The BA.2 variant has caused a rise of COVD-19 cases in Europe and experts say a similar pattern may be coming to the U.S. And the most vulnerable group are those over 65. As many as 28 million seniors remain at risk of becoming severely ill, either because they are not vaccinated, or not fully vaccinated, or because it has been more than five months since their second or third dose of the vaccine.

According to CNN, it is the over 65 population who will determine how future variants affect the nation because the risk of severe outcomes rises dramatically with age.

“It’s really looking at that older age group and how much prior immunity they have, either from previous infection or vaccination, that I think has been the best indicator so far of how severe a given number of cases is going to end up being in terms of hospitalizations and deaths.” said Stephen Kissler, an infectious disease expert at Harvard’s T.H. Chan School of Public Health.

In the U.K., a recent analysis by the Health Security Agency found that the BA.2 subvariant of omicron is growing 80% faster than BA.1, the virus that caused the wave of infections in the U.S. last winter. Both cases and hospitalizations are rising in several European countries, as well as in the U.K., where the BA.2 strain is dominant.

Stating that the most vulnerable group are seniors, the pharmaceutical company Pfizer asked the Food and Drug Administration (FDA) to approve a fourth shot for those over the age of 65, according to CBS News. Rival company Moderna asked the FDA to authorize a fourth shot of its COVID-19 vaccine as a booster dose for all adults last week.

But U.S. surgeon General Vivek Murthy said that while the new variant could cause a spike in cases, the country is in much better shape to cope with a wave of cases than it was two years ago, when COVID-19 “defined our lives.”

“We should be prepared, COVID hasn’t gone away,” Murthy said, according to CNBC. “Our focus should be on preparation, not on panic.”

Dr. Scott Gottlieb, a former head of the FDA and a board member of Pfizer echoed these sentiments.

“I think we’re going to continue to see low levels of infection through the summer. But before we get there, we’re probably going to see some up-tick of infection like the Europeans are seeing right now, maybe not as pronounced,” he said.

But other experts point out that more than 82% of adults in the U.K. have had their third shot of a COVID-19 vaccine, which is crucial to prevent severe illness and hospitalizations, while only 36% of U.S. adults have been boosted, says CNN.

And studies show that the effectiveness of the vaccine wanes over time, falling as much as 70% after six months against the omicron variant. As far as antibody response, a whopping 98% of adults in the U.K. tested positive for antibodies for COVID-19 at the end of February compared to only 43% of Americans.

These figures could mean that the highly transmissible BA.2 variant can cause worse outcomes here than in the U.K.

“I do still think it’s a potential cause for concern that we may still see a higher case fatality rate and higher hospitalizations for COVID in the U.S than the U.K. because of the differences in underlying immunity,” Kissler told CNN. “Every additional layer of protection that we get helps, and so I would highly recommend, especially somebody who’s elderly who has not yet gotten vaccinated to do so, because it can really go a long way towards giving you the durable and robust immunity that you want. This is definitely the time.”