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COVID-19 restrictions may have weakened population immunity, so lifting the restrictions could trigger a flu outbreak. nito100/Getty Images
  • Flu infections dropped by 60% after the introduction of COVID-19 restrictions.
  • This reduced exposure to influenza may mean reduced population immunity.
  • The lifting of COVID-19 controls could lead to a large flu outbreak.
  • Anyone offered a flu vaccination should take it to reduce the risk.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date. Visit our coronavirus hub for the most recent information on the COVID-19 pandemic.

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A study in the Journal of Infectious Diseases has predicted a severe influenza outbreak once COVID-19 control measures are lifted, with increased levels of flu in the following years.

The researchers from Columbia University Mailman School of Health used computer modeling to quantify the reduction in transmission and incidence of flu after the implementation of control measures. They used these data together with data from the Centers for Disease Control and Prevention (CDC) FluView website to project influenza transmission over the next 5 years.

Non-pharmaceutical interventions (NPIs), such as mask-wearing, physical distancing, travel restrictions, and school closures, led to a 60% drop in flu infections during the 10 weeks after their introduction last year. The study suggests that the reduced exposure to flu during the control measures will have led to reduced immunity.

Dr. Jonathan Stoye, head of virology at the Francis Crick Institute in London, United Kingdom, told Medical News Today: “This modeling study […] suggests that the reduced numbers of infections in 2020 will lead to waning population immunity and that this may, in turn, contribute towards a surge in flu infections for several years.”

The researchers predict that relaxing measures could lead to a large-scale flu outbreak, particularly in parts of the United States where there were high levels of COVID-19 control compliance. They also anticipate that the low levels of flu during the pandemic may make it difficult to predict which circulating flu strains they will need to use to inform the future vaccines. This could reduce the effectiveness of influenza vaccines.

On a more optimistic note, the authors concede that the predicted bad flu season is not inevitable. Because of the focus on COVID-19, flu may have been underreported last year, so more people may have been exposed to the virus than their modeling recognizes.

Alternatively, because of reduced flu transmission, the virus will have had less chance to mutate and produce new variants. People might therefore have immunity from earlier flu infections, which would lead to a less severe outbreak.

Prof. William Schaffner, Professor of Preventive Medicine at Vanderbilt University School of Medicine, TN, is not convinced by the suggestion of underreporting: “Last year had the lowest incidence of flu any of us can remember due to the restraints for COVID-19. I don’t think there was underreporting of influenza. We did not detect a major drop in testing, but little flu was detected. I think the low flu rate was genuine.”

But Dr. Stoye agrees that the severity of an outbreak depends on the number of variants: “It will be interesting to see whether such an increase does, in fact, occur as rates of viral infection are driven by multiple factors, such as changes in the rate of appearance of new viral variants.”

Prof. Schaffner points out that other respiratory infections have increased as life starts to return to normal, which would suggest that we need to take the warning of a bad flu season seriously. This summer, with fewer restrictions and the return of children to school, there has been a surge in cases of respiratory syncytial virus, an infection usually seen only in the winter months. “This could be a harbinger of things to come,” he noted.

“This year, it’s more important than ever to get your flu shot. While we’re rightly focused on protecting ourselves against COVID-19, we shouldn’t forget about the flu, which can be fatal.”

– Senior author Dr. Sen Pei, Ph.D, Assistant Professor of Environmental Health Sciences, Mailman School of Public Health, Columbia.

“Predicting flu is a hazardous occupation,” Prof. Schaffner says. “Because we have had such a low previous flu season, has our immunity waned in such a way that we are susceptible to more transmission and more serious disease? Flu is fickle — we’ll just have to wait and see.”

His advice is unequivocal: “Take the jab.”

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