Why flu season is getting so hard to predict

Also: The pitfalls of the FDA's accelerated drug approval program [View in browser](    [❤️]( October 25, 2022 Hi CommonHealth reader, When I called experts to see what the upcoming flu season has in store for us, I kept hearing the same thing: We don’t know. “We really can't predict it very well. Every flu season is different, and especially now in the setting of COVID, the flu seasons have been very different,” Larry Madoff, the medical director of the Bureau of Infectious Disease and Laboratory Sciences at the Massachusetts Department of Public Health, told me. Why? Experts pointed me to three explanations. First, the flu virus is constantly evolving. There can be little changes in the genome — called “drift” — that can make the virus slightly more or less transmissible. These changes can mean the vaccine isn’t quite as effective. There can also be big changes in the genome — called “shift” — that leave people's immune systems very unprepared to fight off the virus. Jennifer Wang, a professor at UMass Chan Medical School, told me this happened with swine flu and avian flu. Second, weather is a big unknown. Changes in temperature and humidity impact how long the virus is able to survive outside the body. “We know the cold, dry weather promotes transmission of influenza and also pushes people indoors into crowded spaces,” Madoff said. Brian Chow, an infectious diseases physician at Tufts Medical Center, added that climate change “makes it even harder to predict” the weather. The final big unknown is how flu interacts with COVID. The past two flu seasons have been particularly atypical. There was almost no flu in 2020-2021, and then 2021-2022 saw an early spike and a late spike, but a big drop in cases when flu usually peaks. For some of this, we can pat ourselves on the back for good pandemic precautions. Lots of good hand hygiene, mask wearing, distancing and staying home. But Wang told me that’s likely not the whole story. “In the beginning of the pandemic,” Wang said, “I remember thinking, ‘We're going to see a lot of dual infections.’ We didn't see that.” Instead, she said, infectious disease experts saw that as COVID took off, flu numbers plummeted. There have been some dual infections — where people got flu and COVID at the same time — but it has been a lot rarer than experts would have guessed. Nobody knows exactly why this is. But Wang thinks “it probably has to do with how our bodies respond to a COVID infection. And that may be providing a hostile environment for the flu.” All of these factors add up to an uncertain outlook. [As I reported yesterday]( experts are both concerned about this coming flu season and optimistic about a new mRNA flu vaccine in the works. For now, one thing they do say is clear is that we should get the standard flu vaccine quickly — ideally before the end of this month. 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