Scientists are beginning to speculate about the possibility that the flu season this year will be particularly bad because of an immunity debt, and experts are encouraging people to get vaccinated for influenza soon. The idea behind an immunity debt is simple: people have been particularly careful to avoid exposure to SARS-CoV-2 and have taken extra precautions like masking and distancing. In doing so, they also limited their exposure to other common pathogens like the cold and flu, leaving their immune system vulnerable to infection since it did not have to fight off any pathogens. The immunity debt may cause an increase in common viral infections, or more rare ones like respiratory syncytial virus (RSV), which usually causes mild symptoms similar to a cold.
While we don't yet know what will happen during the 2021/2022 flu season, data from the Southern Hemisphere has suggested the immunity debt is real. In New Zealand, a country that did very well, comparatively, in controlling the outbreak of COVID-19, there were almost 1,000 cases of RSV in 5 weeks in June and July, which is more than half the average number of cases for a typical, 29-week-long winter season.
A December 2020 study in the Proceedings of the National Academy of Sciences (PNAS) that used computational tools to model both the preventive measures used during the pandemic and the subsequent trajectory of an RSV outbreak also suggested that one coming this winter would be worse than usual. Their projections about influenza were less certain, however.
In August 2020, French researchers came to similar conclusions about future viral outbreaks in children. They cautioned that French kids are not vaccinated for varicella, rotavirus, or Neisseria meningitidis serogroup B and ACYW, and any of these have the potential to cause infections. In addition, vaccination rates all over the world have suffered because people have put off routine health care due to the pandemic, so children that have fallen behind on their vaccine schedule may also be at higher than usual risk due to an immunity debt. This report was published in Infectious Diseases Now.
One way to reduce your risk of serious illness is to get vaccinated for influenza, and to continue to wear a mask. Flu vaccines are already available for this season.