New studies of the BA.2 sub-variant of the omicron strain of COVID — known as “stealth” omicron — show that the lineage is more transmissible than the original omicron, but may not be more severe. And so far, it doesn’t seem to be provoking another surge in cases.
While experts are keeping a close eye on BA.2 as it circulates around the world — it is already in all 50 U.S. states — many aren’t worried that it will upend recent progress in winding down the pandemic.
Here’s what you need to know.
What is BA.2?
BA.2 is a sub-variant of the omicron variant of COVID-19. It’s known as “stealth” omicron not because it’s harder to detect on coronavirus tests, said Dr. Peter Chin-Hong, an infectious disease expert at UCSF, but because it is not always recognized as omicron.
BA.2 lacks the specific genetic feature that allows scientists to quickly identify a COVID-19 strain as omicron when sequencing, which means BA.2 can look like delta in initial screening.
The test will show “positive” for COVID, Chin-Hong explained, but not that it’s omicron.
Because BA.2 is a subvariant of omicron, the California Public Health Dept. does not track it separately, the agency said.
Is BA.2 more infectious than omicron?
Studies estimate that BA.2 is about 30% more transmissible than the original BA.1 omicron version, though that difference in contagiousness isn’t as large as the leap between delta and omicron, the WHO said. On Thursday, South Africa’s top public health body said that while its data show BA.2 does appear to be more infectious than BA.1, it does not cause more severe disease, Reuters reported.
Will BA.2 cause another surge in cases?
While the growth of BA.2 is now outpacing that of BA.1, the World Health Organization nevertheless notes that “the global circulation of all variants is reportedly declining,” meaning that BA.2 does not appear to be spurring renewed outbreaks.
Chin-Hong said that, while it is important to keep an eye on the strain’s spread, highly vaccinated places that have already seen large omicron surges, like the Bay Area, are likely not going to see huge spikes fueled by BA.2.
Still, he said, the omicron sub-variant is very likely to infect people who are not vaccinated and have not had omicron, he said, which could “slow the sense of normalcy” for some.
He also noted that because BA.2 is arising just when many places are dropping mask mandates and other restrictions, it’s hard to know whether increased cases are because of the sublineage or just a natural consequence of the lowered restrictions.
“It’s hard to see what effect there is because of BA.2 specifically,” he said.
Is it BA.2 more severe?
While there are some mixed results in studies, real world data so far show that BA.2 is not causing more severe disease than the original omicron.
A Japanese study on lab mice has found that the sub-variant could potentially cause more severe disease than its parent omicron — but that phenomenon has not played out in humans. The study’s results “look scary,” Chin-Hong said, but doesn’t necessarily translate to humans in large part because “it doesn’t account for previous immunity.”
Chin-Hong added that he is not expecting more people to be hospitalized with BA.2 than with BA.1, especially since vaccines continue to effectively protect against severe disease, hospitalization and death.
However, he also noted that even though omicron and BA.2 are mild for many, UCSF hospital has continued to treat patients with serious illness, most of them unvaccinated.
“The vaccine is still the best protection,” he said.
Can BA.2 reinfect people who’ve already had COVID-19?
Like the original strain of omicron, BA.2 can reinfect people who’ve had COVID before, but it doesn’t appear to be able to reinfect people who’ve already had omicron, especially those who are vaccinated, Chin-Hong said.
A study out of Denmark found that, while BA.2 was able to reinfect a small number of people who already had the original omicron, those were “young unvaccinated individuals with mild disease not resulting in hospitalization or death.”
Chin-Hong added that people who are unvaccinated and had a mild case of omicron may not have produced a robust enough immune response to fight off another infection from BA.2, since mild infections don’t “wake up” as many T cells and B cells — “yet another reason to get vaccinated,” he said.
WHO also said that initial data from real-world infections “suggest that infection with BA.1 provides strong protection against reinfection with BA.2, at least for the limited period for which data are available.”
What does BA.2 mean for the future of the pandemic?
BA.2 is another twist from an unpredictable virus, and researchers and scientists are staying vigilant.
Chin-Hong said that while he isn’t terribly worried about BA.2 itself, people who are unvaccinated and haven’t gotten omicron are very likely to get BA.2 he said, and that opportunity for increased transmission worries him.
“Every time you have a transmission, you can have a mutation,” he said. “The spawn of BA.2 will be something different, but we don’t know what. And there’s no guarantee that it will still be mild.”
“At this point the most worrisome thing about BA.2 is that it demonstrates that it’s possible for a variant to be more infectious than Omicron,” Dr. Bob Wachter, chair of medicine at UCSF, wrote on Twitter. “There’s no guarantee that a future variant won’t be more immune-evasive and/or severe. But luckily, not this one.”
Danielle Echeverria is a San Francisco Chronicle staff writer. Email: danielle.echeverria@sfchronicle.com Twitter: @DanielleEchev