Starting next week, New York City will no longer use COVID positivity as an indicator of the pandemic’s grip on the five boroughs, the mayor announced. Instead, cases, vaccinations and hospitalizations will guide the city’s policy decisions.

“It’s just not going to be how we strategically make decisions,” Mayor Bill de Blasio said Monday of the data point that judges how many tests come back with an infection.

The pivot comes after the Centers for Disease Control and Prevention issued new mask guidance based in part on positivity rate. The agency now recommends that everyone wear face coverings indoors, regardless of vaccination status, in areas where cases rise above 50 infections per 100,000--or if the positivity rate jumps above 8%.

As of August 2nd, community transmission is prominent enough in all five boroughs and all but one New Jersey county to make universal masking a good idea, according to CDC data.

The mayor and city Health Commissioner Dr. Dave Chokshi cited widespread vaccination and low testing rates as a reason to stop announcing positivity rate, which is calculated by dividing the number of positive tests by the total number of tests taken in a given day. Although genetic testing via PCR declined along with cases between April and early July, the seven-day rate has risen dramatically in recent weeks—from just over 22,000 on July 5th to more than 33,000 on July 30th.

Indeed, daily PCR testing this year hasn’t fallen anywhere near the low points witnessed in spring 2020, when the nation struggled to screen people for infections. And, according to city health department data, enough tests are being done to accurately gauge transmission in more than 83% of New York City neighborhoods.

The number of tests being done does affect the accuracy of the positivity rate data, notes Dr. Denis Nash, an epidemiology professor at the CUNY Graduate School of Public Health. But with testing on the rise and widely available, that alone isn’t a reason to abandon the metric—especially when so many people are unvaccinated.

“The positivity rate is not something I would get rid of,” he said. “I don’t find the rationale that the mayor and other experts gave very compelling.”

That’s because unvaccinated people are more likely to catch the virus and get tested--making positivity an important gauge of how the virus is circulating among those who are least protected.

And it’s better to rely on more metrics, rather than fewer, when trying to assess community transmission. The positivity rate, for example, indicates whether an area is running out of resources to keep track of asymptomatic cases. A different metric--called the effective reproduction number or infection growth rate--would explain how fast the outbreak is building.

“It’s always been problematic to rely on any one indicator,” Nash said. “All of these metrics are imperfect in one way or another. When you look at them all at once, they tend to help you see something close to what the true picture might be.”

The health department will still publish positivity rates on its data page, the mayor and city health commissioner said. Currently, their teams release citywide as well as neighborhood-level numbers.

The positivity reached a rolling average of 3% citywide on July 30th, the highest it’s been since late April. Rates vary widely across the city—from just over 1% in parts of Queens to nearly 8% in the Annadale/Rossville and Charleston/Prince’s Bay/Woodrow neighborhoods of Staten Island.