, although no source country was identified for these latter genotypes. In each year of the reporting period, molecular analysis of clinical specimens was able to confirm cases of vaccine associated illness (genotype A).

Completion of the vaccination status field was expected in NNDSS during 2003–2005 for all measles notifications born after 31/12/1969. Overall, 72% of cases born after 31/12/1969 (97/135) had this field completed for this period. None of the 97 cases reported receipt of two or more vaccine doses. Ten were reported as fully vaccinated for age (with half of these cases aged over four years and reporting only one dose), 18 as partially vaccinated for age, and 65 were not vaccinated (with the four remaining cases aged under one year and unvaccinated, with vaccination status recorded as “not applicable”). Vaccination status was validated by written records in 50% of the cases reported to be fully vaccinated, 28% of partially vaccinated cases and 8% of unvaccinated cases.

Comment

In the three year review period covered by this report, measles notifications and hospitalisations continued to decline to new record lows. Measles accounted for only 10 notifications in 2005 and 23 hospitalisations in 2004/2005. The downward trend is similar to that seen in other countries with high vaccination coverage, such as those of the Americas.127 Caution is needed in interpreting hospitalisation data presented from this review period, as highlighted by the zero notifications with four hospitalisations in the 60 years and over age group.

Where endemic measles has been eliminated, enhanced surveillance, including a high level of confirmation, is required and recommended by the WHO.128 By the end of this review period, a national case definition for measles requiring confirmation of all cases (either by laboratory tests or by linkage to a chain of transmission that includes a laboratory-confirmed case) had been adopted by all jurisdictions. The national case definition is more specific than previous jurisdictional case definitions, as notifications are no longer accepted based on clinical criteria alone, and the detection of measles virus-specific IgM antibody now requires confirmation in an approved reference laboratory. These changes are important for a country in the elimination phase and may have resulted in fewer notifications during the reporting period in states other than Victoria, where laboratory confirmation has been a requirement of notification since July 2001.

The current two-dose vaccination schedule began in 1998 following the mass vaccination of primary school aged children as part of the Measles Control Campaign (MCC).129 Progress towards elimination of endemic measles in Australia has been underpinned by consistently high vaccination coverage with this schedule prior to school entry. As a result of these efforts, birth cohorts now aged 5–9 and 10–14 years have good coverage with two doses of MMR vaccine and notification rates in these age groups have reached record low levels; in the final year of the review period there was one notification and no hospitalisations from 5–14 year olds. Good vaccination coverage in these birth cohorts has also led to increased herd immunity and may explain why notification rates have remained low in children under 12 months of age (who are not targeted for vaccination).

In Australia, high vaccination coverage in children and reduced exposure to naturally circulating measles virus have left a residual cohort of susceptible young adults born in the 1970s and early 1980s, when measles vaccine was first introduced but coverage was low. Since the MCC, most outbreaks have involved a high proportion of young adults in this birth cohort. To improve immunity in this age group the young adult MMR vaccination campaign was conducted during 2001.129 In 2002, a serosurvey, conducted by the NCIRS to evaluate this targeted program, indicated that the proportion immune in the young adult birth cohort had not increased and that a relatively high proportion of targeted individuals remained susceptible.130 The notification rate for 20–34 year olds in this review period was the highest of all age groups for the first time since the MCC in 1998.

The USA has recently declared their elimination of endemic measles.131,132 The WHO Regional Committee for the Western Pacific determined 2012 as the target date for measles elimination in the Western Pacific region.133 Modelling, using Australian serosurvey data, suggests elimination may already have been achieved130,134 and, using estimates of vaccine uptake, the models predict endemic measles transmission will remain absent from Australia until at least 2012.130 Continued monitoring is, however, still required as different geographic regions within Australia were projected to exceed the epidemic threshold at different times if vaccination coverage remained at current levels.134

Australia, and other countries of the region which have already eliminated indigenous measles transmission, remain at risk of measles importation from neighbouring countries where measles is still endemic. In Australia, the primary case of most recent measles outbreaks acquired their infection in another country and this is supported by molecular genotyping of clinical specimens or measles isolates from the resulting outbreaks.126,135 However, during the review period, there have been several outbreaks for which the source case has not been identified, including a measles outbreak amongst Indigenous people in Western Australia in 2004 (n=6). In summary, the young adult MMR vaccination program does not appear to have increased the proportion immune to measles in this age group and young adults account for a higher proportion of notified cases than ever before. Therefore, to maintain elimination in the longer term, additional efforts may be needed to target this cohort. The other age group most at risk are children aged 0–4 years who continue to have the second highest notification and the highest hospitalisation rates of any age group. Therefore, high coverage with two doses, including better timeliness and completeness of childhood vaccinations among preschool children, should remain an important goal of Australia’s measles control strategy.

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