Two new omicron subvariants now account for nearly a quarter of all COVID-19 cases in New England, according to an update from the U.S. Centers for Disease Control and Prevention Tuesday.
Until this month, BA.4 and BA.5 hadn't made major inroads in the U.S., where BA.2 and its descendant BA.2.12.1 still account for most of the cases.
But recent data shows that the two new subvariants have begun to spread more rapidly, rising from 10% to 24% in New England in the last week alone. Nationally, BA.4 and BA.5 cases account for about 35% of COVID-19 cases, up 5% over the previous week.
BA.2 and BA.2.12.1 still account for the bulk of the cases -- 65% nationally and 76% here in New England. But doctors have warned that BA.4 and BA.5 could result in another upswing here in the near future.
Three top Boston doctors spoke about what the rise of subvariants means for New England during a "COVID Q&A" discussion with NBC10 Boston last week.
COVID cases in the U.K. are rising for the first time in two months, which health officials there said is likely driven by the original omicron variant BA.1 and the newer variants BA.4 and BA.5.
Similarly, cases had been dropping in South Africa for over a month before the BA.4 and now BA.5 strains began pushing up cases. The BA.4 subvariant now accounts for about half of new infections there.
"They're creeping up," Dr. Shira Doron of Tufts Medical Center said of the strains in New England, noting that Massachusetts has seen decreasing cases, wastewater COVID levels and hospitalizations.
"So will BA.4 and BA.5 reach a proportion at which we see a turnaround? Like South Africa, Portugal? Or, based on our recent experiences and our vaccinations, will we not see that turnaround? Or will it be regional and dependent on the part of the country? Time will tell," Doron continued.
Experts say the strains seem to be more transmissible than both the original omicron variant and an omicron relative known as BA.2. Scientists are still studying the new mutants, but it doesn’t appear they cause more severe disease than other versions of the virus, the World Health Organization said in a recent report.
"It definitely appears to be more immune evasive. It appears to be more transmissible," Boston Medical Center's Dr. Sabrina Assoumou said. "We seem to be getting variants that that are gonna cause us more trouble, but what is the good news? The good news is that it is still omicron -- BA.4 and BA.5 are omicron. Vaccines provide protection."
The new subvariants could lead to a new round of breakthrough cases among people who have already had COVID, Assoumou said.
Dr. Daniel Kuritzkes, chief of infectious disease at Brigham and Women's Hospital, noted that it will be "particularly important" to see whether people who have been infected with BA.1 or BA.2 can now become infected with BA.4 or BA.5 and to determine whether that's due to waning immunity or because there's no "meaningful cross protection" against BA.4 and BA.5.
"Like so much else, we'll just have to see what happens and then we'll know," Kuritzkes said.