NEW YORK — New York City suddenly removed its color-coded coronavirus alert system Thursday just as newer omicron subvariants are fueling another rise in cases and hospitalizations.

Mayor Eric Adams and health officials quietly took down the city’s high profile alert system that warned New Yorkers when they were at a greater risk of catching the virus and should consider taking more precautions.

Now the city’s website reads: “We are re-evaluating the city’s COVID Alert system. Check back here for updates in the coming weeks.”

The city was at the medium risk level Tuesday, and the city’s website now says “there are currently high transmission levels of COVID-19 throughout the city.” Virus cases have started to rise in the city again after dropping last month and the seven-day test positivity rate increased to 10.3% citywide, the highest level since January, according to city data.

Hospitalizations in the city have also risen, from 725 people hospitalized with the virus in early June to 877 people now, according to state data.

The rapidly spreading omicron subvariants known as BA.4 and BA.5 accounted for about 46% of virus cases in the city as of June 18, and those figures are rising, according to city data.

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BA.4 and BA.5 are able to elude some of the antibodies produced after coronavirus vaccinations and infections, including infections caused by some earlier versions of omicron. But there is not yet much evidence that they cause more severe disease.

The city’s health commissioner, Dr. Ashwin Vasan, defended his decision to remove the alert system Thursday and said that the city was “reevaluating” its approach, although he did not offer details about what the city might do instead.

“While we continue to see a lot of transmission here in New York, we’re also at a different phase of the pandemic that demands new analytical tools to match our assessments and communications about risk with reality,” Vasan said at a news conference in New York City to announce new mobile testing units that will provide antiviral treatments.

Fabien Levy, a spokesperson for the mayor, said in a statement Friday that the city always intended to periodically reevaluate the alert system as conditions changed and that Vasan proactively mentioned the change at the news conference.

“What we have seen recently is bouncing between alert levels, which doesn’t provide clear or useful information to New Yorkers,” Levy said. “All of the COVID-19 indicators continue to be updated on the Health Department’s website and we will continue to communicate our recommendations clearly.”

Even as virus infections nationally are thought to be undercounted, BA.4 and BA.5 have together become dominant among new U.S. cases, according to new estimates released this week from the Centers for Disease Control and Prevention. BA.4 made up 15.7% of new cases, and BA.5 was at 36.6%, numbers that experts said are likely to rise in the weeks to come.

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Federal regulators recently recommended that coronavirus vaccine manufacturers update their boosters to target BA.4 and BA.5, despite the uncertainty regarding what form of the virus may be circulating later this year.

New York City’s “Alert Levels” website said Friday that the city was facing high transmission levels and that residents should continue to wear masks in public indoor settings and around crowds outdoors and test before and after travel or gatherings.

Private companies and schools have relied on the city’s alert system to make decisions over requiring masks in offices and classrooms. Without guidance from the city about the growing risk level, organizations will have to monitor the data to make decisions on their own.

Under the color-coded alert system introduced by Adams in March, certain health policy measures were recommended at each level of risk. At the high risk level, the mayor was encouraged to require face masks in all public indoor settings and to bring back a vaccine mandate for restaurants and bars that was in place under the last mayor, Bill de Blasio.

When the city reached the high risk level in May, Adams chose not to reintroduce mask and vaccine mandates. Adams, a Democrat who took office in January, has focused on reopening the city and convincing workers to return, and he has argued that only a significant rise in hospitalizations would prompt more aggressive action.

Starting on Friday, Broadway theaters will be allowed to drop their mask mandates, and Adams recently removed a mask mandate for toddlers.

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Dr. Jay Varma, a public health researcher and former senior health adviser to de Blasio, said Friday that he believed that a BA.5-driven wave was starting in the city and that health officials should provide more clear guidance to New Yorkers about how to change their behavior during regular news briefings.

“I worry that removing the alert system is a sign that the city prefers to wait until there is a marked increase in severe illness and death before any further actions will be considered,” Varma said.

Adams attended the news conference this week about mobile testing sites offering Paxlovid, and he visited a vaccine site for children younger than 5 last week as they began to receive shots. But the mayor has not held regular news briefings to update New Yorkers about the virus like de Blasio did.

Some other cities still appear to be relying on color-coded alert systems to describe COVID-19 levels. Chicago, for instance, entered the “high” — or orange — level last month based on rising coronavirus cases and hospitalizations across Cook County. Los Angeles County is currently in the medium — or yellow — level, reflecting a lower rate of new COVID-19 hospitalizations than Cook County or New York City.

The color-coded systems are based on parameters set by the CDC that incorporate data on both new cases and hospitalizations. In Los Angeles, officials have indicated that indoor masking will be required should the county enter the “high” level.