Just as summer vacation season gets underway, COVID-19 appears to be making a comeback in the U.S.

Nationally, the amount of SARS-CoV-2 virus in wastewater is still low, according to the Centers for Disease Control and Prevention (CDC), but levels have been gradually increasing in recent weeks. Wastewater data show COVID-19 hotspots have already emerged in parts of the Northeast, West, and South, as well as Hawaii.

The increase is apparently driven by the so-called FLiRT variants, which began circulating in the U.S. earlier this spring. FLiRT variants now account for the majority of new U.S. cases, according to the CDC, and seem to be driving an increase in transmission as well as a 16% rise in COVID-related emergency-department visits. Hospitalization and death rates are, for now, holding steady.

It’s not surprising that the U.S. would see a summer COVID-19 spike, says Dr. El Hussain Shamsa, an internal-medicine physician at University Hospitals in Ohio. In fact, as a study that Shamsa co-authored shows, that’s been the pattern in previous years: a big winter wave, followed by smaller upticks in the spring and summer.

In their study, Shamsa and his colleagues concluded that that pattern can’t be totally explained by external factors like weather, human behavior, or public-health campaigns, which suggests there’s something inherent to the virus that drives the seasonal pattern. “That’s what you see in many different types of viruses,” Shamsa says.

But why does COVID-19 seem to spread throughout much of the year, when other common respiratory viruses—like those that cause the flu and common cold—are predominantly fall and winter problems? The science isn’t settled, and not all experts are convinced those patterns will hold true in the future.

SARS-CoV-2 is still a new virus that’s evolving quickly, says Ilan Rubin, a postdoctoral research fellow at the Harvard T.H. Chan School of Public Health’s Center for Communicable Disease Dynamics. It’s possible that it will settle into a more traditional seasonal pattern over time, with most of its spread concentrated in winter, Rubin says.

For now, though, summer peaks seem to be happening not only because the virus is new and evolving, but also because immunity wanes after a previous infection or vaccination. While vaccines shield against severe disease and death for a long time, studies have shown that their ability to protect against all symptomatic cases drops significantly after about six months.

“If everybody’s getting vaccinated in November and December and then everyone is getting sick in December and January, the population is all becoming susceptible around the same time in the summer,” Rubin explains. Throw in increased travel and socializing during the summer, and you have “perfect conditions for whatever variant happens to be circulating at the time to start an increase [in cases],” he says.

It’s not clear yet whether the FLiRT variants will cause a significant summer spike or a smaller blip, Rubin says, but there are some reasons for optimism. The FLiRT variants are similar to Omicron, the previous dominant variant, which also overlapped with the older Delta variant. “That’s a good thing,” Rubin says, because “even if our immunity is lapsing, we probably had some exposure to something similar in the past,” which should help minimize the new variants’ impact.

It’s never possible to predict exactly what SARS-CoV-2 will do, but Dr. David Hirschwerk, an infectious-disease specialist at North Shore University Hospital in New York, says he’s not anticipating a massive surge this summer. “Right now, compared to where we’ve been in the past with the pandemic, the rates are very low in spite of the uptick,” he says. Transmission rates may be rising a bit, but they started from one of the lowest points of COVID-19 spread since the pandemic began, he says.

People at increased risk of severe disease, such as elderly adults and people with underlying health conditions, may want to consider taking extra precautions as the virus begins to circulate more widely, Hirschwerk says. But for most people, he says, there’s no need to take drastic measures.

“It’s an individual decision,” Hirschwerk says. “But for the vast majority of people, they can go on without worrying about things—at least the way they are on June 12, 2024.”