Warning: Flu Season Ahead

The prognosis for future influenza seasons is always uncertain — but this year, it is even more uncertain than usual. While influenza seasons have varied in the past, there were generally some predictable factors. For example, the influenza season typically peaks in the U.S. between December and March. However, the intensity of the influenza season often varies. In the U.S., from 2010-2011 to 2019-2020, case burden varied nearly fivefold, from 9.3 million to 45 million cases of symptomatic illness.

The flu season last year (the 2020-2021 season) was quite unusual: there was essentially no influenza activity detected by surveillance around the world for the whole season, despite the usual levels of testing. This was most likely caused by the interventions put in place to combat the spread of COVID-19. While this was beneficial for the already stressed healthcare system, it raises a concern for the 2021-2022 season — those low case numbers could potentially produce a decrease in population level immunity to influenza, since the immunity increase normally attributed to prior season infections is missing.

So, what is the role of population level immunity? Each year, many individual influenza strains are introduced into the U.S. Based on the characteristics of the strains, the number of introductions and a host of other factors, particular strains become predominant. Population immunity is one of the most important factors determining which strain will be most successful in a given season. Each individual in the population has a history of influenza encounters, including illness and vaccination, giving that person a level of immunity to infection that differs depending on the influenza strain they encounter. The strain that can overcome immunity in the largest proportion of individuals in the population is liable to be the most successful in any given year.

Given the flu trends last season and a potential decrease in population level immunity, is there cause for concern?

Predictions for the Coming Flu Season

Unfortunately, it’s difficult to know exactly what the coming flu season will look like. Modeling an influenza season that follows an extremely light season suggests there is a potential for a substantial increase in cases in that second season, depending on the transmissibility of the predominant strain and the immunity remaining from infections in the season prior to 2020-2021. But modeling also identifies the possibility of less severe impacts.

Some factors may mitigate the expected decrease in population immunity from the non-existent 2020-2021 season. Immunity due to infection is believed to last longer than one season, so there should not be complete loss of immunity that would lead to a pandemic level influenza season. Also, individuals appear to develop long-term immunity, lasting for decades or even throughout their lifetime due to first exposure to influenza.

Limited influenza seasons in all areas of the world implies more limited possibility for evolution of prior circulating strains since the last normal influenza season, which may make it more likely that vaccines are effective and that immunity from 2019-2020 infection will be more protective than if there had been significant antigenic drift. However, the circulation of influenza in non-human hosts and the ability of novel strains to emerge due to recombination also contribute to unpredictability. Our next influenza season could be dominated by a strain for which there is limited existing immunity and for which vaccines are less effective.

So, while increased intensity of influenza in 2021-2022 is a possible outcome of last year’s missing influenza epidemic, there are several other possible outcomes. Take Australia for example: the country saw a second low influenza season in June to September 2021 (wintertime in Australia), possibly due to decreased seeding of cases from other areas because of limits on travel in addition to continued COVID-19 interventions. The U.S. has lifted travel restrictions on international visitors into the country, suggesting that the potential for introduction of cases exists. However, that will depend on both the rate of travel and the level of circulation of influenza in other countries. It’s even possible that because of gradual reductions in COVID-19 interventions and continued individual level behaviors to decrease transmission, influenza will not rebound immediately but may take several seasons to reach pre-pandemic levels. The result of sequential low influenza seasons could make future season immunity even lower. It is also possible that influenza cases will expand and contract in a cyclic fashion, as each year reflects the prior year, with high years followed by low years, until a new equilibrium is established.

Planning for the Unpredictable

With so many factors at play, we just can’t say for certain what the coming flu season will look like. But one thing is sure: we need to be prepared for the worst. In winter, there are other illnesses that stress the healthcare system, making it critical that our hospitals are ready. COVID-19 is the most notable stress at the moment, and we can only hope it will become less of a strain in the coming months. Population immunity to COVID-19 has continued to increase through both vaccination and infection, but we cannot be sure if new immunity escape variants will emerge. Another concern is respiratory syncytial virus (RSV), which typically peaks during cooler months. While RSV, similar to influenza, was also depressed during the COVID-19 pandemic, it began to re-emerge during summer 2021. It is too early to tell if RSV will peak earlier than usual due to the summer start of the season or if it will continue to increase. Essentially any disease spreading by the respiratory pathway would have been limited in the fall and winter 2020-2021 and may now rebound, increasing the possibility of more stress on the healthcare system.

Despite the increased uncertainty surrounding the nearing influenza season, the importance of encouraging influenza vaccination is certain. Vaccination is a highly effective strategy for controlling infectious diseases and the best population-level tool to prevent illness. Influenza vaccination is safe and effective for preventing influenza and for decreasing the severity of cases. In this uncertain time, it is more important than ever for public health professionals and health providers to encourage influenza vaccination.

Mary Krauland, PhD, is a research assistant professor in Health Policy and Management and the Public Health Dynamics Lab at the University of Pittsburgh.