As temperatures drop in the Northern Hemisphere and people begin to move gatherings indoors, scientists and public health experts worry that COVID-19 cases could surge for a third year in a row, Science News reports.

The concern comes as cases begin to rise in the UK and other European countries.

“In the past, what’s happened in Europe often has been a harbinger for what’s about to happen in the United States,” Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, tells NPR. “So I think the bottom line message for us in this country is: We have to be prepared for what they are beginning to see in Europe.”

See “Is COVID-19 Seasonal?

This fall and winter, experts are unsure whether an entirely new variant of SARS-CoV-2 will emerge to dominate cases or whether Omicron subvariants will remain on the playing field, Science News reports. As of October 1, all of the SARS-CoV-2 lineages in the United States were Omicron variants, of which BA.5 was the most prevalent, causing an estimated 67.9 percent of new cases, according to the US Centers for Disease Control and Prevention (CDC). BA.5’s success lies in its ability to circumvent host immunity more effectively than previous variants. Similarly, researchers report in a preprint posted October 4 on bioRxiv that new Omicron variants such as BQ.1.1 and BA.2.75.2 can evade antibodies present in people previously infected with BA.2 and BA.5. Particularly if multiple immune-evading variants circulate in the coming months, people could be vulnerable to infection even if they have some level of immune protection, Science News reports.

No matter which variants spread through the remainder of this year, getting boosted could still be helpful, research indicates. Last week, Pfizer released the first human data on its new bivalent booster, which the company says provided a strong immune response against BA.4 and BA.5. Moreover, a paper posted on bioRxiv on September 22 indicated that booster shots that target current viral variants could also bolster immune responses against variants that don’t yet exist, or haven’t yet become widespread.

“It’s good to see evidence that, even when it’s imprinted, the immune system is adapting in ways that are helpful in redirecting to the newer variant,” Jesse Bloom, a computational virologist at the Fred Hutchinson Cancer Research Center in Seattle and coauthor of a recent preprint on immune response after a breakthrough infection with BA.1, tells Nature.

See: “What We Know About Getting a Second Booster Shot of COVID-19 Vaccines

Unlike in the winter of 2020, most people have been exposed to SARS-CoV-2 at some point either through infection or vaccination. However, the most recent booster shot, which was authorized for rollout in the US in early September, has not been as widely adopted as past vaccine doses. Only 11 million Americans—4 percent of the eligible population—had received the most recent shot as of last week (October 7), The Washington Post reports, and just one-third of US adults say they plan to get it. By comparison, more than 40 percent of Americans received the third booster that was first offered late last year, the Post adds, which itself is low compared to the 70 percent third-shot rates seen in countries like the UK.

See: “Pfizer and Moderna COVID-19 Omicron Boosters Get OK from the FDA

“Working on the vaccine development early on in the pandemic, I felt . . . I should just step away and go on tour and just convince people to get the vaccine,” Kizzmekia Corbett, a Harvard University immunologist who helped develop Moderna’s vaccine, tells the Post. “And I wish I could do that here with the boosters as well.”