What is BA.2 and Why Are You Hearing About the Omicron Subvariant?

News about BA.2 omicron subvariant continues to grow as the variant begins to dominate COVID cases in parts of the U.S.

Chicago's top doctor said BA.2 will likely make up a majority of the city's cases in the coming days and weeks, a transition health officials are watching closely as metrics begin to see small signs of an increase.

Experts say what happens in the next few weeks in the U.S. could be critical to whether or not the U.S. will follow in Europe's footsteps.

So what is BA.2, what are the symptoms associated with it, where has it been detected and how contagious is it? Here's a breakdown.

What is BA.2?

BA.2, also known as "stealth omicron," is considered a subvariant of omicron.

BA.2 has several key mutations, with the most important of those occurring in the spike protein that studs the outside of the virus. Those mutations are shared with the original omicron, but BA.2 also has additional genetic changes not seen in the initial version.

So far, it has not yet been declared a variant of concern on its own.

"BA.2 is part of omicron," Dr. Isaac Ghinai, medical director for lab-based surveillance at the Chicago Department of Public Health, said Thursday. "Omicron is a variant of concern, therefore BA.2 is a variant of concern. Same as BA.1 is a variant of concern."

But that could change.

"People are looking very closely at whether or not BA.2 needs to be classified separately and monitored separately," Ghinai said. "But even without that it is a variant of concern, it's being monitored very closely at the local levels, at the state levels and at the national levels."

How contagious is BA.2?

According to several health experts, BA.2 appears to be more transmissible than omicron.

"There's four unique mutations in the spike protein that are distinct in BA.2, and different from BA.1. ... It seems that these mutations will propel the transmissibility to about a 30% to 50% higher degree of contagiousness than the BA.1 variant," said Dr. Gregory Huhn, an infectious disease physician and the COVID-19 vaccine coordinator for Cook County Health.

White House chief medical advisor Dr. Anthony Fauci said BA.2 is about 50% to 60% more transmissible than omicron, but it does not appear to be more severe.

"It does have increased transmission capability," Fauci said Sunday on ABC's "This Week." "However, when you look at the cases, they do not appear to be any more severe and they do not appear to evade immune responses either from vaccines or prior infections."

Northwestern's Dr. Michael Angarone, an associate professor of medicine in infectious diseases, said the increased transmissibility could be particularly strong in close contacts of those infected, but it's still too early to tell.

"We're still trying to figure out why are we seeing this rising number of cases in some of these countries in Europe and that is because there's something markedly different about the virus," he said. "So is it more transmissible? Are more people going to become infected from one infected individual? There might be some markers of that."

What are the symptoms of BA.2?

According to Angarone, the symptoms for BA.2 are similar to those seen in many COVID infections.

"So this is the same virus, so SARS Coronavirus 2, so we're seeing the same symptoms," he said.

Huhn noted that while omicron led to more upper respiratory symptoms, it remains too early to tell.

"I don't know if we, right now, know the particular features that are distinct for BA.2 versus BA.1. I mean, for BA.1, we knew that it was mostly an upper respiratory-type infection rather than the lower respiratory infections that can lead toward pneumonia and further and greater complications," he said.

For some people, coronavirus causes mild or moderate symptoms that clear up in a couple weeks. For others, it may cause no symptoms at all. For some, the virus can cause more severe illness, including pneumonia and death.

Most vaccinated people either have no symptoms or exhibit very mild symptoms, according to health officials, and the virus rarely results in hospitalization or death for those individuals.

Still, omicron presented a shift in common symptoms for many.

Dr. Katherine Poehling, an infectious disease specialist and member of the Advisory Committee on Immunization Practices, told NBC News in January that a cough, congestion, runny nose and fatigue appear to be prominent symptoms with the omicron variant.

But unlike the delta variant, many patients were not losing their taste or smell. She noted that these symptoms may only reflect certain populations.

Will BA.2 cause a surge in the U.S.?

Experts said they're watching closely, particularly as restrictions continue to lift across the U.S., but many say that even if a surge is seen, it likely won't be to the level seen earlier this year.

"I'm not expecting a big surge here, but we're gonna have to pay close attention and really be driven by data as we have throughout the whole pandemic," White House COVID-19 Response Coordinator Dr. Ashish Jha said last week.

Fauci also said he expects "an uptick in cases" due to BA.2, but not necessarily a massive surge like other variants have caused.

"This is different than the switch from delta to omicron. That truly was a much more drastic change in the virus and that's where we really saw transmissibility increase magnitudes above delta," Huhn said. "This is more of a slight variation and so we're not seeing the transmissibility really accelerate as we did when we switched from delta to omicron."

Ghinai said he doesn't believe BA.2 will lead to a surge like the one previously seen during the omicron peak, but changes are expected "at some point."

"We're at a nearly historic low in terms of COVID here in Chicago, it's very likely that there may be some changes in transmission," he said. "I don't expect it to be a surge like we saw the last few months because of omicron, because of delta."

Huhn said experts will be closely watching as restrictions continue to loosen and some booster immunity may start to wane.

"Now that we've rolled back, you know, we'll see again in the next few weeks if there will be an uptick in cases," he said.

Where has BA.2 been detected?

As most COVID-19 restrictions are relaxed across Europe, including Austria, Britain, Denmark, Germany and France, the numbers of infections have inched higher in recent days. In the last two weeks, COVID-19 hospitalizations and deaths have both risen slightly in Britain.

The uptick is driven in part by BA.2 and by people largely abandoning masks and gathering in bigger groups.

The variant had already been detected in Illinois earlier this year, and though the original omicron strain continues to make up a majority of the area's COVID cases, that is expected to soon change.

Northwestern Medicine's Center for Pathogen Genomics and Microbial Evolution said the subvariant was found in a Chicago resident who was tested for COVID-19 on Jan. 18.

Chicago Department of Public Health Commissioner Dr. Allison Arwady said Tuesday while the COVID risk remains low in the city, data shows the number of cases of the BA.2 subvariant are doubling every week.

"All we're seeing right now in Chicago and across the Midwest is omicron. We are seeing two primary subvariants of omicron: the B1, which is the one that hit us through the surge, is still about 69% and BA.2, which is the one that we're watching and is more contagious than B.1 and probably some of what is driving that increase around the world, is just over 30% at this point. In Chicago, we've seen that BA.2 doubling about every seven days, so we do think we'll see predominance, meaning most of our cases will be that BA.2, by the end of the month."

While numbers are still low in the city in terms of new COVID cases, there is an uptick in cases that has been noted by health experts. As of Thursday, the city is averaging 177 new cases of COVID per day, a 29% increase over a week ago when the city was at 137.

"At this point, we've not seeing major signs like we're seeing in Europe, but while we continue to watch, we'll have a better sense of this honestly, probably by the end of the month when BA.2 will be predominant here," Arwady said.

According to estimates from the Centers for Disease Control and Prevention, the BA.2 variant made up 30.6% of all COVID cases in several Midwest states, including Illinois, Minnesota, Wisconsin, Michigan, Indiana and Ohio.

Arwady highlighted that in some states along the East Coast, BA.2 is representing an even larger portion of cases.

"New York, Connecticut, Pennsylvania, most of their cases already are BA.2," she said.

According to the CDC, the so-called “stealth” omicron variant has been driving the uptick in cases in the New York area, with the strain now accounting for at least 52% of the new COVID cases in the region in recent weeks, estimates suggest.

According to the latest figures from NYC Health, the city is now averaging 892 new cases of COVID per day over the last week.

That represents a 29% increase from a week ago, when the city was averaging 689 cases per day, according to officials.

Nationally, the BA.2 variant is responsible for 35% of new infections.

Why is the U.S. monitoring Europe?

There are signs in other countries currently experiencing an uptick in cases due to the variant that transmission is leading to a rise in hospitalizations and possibly even deaths.

"It is still now taking hold as the dominant variant in many countries ... Denmark, India, Philippines, UK, Switzerland, Sweden, Norway, Belgium, Hong Kong, South Africa and many more regions," said Huhn. "So particularly in the UK, where we now see that this is the dominant strain, we've seen some increased cases over the past several weeks, a doubling in the cases; we've seen increased hospitalizations and a little bit of signal for increased deaths. So with all these variants that have circulated throughout the past two years, many of the trends that start in Europe, and particularly in the UK, eventually, we see that here in the U.S. So that's why we're on heightened alert with the increase in cases of BA.2, increase in hospitalizations in the UK as an early warning signal for what could potentially occur here in the United States."

Angarone said Europe is typically up to a month ahead of the U.S. in its COVID outbreaks.

"We often look at Europe as being that kind of bellwether of 2, 3, 4 weeks kind of ahead of us," he said. "So are we going to see this kind of increase in a month or so? And we just don't know."

Arwady said she's "concerned" about the situation in Europe and her team is in communication with officials there.

Do vaccinations work against BA.2?

Preliminary data indicate vaccinations and boosters are similarly effective in preventing symptomatic cases of BA.1, the original omicron variant, and BA.2.

Already, the makers of the two mRNA vaccines currently approved for use in the U.S. are seeking approval for a second booster shot for certain populations.

Drugmaker Moderna asked the Food and Drug Administration on Thursday to authorize a fourth shot of its COVID-19 vaccine as a booster dose for all adults.

The request is broader than rival pharmaceutical company Pfizer's request for the regulator to approve a booster shot for all seniors.

According to Ghinai, evidence so far also suggests infection with one omicron sublineage is believed to provide protection from other omicron sublineages, but Huhn noted that reinfection is possible, though rare.

"I think it's very likely that what we've seen in Europe, where BA.2 is increasing in relative proportion, is going to happen here," Ghinai said. "We're going to see BA.2 causing an increase in proportion of the number of cases. We've already seen that."

Arwady stressed that while the rise of COVID in other countries could be a sign of what's to come, it's not a guarantee.

"It is not a for sure thing, to be clear, that we will follow. We may, but there are some other countries that have gone through an omicron surge and we've not seen that resurgence yet," she said. "So we're still watching."