No one needs to be reminded of the negative impact the COVID-19 pandemic has had on our lives. But one could argue that the SARS-CoV-2 virus had one benefit: It sent influenza viruses (and some others) packing. Global influenza activity was historically low through 2020 and 2021, likely because people wore masks or stayed home. Influenza has only started picking up again this year.
So where does that leave us as we head into the 2022-2023 northern hemisphere flu season and what should we expect?
The entirely unsatisfactory answer is that no one knows. Predicting the nature of influenza seasons is a global public health challenge. While the World Health Organizations’ Global Influenza Surveillance and Response System (GISRS) and its more than 150 laboratories around the world — who incidentally are celebrating their 70th birthday this year — track and anticipate the evolution of the virus, our ability to predict the magnitude of an upcoming influenza season is poor. The COVID-19 pandemic has made this even more challenging. For starters, one sure bet used to be that we would have an influenza season, but that has not necessarily been the case of late.
So, what do we know? Unlike last year’s northern hemisphere season, we know that influenza is back. The southern hemisphere flu season, which occurs while those of us in the north enjoy summer, was a mixed bag. But in countries such as Australia and New Zealand, influenza returned with activity at or even above pre-2020 levels. While the magnitude of influenza activity in countries south of the equator doesn’t necessarily foretell of similar trends north, the more widespread activity cannot be ignored.
The influenza virus’ reappearance, combined with the return of international travel, ensures that northern hemisphere countries, except maybe those who still have tightly controlled borders, will have influenza viruses arrive at their doorstep. They probably already have. The CDC website showed during the week ending September 24, 2022, there was a small but steady rise in the number of influenza cases detected nationally. The early rise of case numbers in the U.S. was something experienced by some southern hemisphere countries who recorded very early influenza seasons, some weeks earlier than usual. One factor that likely contributed to this atypical seasonality is a change in population immunity to the influenza virus. Every year, somewhere between 10 to 30% of the population gets exposed to the influenza virus and, in doing so, get a bump in their immunity. We have now gone through a couple of years where, while vaccinations have continued, widespread natural infections haven’t. It is again difficult to quantify this lower immunity and conclude what impact it may have, but there is enough evidence to suggest that it could lead to atypical peaks of activity and potentially a more susceptible population. A more susceptible population would equate to more influenza activity.
Perhaps the biggest variable that will dictate the upcoming influenza season is the SARS-CoV-2 virus itself. The factors that drove influenza activity to low levels during the past 2 years likely included masking, social distancing, lack of international travel, and school closures, but also direct virus-virus interference with rampant SARS-CoV-2 spread keeping the lid on influenza virus transmission at the population level. There are biologic and immunologic phenomena that might explain why this is the case. Influenza virus activity was starting to pick up in the U.S. in early 2022 when the Omicron SARS-CoV-2 variant emerged and started rapidly spreading. Once that started, influenza activity dropped off.
I, for one, am becoming more inclined to believe we are less likely to see the “twindemic” of rampant COVID-19 and influenza at the same time. One may follow the other, but an overlap in peak activities might be biologically less likely. With several mutations starting to appear in the Omicron variants, and winter conditions such as crowding and other more favorable conditions for transmission coming, we may well experience another COVID-19 wave. If this occurs all bets are off as to what will happen to influenza activity.
Coming back to the question at hand, where does that leave us as we head into the 2022-2023 northern hemisphere flu season and what should we expect? There is a considerable amount of gazing into a crystal ball in these following comments. But I do believe we will have an influenza season, one that will be more like pre-pandemic seasons than post-pandemic. There is also a good chance the season will be early. The biggest unknown that could make a mockery of these predictions is SARS-CoV-2. If this virus comes roaring back with another variant and another wave, the crystal ball shatters.
I have left the “what can we do?” question until last. The answer is simple: Get vaccinated and encourage your patients to do so too. Make sure they get them both — influenza and COVID-19 (including all boosters for which they’re eligible). We are poor at predicting epidemic magnitude, but as cliched as it is, preparing for the worst and hoping for the best is prudent. Vaccination is central to preparing.
Richard J. Webby, PhD, is an influenza specialist at St. Jude Children’s Research Hospital, and a member of the WHO Global Influenza Surveillance and Response System network.