AISR – 2021 national influenza season summary
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The Australian Influenza Surveillance Report (AISR) is compiled from several data sources used to monitor influenza activity and severity in the community. These data sources include:
- laboratory-confirmed notifications to NNDSS
- influenza associated hospitalisations
- sentinel influenza-like illness (ILI) reporting from general practitioners
- ILI-related community level surveys
- sentinel laboratory testing results.
The AISR is published fortnightly during the influenza season, typically between May and October. Influenza activity updates may be published outside of the seasonal period.
It is important to note that due to the COVID-19 epidemic in Australia, data reported from the various influenza surveillance systems may not represent an accurate reflection of influenza activity. Results should be interpreted with caution, especially where comparisons are made to previous influenza seasons. Interpretation of 2020 and 2021 influenza activity data should take into account, but are not limited to, the impact of social distancing measures, likely changes in health seeking behaviour of the community including access to alternative streams of acute respiratory infection specific health services, and focussed testing for COVID-19 response activities. Current COVID-19 related public health measures and the community’s adherence to public health messages are also likely having an effect on transmission of acute respiratory infections, including influenza.
Reporting Period: 01 January – 07 November 2021
Influenza and influenza-like illness (ILI) activity levels remained well below average across all systems. At the national level, notifications of laboratory-confirmed influenza remained at low levels, with a peak in notifications occurring in mid-May.
Given the low number of laboratory-confirmed influenza notifications, low community ILI activity, and small number of hospitalisations due to influenza at sentinel hospital sites, it is likely there was minimal impact on society due to influenza within the reporting period.
Given the low case numbers of laboratory-confirmed influenza, it is difficult to determine the potential severity of the 2021 season. In the reporting period, of the 598 notifications of laboratory-confirmed influenza, there were no laboratory-confirmed influenza-associated deaths notified to the National Notifiable Diseases Surveillance System (NNDSS)—the lowest rate reported in the last 5 years. The number of patients with confirmed influenza admitted to sentinel hospitals was also the lowest reported in the last 5 years.
Adults aged 85 years and older and those aged 60–69 years had the highest influenza notification rates, followed by children under 5 years of age. The notification rate was lowest among children aged 5–14 years.
Within the reporting period, the majority of nationally reported laboratory-confirmed influenza cases were influenza A (67.7%). Influenza B accounted for 21.9% of notifications, 3.5% were influenza A and B co-infection, 0.5% were influenza C, and 6.4% were untyped.
Vaccine match and effectiveness
The low case numbers of influenza across all systems during the 2021 season precludes meaningful analysis to estimate vaccine effectiveness. Of the relatively small number of samples referred to the WHO Centre for Collaboration (WHOCC), there was reasonable matching between the influenza A(H1N1) and influenza B/Victoria sample and the corresponding 2021 vaccine component. The influenza (H3N2) samples were not well matched with the corresponding vaccine component.