On Dec. 16, 2025, researchers from the International Vaccines Access Center (IVAC) reported that as of mid-December, flu, COVID, and RSV activity are relatively low throughout most of the U.S., but rates of all three are climbing just as many Americans travel and gather for the holidays.

And, for the first time in decades, measles outbreaks in parts of the country are adding even more questions for families about how to stay informed and safe.

There are two types of flu viruses—influenza A and influenza B—that are responsible for seasonal outbreaks. Influenza A has two subtypes, H1N1 and H3N2, and influenza B has two lineages, Victoria and Yamagata. Those subtypes and lineages can then be further classified into clades and subclades.

The vast majority of cases since the end of September have been flu A, according to the latest Centers for Disease Control and Prevention (CDC) data. And of flu A cases, most of those sequenced are H3N2, subclade K.

Subclade K—a relatively new strain of H3N2 that was identified this year—has driven an earlier start to the influenza season and higher-than-usual case rates in the UK, Japan, and Canada, said Andrew Pekosz, professor of Molecular Microbiology and Immunology, at IVAC.

Public health researchers are still working to understand whether subclade K is more likely than other flu viruses to cause severe illness. “There have been reports that the disease is particularly strong in children, and certainly there are reports of the elderly having higher hospitalization rates,” Pekosz explains. “Those are two things we expect with any influenza season surge.” It’s not clear yet, he says, whether the high rates of illness and hospitalization are proportional to the higher number of total cases, or whether subclade K is more likely to cause severe symptoms.

What is clear is that “influenza cases [are] increasing across the country, and that’s a trend that we’re sure will continue into the new year,” Pekosz said.