It hasn’t even been a month since New York City rolled back its vaccine requirements and ended mask mandates, but cases of COVID-19 are already back on the rise—thanks to the BA.2 Omicron subvariant that’s making its way through the city right now.

Known as “stealth Omicron,” the BA.2 subvariant has a different genetic sequence from BA.1, the initial Omicron strain that wreaked havoc in the city and the country at large last winter. That difference makes it harder to detect and classify in current PCR tests, according to the U.K. Health Security Agency.

BA.2 is about 50% more transmissible than the original Omicron strain, but it doesn’t cause more severe illness, evade vaccinations, or cause earlier infection, according to Anthony Fauci, the chief White House medical adviser.

In an appearance on ABC’s This Week on Sunday, Fauci warned that COVID cases would likely rise in the U.S. because of BA.2, which has contributed to a steady increase in cases in the U.K. and other European countries in recent weeks.

“The bottom line is we likely will see an uptick in cases, as we’ve seen in the European countries, particularly the U.K.,” Fauci said.

Just three days later, we’re already seeing that uptick in New York.

Cases on the rise in NYC

BA.2 cases now account for upwards of 30% of new cases of COVID-19 in New York City, according to the city’s health officials.

Overall in the state, BA.2 now accounts for 42% of cases overall, according to state health officials at a March 21 press conference in Albany. Cases in the state have risen almost 2% over last week, according to Gov. Kathy Hochul’s daily COVID update.

Case totals in New York City, which has one of the highest vaccination rates of any U.S. city, rose 35% last week over the week before, according to the New York Times database.

But state health officials at a March 21 press conference in Albany say the uptick is “no surprise.”

“We don’t expect to see a steep surge in cases in New York State,” said State Health Commissioner Dr. Mary Bassett on Monday. “We will be watching, so I don’t want to promise you that it won’t change.”

Mayor Eric Adams, meanwhile, announced on March 22 that if COVID numbers in the city remain relatively low, mask mandates for 2- to 4-year-olds in school and day care would be lifted starting on April 4.

“We'll continue to peel back requirements. But we won't be rushed,” Adams said in a tweet on March 22. “New Yorkers can trust me to follow the science and put the health and safety of our kids first.”