Extended Flu Season

June 15, 2022

Flu is circulating at unseasonably high levels.

Scott Hensley, Ph.D., professor of microbiology at the University of Pennsylvania, on the extended flu season.

Why It Matters: Peak flu season is typically December-February, with cases waning in the spring. Dr. Pritish Tosh, an infectious disease specialist for Mayo Clinic, noted: “It’s quite uncommon to see cases this far into June.” The CDC’s weekly influenza report states that “seasonal influenza viruses continue to circulate, and activity is increasing in parts of the country.” Cases remain high in certain parts of the country (such as New Mexico, Washington D.C., and Florida) but still relatively low in the majority of states.

What is contributing to the extended flu season?

  • Dr. Tosh explained that this year is a “perfect storm” of reasons. “In any given flu season, we expect about 10% of the population to become infected and develop some degree of natural immunity for the next flu season. But because of a near non-existent flu season the past two years (due to COVID restrictions), that natural immunity never happened, so more people have been susceptible to getting sick.”
  • Evolving pandemic practices and responses to different COVID-19 waves likely impacted the typical rhythm of flu season. Cases were considered at normal levels in December, but “then began to rise again in March, which correlated with the end of mask mandates and relaxing of other distancing measures,” said Dr. Sandra Nelson, associate clinical director of the infectious diseases division at Massachusetts General Hospital.
  • The flu vaccine, which is tweaked each year to help combat the evolving flu strains that circulate, is said to reduce infection by 40-60%. Because of the extended season, those who got their flu vaccine in late summer/early fall of last year are “more susceptible with the virus still spreading this close to the summer” (Today).

Something to consider: Though the flu season is longer than usual, it doesn’t necessarily mean the flu is more aggressive than in past years. Dr. Tosh said, “Though we’re seeing a prolonged number of cases, there’s been a lower amplitude overall.”

Something to watch: Australia reported record-breaking flu cases in May; this is notable because “Australia is in a unique position in that we are one of the first countries to face Covid and a simultaneous flu season that is similar to pre-Covid levels,” said Dr. Jonathan Anderson of the pharmaceutical company Seqirus. Experts are monitoring the situation to see how to best deal with simultaneous diseases.

Weekly U.S. Influenza Surveillance Report (CDC)

2021-2022 flu season: Why are cases still on the rise in the US? (USA Today)

Current U.S. flu activity map (CDC)

by Jenna Lee,