The Twindemic

The Covid-19 pandemic has shown us that non-pharmaceutical interventions such as masks, social distancing, border controls and lockdowns are useful in preventing influenza and other respiratory viruses.

Like most countries of the world, also Australia experienced a sharp decline, if not disappearance, of flu during 2020 and 2021.

In 2022, everyone abandoned main individual protection measures, reopen the borders, normal social contacts increased and decreased the protection of the population from exposure to seasonal flu. All those things lead a "twindemic", the possibility of being faced with a simultaneous infection of Covid-19 and flu or an exceptionally severe flu season.

The 2022 Australian flu season was, however, not as bad as assumed. Yet, it still put a strain on a health system that was under pressure from the winter wave of Covid-19 and its new variant.

Australia experienced an unusually early and short flu peak in June 2022, dominated by subtype A H3N2, while data from the last decade clearly shows that the Australian flu season typically peaks around mid-August.

The number of laboratory-confirmed influenza notifications began to increase in the last two weeks of April with a peak in mid-June, with more than 30 000 notifications per week, and then rapidly declined to less than 5,000 cases per week in mid-July.

Australia's unusually early flu season may be related to the full reopening of its international border in February 2022, reduced population immunity to flu due to the absence of community transmission in 2020 and 2021, immune dysregulation due to infection with Covid-19 or the lifting of the obligation to wear a mask in public places.

Australians typically receive their seasonal flu vaccines from March to May, so infection started spreading while only a smaller proportion of the population was being vaccinated, which may partly explain the fast rise in community cases.

An interesting aspect to consider is that the timing of the peaks of Covid-19 and flu viruses in Australia did not occur simultaneously in 2022. As flu case numbers began to decline, the number of Covid-19 cases began to increase, a trend also observed in early 2022 in the United States.

It is possible to presume that there is some sort of "competition" or interference between the two viruses, which is why both viruses do not peak at the same time.

Case notification data allow to understand the timing and length of the Australian flu season,but they are not usefull to understand severity of this infection.

In fact, the number of flu tests conducted each year increased more than 10 times from 2010 to 2019 due to the increased accessibility and diffusion of PCR tests and has probably increased further during the covid-19 pandemic.

The large number of flu notifications that were recorded during 2022 does not indicate a more severe season. As a matter of fact data on hospital admissions suggest that the 2022 Australian flu season was a moderate one.

No matter how severe Australia's flu season is, however, it's good to remember that the pandemic has provided us all with an opportunity to implement long-term interventions that would not only slow the spread of Covid-19, but also prevent flu transmission and spread of other respiratory viruses.

Several states have implemented systems to reduce indoor transmission of Covid in public settings, such as indoor ventilation systems and the installation of HEPA filters. Programs that now, however, seem to be abandoned.

Recent experience has shown us that covid-19 does not have the seasonality of the flu: it has multiple peaks distributed throughout the year, unlike the typical winter peak of the flu.

The burden of covid-19 still eclipses that of the flu, but both viruses are preventable. The use of an adequate vaccination strategy, enhanced by the reintroduction of the main individual protection measures and non-pharmaceutical interventions, would be desirable, also because it is extremely probable that the two viruses will continue to co-circulate in the near future.

REFERENCE

Mallory J Trent, Aye Moa, C Raina MacIntyre. “I’ll be back”: Australia’s experience of flu in 2022. BMJ 2022;379:o2998, https://doi.org/10.1136/bmj.o2998 

Bruce Y. Lee. Australia’s Bad Flu Season Raises ‘Twindemic’ Concerns For U.S. Winter 2022. Forbes Aug 20, 2022. https://www.forbes.com/sites/brucelee/2022/08/20/australias-bad-flu-season-raises-twindemic-concerns-for-us-winter-2022/?sh=5c47858f10a1