Even as COVID-19 cases and hospitalizations driven by the omicron subvariant BA.5 continue to fall in the United States, the proportion of infections attributed to other omicron offshoots is rising.
According to data from the Centers for Disease Control and Prevention, BA.5 currently makes up 67.9% of new virus cases as of last Friday.
The remaining cases are made up of BA.4.6 -- which has been circulating in the U.S. for weeks -- as well as newly identified subvariants like BQ.1, B.Q.1.1 and BF.7, which have been spreading mainly in Europe.
The subvariants have been attributed to an increase in infections in Europe. According to the latest weekly report from the World Health Organization, published Oct. 12, there were more than 1.66 million COVID-19 cases recorded across the continent, up from more than 1.53 million cases recorded in the previous report published on Oct. 5.
Throughout the course of the pandemic, Europe has been considered a bellwether of what's come in the U.S.
Should we be concerned about the new subvariants? Experts told ABC News that the U.S. may see a new wave as we enter the colder weather months and move indoors and it's important to remain vigilant.
"In the next few months, I think there's reasonable expectation that we'll probably see a fall wave," said Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital and an ABC News contributor. "But the magnitude of that wave, I think, is still very much in question."
What are the new subvariants?
The subvariant that makes up the most cases after BA.5 is BA.4.6. An offshoot of the omicron subvariant BA.4, it's estimated to account for 12% of new cases, according to the CDC.
Then there are newer subvariants, which have recently begun steadily spreading in the U.S: BQ.1 and BQ.1.1, Both are offshoots of BA.5 and make up 12% of new cases together. This is a rapid rate of increase considering they made up just 3% of new cases for the week ending Oct. 1.
"It seems to have come out of nowhere," Dr. Peter Chin-Hong, an infectious diseases specialist at the University of California, San Francisco, told ABC News. "But it didn't really come of nowhere. It was first seen in Nigeria in summer and then spread to other countries in Europe and Asia, and now in the United States."
Another offshoot of BA.5 that is picking up speed is BF.7. It makes up 5.3% of new cases in the U.S., up from 3.2% just two weeks prior.
Meanwhile, there have been reports of another subvariant called XBB, which combines multiple strains of the omicron variant and other COVID-19 variants and has been detected mostly in Asia. So far, it has not been detected in the U.S.
Not much is known about any of these newer subvariants. Preliminary data suggests they have mutations that are better at evading immunity, whether from vaccination or prior infection, and may be more transmissible.
Another reason these offshoots could be spreading more rapidly is because human behavior has changed. Travel is reaching pre-pandemic levels, children are back in school and most restrictions -- including masking, social distancing and vaccinations requirements -- have ended.
There is some good news. Since September, bivalent boosters that target BA.4 and BA.5 have been approved in the U.S. for those aged 12 and older and for those aged 5 and older since last week. Early data suggests the booster increases antibody response.
Because these subvariants are related to BA.4 and BA.5, it's likely that the booster will provide at least some protection against them as well.
"I think there's a very reasonable expectation that the vaccines, especially with the new boosters, will provide good protection," said Brownstein. "But we're still missing a lot of real-world data, especially given that these new sort of omicron variants of BA.5 and BA.4 are starting to take hold in various parts of the world."
"We'll hopefully have reasonable insight from other countries that will help inform some of the forecast for the coming months, but that data is still being compiled," he continued.
Chin-Hong said it's also reasonable to expect, as with previous variants, that those most at risk are Americans with the least amount of protection.
"For people who are unvaccinated, those over age 65 who've never gotten a booster, or people who are immunocompromised, they may become more seriously ill and die if they don't protect themselves," Chin-Hong said.
Should we be concerned?
Brownstein said he thinks public health officials should remain vigilant and keep tracking the spread of these subvariants but the public should not yet worry.
"I think it's too early to tell the level of concern," Brownstein said. "Of course, everyone should be concerned about how new variants emerge, especially when there's uneven vaccination across the U.S and then across the globe."
He added, "The brunt of that concern should be on public health and scientists right now, as we try to work out what's happening."
Brownstein said Americans should be willing to modify their behavior -- like masking indoors again -- if cases spike or a new variant emerges.
Brownstein said if the health care system becomes overwhelmed by a surge, city- or state-level officials might consider masking to reduce the number of patients entering hospitals.
"I think that clearly targeted masking, especially in high-risk places like health care settings and long-term care facilities, will be critical, because those places are charged with protecting the most vulnerable," Brownstein said. "Beyond that, I think it's going to be really driven by local level cases."
ABC News' Sony Salzman contributed to this report.