Covid-19 test positivity rates in New York City have doubled in just three days, Dr. Jay Varma, a top health advisor to Mayor Bill de Blasio, tweeted Thursday.

“Um, we’ve never seen this before in #NYC,” he wrote.

Positivity rates are rising as the omicron variant spreads. According to CDC data released Wednesday, New York and New Jersey are the two states with the most rapid spread of the variant. The data shows that 13.1% of cases in the CDC region that includes New York and New Jersey are omicron, compared with a national average of 2.9%.

“Omicron is here in New York City and it is spreading quickly,” Dr. Dave A. Chokshi, commissioner of the New York City health department, said at a news conference Thursday.

De Blasio and his health advisors at the briefing expressed concern ahead of the holiday season. Demand for testing has increased following a few major events, including last weekend’s annual SantaCon pub crawl event.

“We’re seeing a surge ahead of the holidays,” Chokshi said. “I do expect cases will continue to increase in the coming days,” he added.

The variant was first detected in South Africa at the end of November and has quickly spread across the world. In the U.K., it’s now the dominant strain and the country saw the highest daily number of new cases since the pandemic began on Wednesday.

From Dec. 9 to Dec. 12, the percentage of positive tests in the city went up from 3.9% to 7.8%. “This is #SARSCoV2 evading both vaccine & virus induced immunity against infection unlike any variant before,” Varma added.

He noted, though, that about 67,000 PCR tests are administered in New York City per day, which is higher per capita than many other communities or cities across the country. Additionally, the data reported by the city has a lag of about three days, meaning current positivity rates could be higher and previous days’ data may still be adjusted.

At the news conference Thursday, de Blasio announced that the city will be increasing it’s testing capacity moving forward. “That means the mobile sites we’ve talked about, you’ll see more of those in all the five boroughs, but we will also be doubling down on our brick and mortar sites,” he said.

Hours and capacity at existing fixed sites will be expanded, de Blasio said, even as new sites are added.

Still, the mayor emphasized, getting vaccinated is the best way to protect against omicron and severe disease. “Don’t wait, get your booster shot right now,” he said.