Since the novel coronavirus began its global spread, influenza cases reported to the World Health Organization have dropped to minuscule levels. The reason, epidemiologists think, is that the public health measures taken to keep the coronavirus from spreading also stop the flu. Influenza viruses are transmitted in much the same way as SARS-CoV-2, but they are less effective at jumping from host to host.
As Scientific Americanreported last fall, the drop-off in flu numbers was both swift and universal. Since then, cases have stayed remarkably low. “There’s just no flu circulating,” says Greg Poland, who has studied the disease at the Mayo Clinic for decades. The U.S. saw about 600 deaths from influenza during the 2020–2021 flu season. In comparison, the Centers for Disease Control and Prevention estimated there were roughly 22,000 deaths in the prior season and 34,000 two seasons ago.
Because each year’s flu vaccine is based on strains that have been circulating during the past year, it is unclear how next year’s vaccine will fare, should the typical patterns of the disease return. The WHO made its flu strain recommendations for vaccines in late February as usual, but they were based on far fewer cases than in a common year. At the same time, with fewer virus particles circulating in the world, there is less chance of an upcoming mutation, so it is possible the 2021–2022 vaccine will prove extra effective.
Public health experts are grateful for the reprieve. Some are also worried about a lost immune response, however. If influenza subsides for several years, today’s toddlers could miss a chance to have an early-age response imprinted on their immune system. That could be good or bad, depending on what strains circulate during the rest of their life. For now, future flu transmission remains a roll of the dice.
Influenza Cases Worldwide, by Region
The World Health Organization tracks influenza transmission in 18 zones. Three of those regions appear here. Only people who get tested for influenzalike illnesses—typically about 5 percent of individuals who fall ill—are tallied.
Creating the influenza vaccine this year has been more difficult than in the past.
Every year, scientists evaluate the strains of influenza that are circulating around the world, and meet to decide which strains to protect against in that year’s vaccine. They look at the strains that are getting people sick, and use that information to predict which strains are most likely to infect people when flu season sets in.
“We met at the end of February to make those recommendations,” said Dr. Webby, referring to the World Health Organization panel that assesses the flu vaccine. “And it was tricky. The amount of data was orders of magnitude less than it typically is.”
Dr. Olsen, the C.D.C. epidemiologist, pointed out that the vaccine choices are based on more than just existing strains. Scientists also consider other data, including forecasts of “the likelihood of any emerging groups of influenza viruses becoming more prevalent in coming months.”
And, she said, the uncertainty around the return of influenza makes getting vaccinated against the flu more important, not less.
There’s another hard-to-predict factor that could play a significant role when the flu comes back: whether society will carry on behaviors learned in the pandemic that benefit public health. Will mask-wearing become the norm? Will employers give their employees more physical space?
The last time Americans had a chance to make those behaviors part of the culture, Dr. Baker pointed out, they did not.
“The 1918 influenza pandemic should have been something that gave us some sort of societal learning,” said Dr. Baker, but behavior did not change. “So what is the journey you are about to go on from the Covid-19 pandemic, along that axis?” she added. “Will you wear your mask, even if no one else is?”