Background and aim: Influenza causes significant disease burden that is preventable through vaccination. This study examines contemporary trends in the epidemiology of influenza in Australia.

Methods: We descriptively analysed Australian national data on influenza notifications (2015–2024), hospitalisations (2010–2022) and deaths (2010–2023), obtained from the National Notifiable Disease Surveillance System, National Hospital Morbidity Database, and the Australian Coordinating Registry, respectively. We calculated age-specific rates by year and examined trends over time.

Outcomes: In the period analysed, rates of influenza notifications, hospitalisations and deaths were highest in 2017 (1022, 206 and 7 per 100,000, respectively) and 2019 (1237, 208 and 5 per 100,000, respectively). Prior to the COVID-19 pandemic onset (2010–2019), the highest hospitalisations rates were observed among infants aged <6 months and adults aged ≥65 years (291 and 273 per 100,000, respectively). Influenza disease burden was diminished following the onset of the COVID-19 pandemic until the end of 2021. Following the easing of public health restrictions, influenza notification rates among all age groupings of children aged <15 years were more than double those prior to the pandemic. Hospitalisation rates in 2022 were highest in young children aged <2 years (<6 months: 371; 6–11 months: 255; 12–23 months: 176, per 100,000 respectively). Throughout all periods, influenza death rates were highest among adults aged ≥75 years (24 per 100,000 in 2010–2019; 14 per 100,000 in 2022–2023).

Conclusions and future actions: Influenza disease burden is consistently highest among young children and older adults. Higher influenza notifications among children following easing of COVID-19 pandemic restrictions likely reflects increased exposure to influenza following a period of diminished transmission and increased testing. The findings support vaccination recommendations targeting children aged <5 years and older adults aged ≥65 years to prevent serious influenza disease, and considering approaches to improve vaccination coverage especially among children.

Background and Aim

Influenza A(H3N2) subclade K appears to have first emerged in typed Australian influenza samples from August 2025. This presentation describes the emergence of influenza A(H3N2) subclade K in Australia, the array of factors that contributed to its rapid spread, and early insights for the 2026 influenza season.

Methods and Analysis

We synthesised data from a range of national surveillance systems presented in the Australian Respiratory Surveillance Reports, to report on the emergence, transmission, antigenic characteristics, vaccine effectiveness, and severity of illness associated influenza A(H3N2) subclade K in late 2025 and early 2026.

Outcomes

Subclade K spread rapidly, contributing to an unexpected increase in influenza cases from October to December, outside of the typical Australian influenza season. Subclade K drifted to accumulate several mutations making the virus less responsive to antibodies produced by the previous season’s vaccines or from natural infections, leading to a larger susceptible population. This was compounded by reduced vaccine uptake (30.7% nationally), waning immunity from seasonal vaccines due to end-of-season emergence, and, in some instances, increased population mixing with end-of-school celebration events. Despite its rapid spread contributing to a small increase in hospitalisations in late 2025, other severity indicators did not suggest that subclade K caused more severe illness than previous H3N2 viruses. Although there was reduced reactivity to the 2025 southern hemisphere vaccine, the subclade K virus demonstrates a reasonable antigenic match to the 2026 southern hemisphere vaccine strain.

Conclusion and future actions

This is the first time that an influenza A(H3N2) variant has emerged and spread so rapidly towards the end of the influenza season in Australia. Findings presented at CDIC 2026 will provide early insight into how subclade K’s out-of-season circulation will impact Australia’s 2026 influenza season.

Background and Aim: In 2025 Influenza activity was high with three waves: early A/H1N1, mid-season influenza B, and finally an unusual late wave of influenza A/H3N2 subclade K (A/H3N2/K). In this report, we describe the epidemiology of hospitalisations in Australia due to the 2025 wave of A/H3N2/K in a hospital-based sentinel surveillance system.

Methods and Analysis: FluCAN is a national hospital-based sentinel surveillance system. Data were collected for all hospitalised influenza patients (and test-negative controls) from 01/01/2025 – 31/12/2025 who were admitted to any of the 22 sentinel hospitals across Australia. Demographic data of A/H3N2/K was compared to other influenza types/subtypes, and vaccine effectiveness (VE) was estimated by conditional logistic regression.

There were 7939 reported influenza-related hospitalisations in the study, including 351 A/H3N2/K. Of the A/H3N2/K cases, 203 (57.8%) were in children and 74 (21.1%) were in people aged 65+; 172 (49%) had chronic comorbidities, and 2 (0.6%) were pregnant; 15 (2.4%) resulted in intensive care admission. No statistically significant differences were observed in the demographics of hospitalised patients with A/H3N2/K and other influenza types/subtypes. However, the estimated VE against hospitalisation was lower for A/H3N2/K at 31.4% (95%CI: 6.7%, 49.5%) compared to influenza B (75.0% [95%CI: 68.8%, 80.0%]) and A/H1N1 (41.6% [95%CI: 34.2%, 48.2%]). VE for A/H3N2/K was highest in adults 65+ (55.0% [95%CI: 17.0%, 75.6%]).

Outcomes: This large Australian study identified that the epidemiology of A/H3N2/K is likely to be similar to other influenza types/subtypes, however, VE is lower. The lower VE may have been due to antigenic mismatch or waning immunity by the time of the A/H3N2/K wave.

Conclusion and Future actions: These data reinforce the value of surveillance in informing communications regarding the value of vaccination late in the season while influenza continues to circulate. It also complements studies assessing vaccine match with circulating influenza strains.

Background and Aim: It is unclear how timing of pregnancy influenza vaccination affects its effectiveness against infant influenza disease. We aimed to assess pregnancy influenza vaccine effectiveness (VE) in preventing infections, and hospital presentations and admissions in infants aged <6 months during the 2022 influenza season by trimester of vaccination and vaccine-season alignment.
Methods and Analysis: Population-based retrospective cohort study of infants born October-2021–September-2022 recorded in the New South Wales Perinatal Data Collection. Influenza vaccination during pregnancy (exposure) was ascertained using the Australian Immunisation Register. Outcomes were assessed through linked influenza notifications, Emergency Department (ED) presentations, and coded hospitalisations from 1 April–30 September 2022. Adjusted Cox models stratified by birth month were used to obtain hazard ratios (HRs) and estimate vaccine effectiveness (VE) up to 6 months of age.
Outcomes: We included 68,323 infants; 37,521 (54.9%) had unvaccinated mothers and 30,802 (45.1%) vaccinated. There were 345 and 106 influenza notifications, 239 and 61 ED presentations, and 117 and 28 hospitalisations among the unvaccinated and vaccinated, respectively. Overall, VE was 32% (13–46%) against influenza notification, 40% (18–56%) against ED presentation and 34% (-6–59%) against hospitalisation. VE against notification was 23% (-2–41%), 39% (6–61%), and 53% (12–75%) when vaccinated in the first, second and third trimester, respectively; 32% (2–53%), 40% (-4–65%) and 64% (19–84%) against ED presentation; and 19% (-46–55%), 60% (-15–86%) and 39% (-38–73%) against hospitalisation. The 2021-season vaccine was 27% (7–43%), 35% (10–53%) and 30% (-19–59%) effective against notification, ED presentation and hospitalisation, respectively; whilst the 2022-season vaccine was 67% (15–87%), 73% (17–92%) and 45% (-40–78%) effective.
Conclusion and Future action: Influenza vaccination during pregnancy prevents infant influenza disease with effectiveness appearing greater when vaccination occurred later in pregnancy. The 2021-season vaccine remained effective in 2022. Antenatal care and immunisation providers should encourage influenza vaccination during pregnancy year-round.

Background and Aim:
Several Australian jurisdictions are introducing seasonal influenza immunisation programmes in 2026, using an intranasally delivered live-attenuated influenza vaccine (LAIV), targeting pre-school aged children. Western Australia will also include primary school children. Given the key role played by children in influenza transmission, such immunisation programmes are expected to provide both direct and indirect (population-level) benefits. School-based LAIV programmes have been implemented internationally, most notably in the UK. As part of the Australia–Aotearoa Consortium for Epidemic Forecasting and Analytics (ACEFA) 2025-26 scenario modelling exercise, we estimated the potential health impacts of a school-based LAIV programme in Australia.

Methods and Analysis:
We developed a dynamic, age-stratified model of temperate seasonal influenza transmission, calibrated to Australian surveillance data using Approximate Bayesian Computation. We simulated a range of LAIV intervention scenarios, varying targeted age groups, vaccine effectiveness, and coverage levels. Simulations were performed over a one-year horizon with a single LAIV campaign rolled out before winter.

Outcomes:
All LAIV scenarios projected reductions in cases and hospitalisations across all age groups (not only in those receiving LAIV), indicating indirect effects. The more substantial indirect effects were modelled in the school-based LAIV programs. Including both primary and secondary schools approximately doubled the projected impact relative to a primary-school only rollout. A preschool-only program would likely have limited impact.

Conclusion and Future actions:
A school-based LAIV program in Australia could substantially reduce severe illness resulting from influenza infection across all age groups, particularly if high uptake is achieved, and both primary and secondary schools are included.

Background and Aim
Based on immunogenicity studies and the premise that young children are likely to be influenza-virus-naïve, the World Health Organization recommends previously unvaccinated children <9 years receive two influenza vaccine doses at least four weeks apart in the first year of vaccination. Information on the real-world vaccine effectiveness (VE) of this schedule, by dose, for influenza-virus-naïve children, is needed.
The temporary elimination of influenza in Australia in 2020–2021 provides an opportunity to investigate this issue.
Methods and Analysis
We used a Victorian birth cohort to emulate a three-arm target randomised controlled trial of one and two doses of inactivated influenza vaccination compared with no influenza vaccination amongst previously unvaccinated influenza-virus-naïve children. The cohort was constructed using routinely collected data for children born between 1 May 2020 and 30 October 2020.
We conducted 12 nested sequential trials between 1 April 2022 and 30 June 2022 that followed trial participants until 30 September 2022. Participants were eligible for inclusion in each trial provided they had no prior influenza vaccinations or infections. Participants receiving their first influenza vaccine dose during the first week of the trial were cloned and allocated to both intervention arms. Weighted matching was used to control for confounding and selection bias.
The primary outcome was laboratory-confirmed influenza. VE was calculated as 1 minus the risk ratio.
Outcomes
Of the 76,958 individuals in the cohort, 76,576 contributed to the unvaccinated arm at least once and 20,033 contributed to the intervention arms. Inactivated VE estimated in the sequence of target trials ranged from 1.5%–62% for one-dose and 19%–54% for two-dose interventions.
Conclusion and Future Actions
We plan to pool results from the trials to obtain an overall VE for the season and cohort which will provide important evidence about the value of this intervention with implications for vaccine policy.

Background and Aim
Influenza in older adults are at increased risk of severe complications, hospitalisations, and mortality. Two large-scale individually randomised trials comparing high-dose inactivated influenza vaccine (HD-IIV) versus standard-dose inactivated influenza vaccine (SD-IIV) against hospitalisation outcomes have been conducted in Denmark and Spain. Data was pooled from these trials to enhance generalisability and assess the relative vaccine effectiveness (rVE) of HD-IIV versus SD-IIV against severe clinical outcomes in 65+.

Methods and Analysis
FLUNITY-HD was a prespecified pooled analysis of two harmonised pragmatic individually randomised trials (DANFLU-2 and GALFLU) comparing HD-IIV (60 μg haemagglutinin per strain) with SD-IIV (15 μg per strain) in adults aged 65+ years. DANFLU-2 was conducted in Denmark (2022-25) and GALFLU in Galicia, Spain (2023-25). Participants were randomly assigned (1:1) and followed from 14 days post-vaccination using routine health-care databases. The primary endpoint was hospitalisation for influenza or pneumonia, with hierarchical secondary endpoints including cardiorespiratory hospitalisation, laboratory-confirmed influenza, and all-cause hospitalisation.

Outcomes
The analysis included 466,320 participants, of which 48.9% had at least one chronic condition. The primary endpoint of hospitalisation for influenza or pneumonia demonstrated a significantly reduced incidence in the HD-IIV group compared with SD-IIV group (rVE 8.8%; 95% CI 1.7 to 15.5). HD-IIV also significantly reduced the incidence of cardiorespiratory hospitalisation (rVE 6.3%; CI 2.5 to 10.0), laboratory-confirmed influenza hospitalisation (rVE 31.9%, 95% CI 19.7 to 42.2,) and all-cause hospitalisation (rVE 2.2%; 95% CI 0.3 to 4.1). Benefits were consistent by season. Safety profiles were comparable between groups.

Conclusion and Future Actions
In this prespecified pooled analysis, HD-IIV demonstrated superior protection compared with SD-IIV against hospitalisation for influenza or pneumonia, and also reduced the incidence of the secondary endpoints of cardiorespiratory hospitalisation, laboratory-confirmed influenza hospitalisation, and all-cause hospitalisation. Given wide eligibility for influenza vaccination, implementing HD-IIV could result in substantial public health benefits.