Credit CDC
Under normal circumstances, most non-tropical countries experience a distinct winter respiratory season, which is punctuated by a yearly return of influenza A or B. In tropical climes, influenza spreads at low levels more or less year-round.
There are a few notable exceptions, such as Hong Kong, which often experiences a biphasic flu season (see Seasonality of Influenza A(H3N2) Virus: A Hong Kong Perspective (1997–2006).
A little more unusual, during the summer of 2017 Hong Kong, Taiwan, and parts of Southern China saw an unusually severe, summer H3N2 summer flu season (see Hong Kong's Summer Flu Appears To Have Peaked), which was followed by an unprecedented and intense out-of-season summer flu epidemic in Australia, as described below in a 2019 Eurosurveillance report:
There was a record number of laboratory-confirmed influenza cases during the interseasonal period November 2018 to May 2019 (n= 85,286; 5 times the previous 3-year average) and also more institutional outbreaks, hospitalisations and deaths, than what is normally seen.
The COVID pandemic - which brought about social distancing, the near-universal wearing of face masks in public, and the cancellation of most international travel for more than a year - disrupted the spread of influenza around the globe, making it (and other non-COVID respiratory viruses) effectively a no-show for the 2020-2021 flu season.
The relaxation of some of these COVID restrictions over the summer, however, has led to numerous out-of-season epidemics of RSV around the world (see here, here, and here), along with anecdotal reports of increased summer `colds' (see UK: `Worst Cold Ever' Trending On Social Media).
In recent weeks we've seen an early season arrival of influenza (H3N2) on several university campuses around the United States (see CDC Statement: Tracking Flu in Young Adults), and last week South Africa's NICD Issued A Late-Season Influenza Alert.
With more and more nations opening up their borders and relaxing their COVID restrictions, many of the barriers preventing the spread of influenza, and other viruses, have fallen.
Added to that, most children born over the past 18 months haven't been exposed to these common respiratory viruses, and older individual's immunity has likely waned due to a lack of recent exposure.
All of which brings us to an Editorial, published yesterday in the Medical Journal of Australia, on the prospects for seeing an early and intense flu season as Australia continues to open its borders to the world. Concerns that are based, in part, on the findings of a study (link below) on the drivers of Australia's summer flu surge of 2018-2019
Drivers of the summer influenza epidemic in New South Wales, 2018–19
Celeste K Marsh, Vicky Sheppeard, Sean Tobin, Robin Gilmour and Ross M Andrews
Med J Aust || doi: 10.5694/mja2.51266
Published online: 4 October 2021
(Excerpt)
Conclusions: Travel‐related factors were early drivers of the 2018‒19 NSW summer influenza epidemic; local transmission sustained the outbreak despite unfavourable conditions later in summer. Our findings prompted re‐evaluation of recommendations for pre‐travel vaccination in NSW. The role of travel in out‐of‐season influenza outbreaks should be considered in other temperate zones.
The Editorial, and an accompanying PODCAST (16 min), focus on the likely impact of opening Australia's borders in the months ahead, warning that the experience of 2018-2019 proves that summer isn't always a barrier to the importation and spread of influenza.
A link, and some excerpts, but you'll want to read the entire report (and listen to the podcast). I'll have a brief postscript after the break.
Preparing for out‐of‐season influenza epidemics when international travel resumes
Sheena G Sullivan
Med J Aust || doi: 10.5694/mja2.51340
Published online: 22 November 2021
In this issue of the MJA, Marsh and colleagues1 report that inter‐seasonal influenza activity in New South Wales was unusually high during the 2018–19 summer, and provide compelling evidence for the role of international travel in seeding local influenza epidemics. During December 2018 – March 2019, people with notified influenza infections were 3.7 times as likely to report recent overseas travel as other people; however, the odds were almost seven times as high for people with infections identified during the first two months of summer, suggesting that some of these cases initiated local outbreaks, with subsequent community transmission.1
(SNIP)
The report by Marsh and colleagues1 indicates that, as borders open in late 2021, we should expect the introduction of influenza viruses into Australia and possibly intense out‐of‐season activity. Re‐opening will precede our seasonal influenza vaccination campaign, which usually begins in April. To restrict the entry of influenza viruses, all returning travellers should be required to be vaccinated against influenza prior to travel, as recommended by the New South Wales Ministry of Health in response to the study by Marsh and colleagues.1
(SNIP)
Any resurgence of influenza will be most harshly felt by older people,7 particularly those in aged care facilities, among whom influenza mortality is high8 and the COVID‐19 pandemic has already exerted a heavy toll.9 It behoves us as a nation to protect our older citizens, and it reflects on us poorly when we fail to do so. In addition, influenza is likely to infect a considerable proportion of young children. In contrast to those infected with SARS‐CoV‐2, many children infected with influenza, especially those less than six months old, may need hospital care.7 A resurgence of respiratory syncytial virus outbreaks in late 2020/early 2021 has already challenged some paediatric hospitals in Australia.10 Increased severity of disease among patients co‐infected with SARS‐CoV‐2 and influenza was reported in China in 2020,11 and this could also have consequences for our healthcare system.
The disruption caused by the international response to the COVID pandemic may cause ripples in other disease patterns for years to come. Some potentially good, and some - as outlined above - potentially bad.
Pandemic inspired curbs international travel have made the importation of exotic infections - like MERS-CoV and avian flu - far less likely, but at the same time, surveillance for these types of infections has suffered in many parts of the world.
There is some hope that at least one strain of influenza B (Yamagata lineage) may actually have gone extinct (see Nature Influenza lineage extinction during the COVID-19 pandemic?), but it is probably way too early to pop open the champagne bottles.
Besides, nature abhors a vacuum, and another strain or subtype could emerge to take its place. Rates of other infectious diseases, like Scarlet Fever or EV-68, have plummeted over the past year, but could easily make a comeback next spring or summer.
Since COVID isn't over, the impact of the pandemic on the spread of other infectious diseases hasn't ended. A new variant, or a massive winter resurgence of Delta, could reset or change the trajectory of the next influenza season. Or, alternatively, a `twindemic' of COVID and Flu could spread globally.
We'll simply have to wait and see.
But we shouldn't expect an immediate return to normal seasonal patterns of disease spread once the pandemic has ended. The ripple effects of COVID are likely to remain with us for some time - including the impacts of `Long COVID' - and that could produce a number of unpredictable public health challenges for years to come.
