Over 500 people at the University of Michigan have recently come down with influenza, with a large majority of cases being unvaccinated.

An outbreak of flu at the University of Michigan has sickened more than 500 people since early October. Local health officials have now brought in help from the Centers for Disease Control and Prevention to investigate the matter, which may offer a preview of the flu season this winter. One important consideration is that a majority of those sick so far have been unvaccinated.

As of this week, 528 cases of influenza have been reported by the university since October 6, according to a news release from the university. Most of these cases have been diagnosed recently, however, with over 300 reported last week alone. These cases have been tied to a commonly circulating subtype of influenza A known as H3N2.

The typical flu season in the U.S. can start as early as September and run through April. But the uncharacteristic size and spread of this outbreak at this point of the year was enough to prompt Michigan officials to call the CDC.

“While we often start to see some flu activity now, the size of this outbreak is unusual,” Juan Luis Marquez, a medical director with the Washtenaw County Health Department, is quoted as saying in the university release.

Last winter, the seasonal flu took an unexpected hiatus, thanks largely to social distancing and other measures like mask-wearing that were implemented to slow the spread of the covid-19 pandemic. Though covid-19 still managed to cause the largest peak of illness seen in the U.S. to date during those months, the less contagious flu effectively disappeared.

But states and the federal government have made it clear that mass vaccination, not restrictive measures, will be the primary strategy for controlling the pandemic moving forward. So Americans are likely to spend more time around others and in large gatherings this winter. Many experts remain wary that we may see a resurgence of both the flu and covid-19—a so-called twindemic. Cases of covid-19 in the U.S. have started to climb again recently after a dropoff, while deaths and hospitalizations remain stubbornly high. While it’s too early to tell whether the flu will return to full strength this year, the current outbreak at the University of Michigan isn’t an encouraging sign.

“This outbreak doesn’t necessarily have an immediate impact on the broader local community, but it does raise concerns about what the flu season may bring,” said Marquez.

Of course, we have vaccines available to train our immune systems against both viruses. The original covid-19 shots continue to provide high protection against the worst outcomes of the disease, and it’s become clear by now that booster doses are restoring the vaccines’ high level of effectiveness against infection and transmission, at least temporarily. The seasonal flu vaccine is generally less effective against the fast-evolving flu, but it still prevents a sizable number of sick days, hospitalizations, and deaths every year. Notably, 77% of the cases during this latest outbreak in Michigan were unvaccinated.

Officials are recommending that everyone at the university get vaccinated as soon as possible, and the university is holding a vaccination drive on Tuesday.