The WHO says the measures people and governments are taking to prevent the spread of Covid-19, such as wearing masks and limiting public gatherings, have probably helped keep the flu in check. Increased flu vaccination rates may also be contributing, it says.

Another hypothesis holds that the broad spread of SARS-CoV-2, the virus that causes Covid-19, in countries like the U.S. may play a role in blocking the flu by lifting people’s immunity against other viruses. One study in the spring of 2020 in New York City found that people testing positive for SARS-CoV-2 were far less likely to be carrying other common viruses such as influenza viruses. Still, research into that hypothesis is just beginning.

What is clear is the historically low number of people with the flu.

“In a normal year, there are 50 to 100 flu patients every day around this time of the year,” said Sho Naito, who runs five clinics in the Tokyo metropolitan area. “But we recently have only two to three a week” at each site, he said.