(NEXSTAR) – On Tuesday, the Centers for Disease Control and Prevention added two new subvariants to its online tracker: BA.4 and BA.5. Both are subvariants of the omicron variant of COVID-19, which has been dominant in the U.S. since omicron overtook delta in last winter’s surge.

BA.4 and BA.5 now make up 5.4% and 7.6% of new COVID cases, respectively, but their presence is larger out West and in the South. In parts of the country, the two subvariants combined already make up more than 20% of cases.

The two new strains have already had a “big impact” on other parts of the world, and U.S. health officials need to closely monitor their impact on the virus’ spread over the next few weeks, said Dr. Tom Inglesby, director of The Johns Hopkins Center for Health Security. Here’s what we know so far.

Where are BA.4 and BA.5 spreading?

BA.4 and BA.5 were both first detected in South Africa early this year, according to the European Centre for Disease Prevention and Control. Since then, the strains have been detected in several other countries, and quickly became dominant in Portugal.

In the U.S., the CDC tracks variants by region. The region that currently has the highest proportion of BA.5 and BA.5 is Region 6, made up of Arkansas, Louisiana, New Mexico, Oklahoma, and Texas.

The new subvariants have what scientists call a “growth advantage” over older types of omicron, which means they’ll grow more dominant by the day.

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“Given the data we’ve seen so far, I’d expect BA.4 and BA.5 would continue to replace BA.2.12.1,” said Inglesby.

Are BA.4 and BA.5 more contagious than other types of omicron, like BA.1 and BA.2?

Early evidence suggests BA.4 and BA.5 aren’t more contagious. However, it appears they are better at evading prior immunity.

So if you already had omicron, can a new subvariant like BA.4 or BA.5 get you sick again?

It’s possible, especially if you were sickened with omicron back in December 2021 or January 2022 — six months and several subvariants ago.

Limited studies show BA.4 and BA.5 are likely to reinfect unvaccinated people whose only immunity from the virus comes from a prior infection with BA.1 (the first type of omicron).

The same studies showed vaccinated people appeared to be better protected against reinfection with BA.4 and BA.5, however, “protection derived from currently available vaccines does wane overtime against the Omicron variant,” writes the ECDC.

A booster shot can help with that waning immunity, Inglesby said. Only one-third of Americans have received a booster shot against the virus.

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These studies also only looked at prior protection from BA.1, and right now the U.S. is in the middle of a surge of BA.2 — yet another omicron strain.

“It’s possible BA.4 and BA.5 could create a surge on top of a surge, but it’s very difficult to know this because we don’t understand whether the collective immunity provided by BA.2.12.1 will protect against BA.4 and BA.5,” said Inglesby.

Are these subvariants more dangerous or deadly?

“There is currently no indication of any change in severity for BA.4/BA.5 compared to previous Omicron lineages,” according to the ECDC.

That doesn’t mean BA.4 and BA.5 should be ignored, the European health agency warns. Even if the variant is less deadly than a prior iteration like delta, omicron is so contagious that BA.4 and BA.5 still threaten to sicken millions and overwhelm health care systems.