If you haven’t caught the flu this season, perhaps you know someone who has, or are concerned about the virus infiltrating your household. We are, by at least one measure, in the midst of the nation’s worst flu season in recent decades.
At least 24 million illnesses, 310,000 hospitalizations, and 13,000 influenza-linked deaths—including 57 children—have plagued the U.S. this season, according to the Centers for Disease Control and Prevention (CDC). Flu-related emergency department visits necessitated the CDC’s most severe “very high” ranking as of Feb. 7, as did influenza virus activity in national wastewater samples. What’s more, the percentage of outpatient and emergency department visits concerning patients with influenza-like illness (ILI) hit a season high of 7.8% the week ended Feb. 1, greater than any other week since at least the 1997–98 flu season.
Alarming though these numbers may seem, Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, doesn’t see a need for panic.
“We are in the midst of respiratory virus season, and influenza activity typically peaks in February,” Adalja tells Fortune. “There has been a respite in influenza during the early COVID years and now the virus is back in its usual pattern.”
From the 1982–83 through 2023–24 seasons, flu activity peaked in February 17 times, CDC records show. That’s nearly double the nine times of runner-up December. Recently, as Adalja pointed out, COVID altered traditional patterns of flu spread. In 2020–21, the first full flu season of the pandemic, flu activity was so unusually low that the CDC couldn’t discern a peak. But there’s a downside to the coronavirus effect, according to Dr. Mark Mulligan, director of infectious diseases and immunology at NYU Langone Health.
“After the masking and isolation of the COVID years, our population immunity [to flu] may be decreased since we haven’t been getting natural exposure and the immunity boosting that comes with that,” Mulligan tells Fortune.
The caveat to the soaring percentage of ILI outpatient visits is that it doesn’t necessarily represent laboratory-confirmed flu cases. Because ILI symptoms include fever and cough or sore throat, it’s possible the percentage of ILI cases includes people with other respiratory diseases such as COVID or respiratory syncytial virus (RSV). In terms of clinically confirmed flu cases, the national test positivity rate was 31.6% the week ended Feb. 1. That’s on par with the 30.2% recorded the week ended Feb. 1, 2020, just ahead of the COVID pandemic.
“The numbers this year are actually in line with the past few decades,” Mulligan says.
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Could low flu shot uptake be to blame?
The 2024–25 seasonal flu shot has been widely available at pharmacies, doctor’s offices, urgent-care clinics, and other health care providers since early September. If you missed the unofficial “vaccine before Halloween” memo, you’re not alone. As of the week ended Feb. 1, less than half of adults (45%) and children (46%) had been immunized. But Adalja isn’t convinced low uptake is the cause of this year’s flu activity.
“The influenza vaccination does not provide sterilizing immunity; people can still be infected with the virus despite being vaccinated,” Adalja tells Fortune. “The vaccination rate traditionally has been in the 50% vicinity. I don’t think the low vaccination rate has a major impact on the dynamics of the season, but it does have an impact on the severity, especially pediatric hospitalizations and deaths.”
As spring draws near, it’s not too late to get your flu shot, says Dr. Erica Shenoy, chief of infection control at Mass General Brigham. She gives the same advice to her own patients. Keep in mind that no vaccine is 100% effective, and your body takes about two weeks post-jab to develop immunity.
“Vaccination rates combined with vaccine effectiveness do impact how severe the flu season is, as measured by the number of illnesses, medical visits, hospitalizations, and deaths,” Shenoy tells Fortune. “For example, during the last flu season, based on vaccination rates and effectiveness, the CDC estimated that 9.8 million illnesses, 4.8 million medical visits, 120,000 hospitalizations, and 7,900 deaths were prevented by vaccination.”
At this time last year, flu vaccine uptake was just as poor for adults (45%) and slightly better for children (50%). The CDC’s Healthy People vaccination target is 70%.
Will the 2024–25 flu season keep getting worse?
Flu season may feel like it’s in abnormally full swing, but the CDC’s in-season severity assessment was “moderate” as of Jan. 25. That’s across all ages and considers illnesses, hospitalizations, and deaths. Yet with February not yet half over, things may change, Shenoy says.
“I don’t have a crystal ball but based on the national and regional data we have so far, which is about a week or so old, we are still on the upswing,” Shenoy tells Fortune. “This is reflected in our own data in our health care system, where we are seeing week-over-week increases in influenza A diagnoses and hospitalizations.”
Two types of influenza A, H1N1 and H3, accounted for most infections nationwide the week ended Feb. 1. But even when influenza A declines, we should be prepared for influenza B cases to creep up in late winter or early spring, Shenoy says. This year’s flu shot protects against H1N1, H3N2, and a B strain.
As it’s been nearly two years since the federal COVID public health emergency ended and even longer since people started easing back into their pre-pandemic routines, the coronavirus’s continued impact on the seasonal flu, if any, is up in the air.
“It’s not clear how behavior this year differs from last year in terms of masking and social distancing, and I haven’t seen data documenting a change,” Shenoy says. “Flu seasons vary year to year for many reasons, some of which we understand and some we don’t—yet.”
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