New research warns that the United States could experience a severe influenza outbreak after public health measures like face masks and social distancing are lifted. These measures have protected people from COVID-19 and influenza—incidence of influenza declined 60 percent during the first ten weeks following the implementation of the measures—but is also leading to greater numbers of Americans susceptible to the flu as immunity to the virus wanes.

The findings by researchers at Columbia University Mailman School of Public Health are published in the Journal of Infectious Diseases.

The use of public health measures like face masks and social distancing has declined substantially since the 2020/2021 flu seasons, although not yet to pre-pandemic levels. To what extent the flu might reassert itself in the U.S. this winter likely hinges on how much these behaviors continue.

“In the short term, measures to control the spread of the coronavirus will likely continue to suppress the number of influenza infections, but after these measures are relaxed, with greater population-wide immune susceptibility to influenza could lead to a large outbreak,” says senior author Sen Pei, PhD, assistant professor of environmental health sciences. “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.”

Pei and colleagues used a computer model of influenza A/H1 and B, which circulated in early 2020, to quantify the reduction of incidence and transmission after the implementation of non-pharmaceutical interventions (NPI) in most states on March 15, 2020. NPIs include travel restrictions, face masks, social distancing, public education on prevention measures, and school closures. They also projected influenza transmission at the national scale over the next five years. The model used data from the CDC FluView website.

A LARGE OUTBREAK FOLLOWED BY YEARS OF SEVERE FLU SEASONS

According to the model’s projections, as public health measures are relaxed, a large-scale influenza outbreak will likely take place. In subsequent seasons, outbreaks will also be elevated, gradually return to pre-pandemic levels. Over the last decade, U.S. influenza deaths ranged from 12,000 in 2011-12 to 61,000 in 2017-2018. “Our projections show that the downstream, ripple-effects of pandemic public health measures could persist for a number of years,” says Pei

HOW BAD COULD THE FLU OUTBREAK BE?

It could be really bad. The longer the control measures are in place before they are lifted, the greater the number of individuals who will be susceptible to an influenza infection—a situation that could lead to a more severe outbreak in parts of the country that had high levels of adherence to masking and other public health measures during the pandemic. Additionally, the researchers anticipate that the global suppression of influenza activity during the pandemic may make it difficult to predict future circulating strains to inform the production of influenza vaccines. As a result, vaccine effectiveness might be reduced. Furthermore, suppression of influenza during the pandemic could lead to a surge of multiple influenza strains, including the A/H3 strain which has a high mortality rate and hasn’t been widely circulating since the 2018-2019 season. All these factors would contribute to a more severe outbreak.

Or maybe not. The suppression of influenza during the 10-week study period could be overstated in the data due to people’s reluctance to seek medical care for non-emergencies during the pandemic. Another question is whether or not antigenic escape (viral mutations that allow it to evade the immune system) rather than waning immunity is the predominant mechanism in the accumulation of population susceptibility to influenza. If so, influenza would have less opportunity to mutate while public health measures are in place. A more durable influenza immunity would also result in fewer people being susceptible to influenza when public health measures are lifted. All these possibilities would contribute to a less severe outbreak.

REGIONAL VARIATION IN 2020 FLU ACTIVITY

During the study period, changes in Influenza A activity over a simulated season without NFTs varied by public health region, from a reduction of 68 percent in Region 3 (Delaware, Pennsylvania, Virginia, West Virginia, and Washington, D.C.) to 88 percent in Region 5 (Illinois, Indiana, Michigan, Minnesota, Ohio). Reductions in Influenza B activity ranged from 32 percent in Region 1 (Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, Vermont) to 91 percent in Region 7 (Iowa, Kansas, Missouri, Nebraska). Unusually, Region 2 (New Jersey, New York, Puerto Rico, and the Virgin Islands) saw an increase in influenza incidence—an outcome the authors attribute to elevated medical visits early on in the pandemic that detected influenza cases that would have otherwise gone undetected. Due to this bias, the authors excluded Region 2 from their projections.

The study’s first author is Yuchen Qi, former MS student in biostatistics. Jeffrey Shaman, professor of environmental health sciences, is a co-author.

Their research was supported by the National Science Foundation (DMS-2027369) and a gift from the Morris-Singer Foundation.

Shaman and Columbia University disclose partial ownership of SK Analytics. Shaman discloses consulting for BNI. Other authors declare no conflict of interest.

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The man died last week at Riverside University Health System-Medical Center in Moreno Valley. There were eight flu-related deaths in the 2019-20 flu season and nine deaths in 2018-2019 for residents under age 65, which is the required age range for reporting by California Department of Public Health.

The death comes as health officials and the community cope with the coronavirus pandemic, which has claimed more than 1,000 lives in Riverside County. There have been more than 53,000 confirmed COVID-19 cases in the county.

Kim Saruwatari, director of Riverside County Public Health, offered condolences to the family and said the death is a reminder that residents should get their flu shot as soon as possible.

“There has never been a more important time for someone to get their flu shot because we are also dealing with the COVID-19 pandemic,” said Saruwatari. “Getting the vaccine is the most effective way to prevent contracting the flu.”

Dr. Cameron Kaiser, Riverside County public health officer, said residents should get their flu shot before the end of October. The Centers for Disease Control and Prevention recommend all persons 6 months of age and older receive a flu vaccination each year, and some providers have this year’s shot already in stock. Children 8 years old and under who haven’t received at least two flu shots in prior seasons may need two shots to be fully protected; consult your primary care provider.

Kaiser also recommends frequent hand washing and avoiding getting close to people who are sick as ways to avoid the flu. If you or your child are sick, stay home from work and school.

Some of the same preventative measure used to slow the spread of coronavirus can also help to stop the flu, including wearing a mask, practice social distancing and frequent hand washing.

“Our condolences go to the family and friends of this individual who passed away from the flu,” said Riverside County Board Chair and Fourth District Supervisor V. Manuel Perez. “As we approach the seasonal flu period, we should all get the flu shot to reduce our chances of catching the flu and spreading it. It’s important and it will help avoid overwhelming the hospitals.”

For more information on where flu shots are being offered throughout Riverside County, www.rivcoimm.org.

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