Intended for healthcare professionals

Opinion

“I’ll be back”: Australia’s experience of flu in 2022

BMJ 2022; 379 doi: https://doi.org/10.1136/bmj.o2998 (Published 14 December 2022) Cite this as: BMJ 2022;379:o2998
  1. Mallory J Trent, research associate,
  2. Aye Moa, research fellow and programme manager,
  3. C Raina MacIntyre, professor and programme head
  1. Biosecurity Program, Kirby Institute, University of New South Wales, Sydney, Australia

Raina MacIntyre and colleagues look at what we can learn from Australia’s 2022 flu season

The covid-19 pandemic showed us that non-pharmaceutical interventions such as masks, social distancing, border control, and lockdowns are effective at preventing flu and other respiratory viruses beyond covid. Like most of the world, Australia found that flu virtually disappeared during 2020 and 2021.12 In 2022—with international borders open, most non-pharmaceutical interventions abandoned, the resumption of normal social contact, and waning population protection from exposure to seasonal flu—many expressed concern about a “twindemic” of severe covid-19 and flu, or at the very least an exceptionally severe flu season. Australia’s 2022 flu season was not, it turns out, as severe as expected. Yet it still placed strain on a health system already under pressure from the winter covid-19 wave of the BA.2 variant.

The timing of the peaks of covid-19 and flu viruses in Australia didn’t occur at the same time in 2022. Australia experienced an unusually early and short lived peak of flu in June 2022, dominated by the subtype A H3N2.3 The number of laboratory confirmed flu notifications began to rise in the last two weeks of April and increased rapidly.4 There were fewer than 5000 flu notifications in the week ending 1 May, which jumped to more than 25 000 a week by 28 May, just as the covid-19 peak of BA.1 cases began to decline and the BA.2 wave began to rise. The incidence of flu peaked in mid-June at more than 30 000 notifications a week and then quickly tapered off. Notifications dropped to fewer than 5000 a week by mid-July.4 By contrast, data from the past decade show that the Australian flu season typically peaks around mid-August.5

Australia’s unusually early flu season may be related to its international border reopening fully in February 2022, a reduced immunity to flu in the population because of the absence of community transmission in 2020 and 2021, immune dysregulation due to covid-19 infection, or the lifting of mask mandates and other covid-19 mitigation measures. Australians typically receive their seasonal flu vaccines from March to May, so the early start to the season meant that a smaller proportion of the population was vaccinated, which may partially explain the rapid increase in cases in the community. As flu case numbers began to wane, the number of covid-19 cases began to rise—a trend also seen in early 2022 in the United States.6 Viral interference from SARS-CoV-2 has been postulated as a reason why both viruses don’t peak simultaneously.7

While notification data can paint a clear picture of the timing and duration of Australia’s flu season, this is less useful for determining its severity. The number of flu tests conducted each year increased more than 10-fold from 2010 to 2019 owing to increased accessibility and uptake of PCR tests8—and this has likely increased further during the covid-19 pandemic. Consequently, the large number of flu notifications we had during 2022 do not by themselves indicate a severe season.

Data on hospital admissions suggest that Australia’s 2022 flu season was moderate. From April to October, 1832 people were admitted to sentinel hospitals for flu—less than half as many as in 2019, which had the last severe H3N2 season before the covid-19 pandemic.39 Of those people, only 122 (6.7%) were admitted to intensive care, which was lower than in recent severe flu seasons.3 Even during the peak of Australia’s flu season, considerably more people were admitted to hospital with covid-19 than with flu in 2022.410 For example, from 5 June to 2 July in New South Wales 2168 patients were admitted to hospital with covid-19 but only 698 admissions were due to influenza-like illness,11 which excludes covid-19 but includes other respiratory viruses such as RSV. In Australia the deaths of 308 people were attributed to flu in 2022, while more than 700 were in 2017, which had the worst recent H3N2 epidemic.312 In contrast, more than 10 000 deaths were attributed to covid-19 in 2022.13

Opportunities to prevent transmission

The pandemic presented countries with an opportunity to implement long term interventions that would not only slow the spread of covid-19 but would also prevent transmission of flu and other respiratory viruses. Several states in Australia, for example, implemented programmes during the pandemic to reduce indoor transmission of covid in public settings such as restaurants. Among these, Victoria had a programme designed to help small businesses improve indoor ventilation and install HEPA (high efficiency particulate air) filters.14 These programmes have now ended, however, and it’s not clear when they will be reinstated, if ever.

Similarly, some countries have retained mask mandates, but there’s a reluctance to reintroduce them in Australia. Despite the emergence of highly immune evasive subvariants of omicron, along with waning vaccine immunity and suboptimal booster rates, mask mandates were removed in public spaces in most parts of Australia, although they were later reinstated in hospitals and care facilities for older people during the current covid-19 wave.

Australia hasn’t made the most of the lessons of covid-19 about masks, ventilation, and optimal vaccination to inform public health strategies to reduce the impact of seasonal flu. For individuals with chronic disease including chronic obstructive pulmonary disease and asthma, and for the health and aged care sectors, masks and other non-pharmaceutical interventions can help prevent respiratory viral infections and outbreaks. Despite an increase in flu vaccination rates in 2020, coverage has not increased considerably beyond pre-pandemic levels,15 and covid-19 booster vaccination has stagnated,16 underlining how Australia needs to maintain momentum to increase vaccination coverage.

Recent experience has shown us that covid-19 doesn’t have the seasonality of flu: it has multiple peaks a year, unlike flu’s winter peak. The burden of covid-19 still eclipses that of flu, but both are preventable. The use of a vaccine-plus strategy, enhanced by non-pharmaceutical interventions, should inform the policies that countries develop to prevent both covid-19 and flu, which will likely continue to co-circulate in the foreseeable future.

Footnotes

  • Competing interests: Raina MacIntyre currently receives funding for investigator driven research on influenza from Sanofi. She also currently receives funding from the Australian grant bodies NHMRC and MRFF. She is on the WHO Covid-19 Vaccine Composition Technical Advisory Group and the WHO SAGE Smallpox and Monkeypox Advisory Group. Nothing else to declare.

  • Provenance and peer review: Commissioned; not externally peer reviewed.

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