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“The fact that we’re already at this high level going into the holiday season makes me nervous,” said Scott Hensley, a microbiologist and flu expert at the Penn Institute for Immunology.

Hensley said flu is hitting harder earlier this year because population immunity is probably at its lowest level in recent history. Flu basically didn’t circulate for two years due to the masking and social distancing measures put in place during Covid, he said. As a result, large swaths of the population didn’t get an immunity boost from infection so they may be more vulnerable to flu this year than in past seasons.

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Seniors and children under age five are the most vulnerable, with hospitalization rates about double the national average. A flu variant that’s more severe for the elderly is also dominant right now, which means the U.S. could be in for a tough season. More than 60% of flu samples tested by public health labs were positive for the influenza A(H3N2) strain, according to CDC.

“It is a well described phenomenon. H3N2 has a more severe buy phone number list impact on older persons so more hospitalization, ICU admissions and deaths,” said Dr. William Schaffner, an infectious disease expert at Vanderbilt University.

Flu vaccines typically aren’t as effective against H3N2, though there’s hope that this season might prove different. The majority of flu viruses tested are similar to the strains included in this year’s vaccine, according to the CDC.

Vaccine efficacy data hasn’t been published yet, but the shots normally perform better when they are matched well to the circulating variants. Flu vaccine efficacy has ranged widely from 19% to 60% in past seasons depending on how well the shots were matched to the strains circulating.