Chasing down the flu

Scientists who study the influenza virus are reporting a potential silver lining from the COVID-19 pandemic. According to a report in STAT, experts who track the prevalence of flu strains say that at least two have apparently died out. If true, the disappearance should give scientists who design flu shots a leg up in predicting which strains to prepare for in any given season. And more accurate predictions could lead to more lives saved by seasonal flu vaccines.

Each year from 2010 and 2017, between 9 million and 45 million Americans contracted the flu and 35,000 died from it on average, according to the Centers for Disease Control and Prevention. That changed this year. While the CDC has yet to release estimates for the 2020-2021 flu season, some states have reported record low numbers of infections. In Pennsylvania, state health officials in June reported just 3,664 confirmed cases over the course of the previous flu season—down some 95 percent from the season before.

Most experts attribute the precipitous fall in flu cases to precautions taken against COVID-19, like mask wearing, social distancing, and school closures. “I’ve been an infectious specialist for 45 years. I’ve never seen anything approaching what we’ve seen this year in terms of the number of influenza cases,” UC Berkeley School of Public Health Professor John Swartzberg told USA TODAY. “There’s only one major [explanation] that can be reasonably entertained—the nonpharmaceutical interventions we instituted to protect ourselves from COVID.”

In fact, those interventions may not only have led to an extraordinarily mild flu season, they might have eradicated a pair of influenza strains. Epidemiologists divide the influenza viruses that most afflict humans into two main categories: influenza A and influenza B. Within A, epidemiologists have identified two major subtypes based on proteins found within the virus: H1N1, which caused the Spanish Flu epidemic a century ago, and H3N2. Within each subtype, scientists further classify particular strains into what they call clades, a smaller and more specific sub-classification for flu viruses.

According to STAT, researchers who study flu strains say that one clade of H3N2 and one strain of influenza B have gone missing from infection databases.

The findings come with a slate of qualifications. The missing flu strains could be hiding out in a portion of the world that didn’t see as much mask wearing or social distancing. And not every country keeps as close track of flu varieties as Western democracies do. “The world is a very big place,” one flu epidemiologist told STAT.

Most years, vaccine makers play a guessing game designing flu shots. In any given seasonal shot, they can include at most four weakened virus strains out of potentially dozens of possible permutations. The World Health Organization and national agencies like the Centers for Disease Control and Prevention collect data from more than 100 countries to see which variants are trending in any given year. Then the U.S. Food and Drug Administration makes the call for U.S. flu vaccines.

Typically, American and European scientists study the Southern Hemisphere’s flu season for clues about what may be coming next. If scientists choose well, the seasonal flu shots can help tamp down the effects of flu season. But viruses can be hard to predict. Immunologist and presidential adviser Anthony Fauci co-authored an article in the New England Journal of Medicine blaming the particularly strong 2017-2018 flu season on a strain researchers failed to predict.

If some flu strains disappear, the task of correctly predicting next season’s flu should become easier.

“Without doubt this is definitely going to change something in terms of the diversity of flu viruses out there,” St. Jude’s Children’s Research Hospital faculty member and influenza expert Richard Webby told STAT. “The extent to which it changes and how long it stays changed are the big question marks. But we have never seen this before.”