The seven-day moving average for cases across the country increased 19.1% this week, according to the Centers for Disease Control and Prevention. The rise in cases has been most severe in East Coast cities like New York, Philadelphia, and Washington, D.C. 

And while case numbers, hospitalizations, and deaths remain at their lowest levels since the summer of 2021, the rise in cases is noteworthy because it is driven by several new coronavirus variants and subvariants, according to the New York Times’ COVID database. 

It remains to be seen whether any of these variants will result in the return of COVID restrictions in many major cities. On Monday, Philadelphia will be the first major American city to reinstate its indoor mask mandate after a 50% increase in cases was reported there in the past two weeks. 

 Here is the latest information about the COVID subvariants responsible for the recent uptick. 

BA.2, or “stealth Omicron” 

The overwhelming majority of cases in recent weeks are the BA.2 Omicron subvariant, which is as much as 60% more transmissible than original Omicron. 

Sometimes referred to as “stealth Omicron,” BA.2 has a different genetic sequence from BA.1, its Omicron predecessor, which initially made it harder to detect and classify in PCR tests, according to the U.K. Health Security Agency. 

In late March, BA.2 became the most dominant form of COVID-19 in the U.S., according to the Centers for Disease Control and Prevention. 

While BA.2 is more transmissible than BA.1, its increased virility has not led to a major increase in hospitalizations or deaths.  

“It simply is not nearly as severe, so it’s less of a public health problem,” Dr. William Schaffner, a professor of infectious diseases at Vanderbilt University Medical Center, told Fortune earlier this week. 

Two new subvariants in New York 

In New York, where cases have increased nearly 40% in the past two weeks, health officials announced on Wednesday they had identified two new subvariants of BA.2 in the city. 

Both subvariants evolved from BA.2, and while New York health officials said they similarly don’t appear to cause more severe diseases than BA.2, they are most likely contributing to the surge in cases throughout the state in recent weeks. 

Known as BA.2.12 and BA.2.12.1, the subvariants have been estimated to have a 23% to 27% growth advantage above the original BA.2 variant, meaning they are even more transmissible than the already highly transmissible stealth Omicron, according to New York State health officials. 

“It’s just a reminder that we’re not out of the woods with regard to this virus, and people should continue to take precautions and to get fully vaccinated if they haven’t completed their course,” Kirsten St. George, a virologist for New York State, told the New York Times about the new subvariants.

BA.4 and BA.5 identified in South Africa

Earlier this week, scientists in South Africa discovered two new sublineages of the Omicron variant, which they have named BA.4 and BA.5. 

The two Omicron subvariants have not caused a spike in infections in South Africa yet, but they have been identified in samples in a number of European countries, including Germany, Denmark, and the U.K., according to Tulio de Oliveira, who led the research team that discovered the subvariants.

“Given the very low infections, hospitalizations, and deaths in South Africa, we are alerted about the continued evolution but not concerned,” de Oliveira told Bloomberg via text message. 

According to de Oliveira, the two new subvariants differ from the previous Omicron strains because of amino acid mutations outside the spike protein. It’s unclear how the two new subvariants compare to the other Omicron variants in terms of transmissibility.

On Friday, the World Health Organization announced it was tracking the two subvariants as they continue to account for new cases in Africa and Europe. Cases of BA.4 and BA.5 haven’t yet been detected in the U.S.