During the city’s second COVID-19 surge, cases exploded from approximately 800 per day at the start of November to 6,400 around the New Year. Likewise, the city’s COVID-19 positivity rate—the percentage of tests confirming infections—rose from around 2% to a peak of 9.76%, per city data. Anything above 5% is typically considered a sign of an uncontrolled outbreak.
Since the January peak, cases have plummeted, with the seven-day average cutting in half. But the positivity rate has remained stubbornly high. For nearly three weeks, it has largely plateaued around 6.5% to 7%, spurring questions about why that positivity level—the one we’ve all been watching for months now—isn’t dropping faster.
The positivity rate on its own has limitations, said Rachael Piltch-Loeb, an associate research scientist at New York University's School of Global Public Health. The percentage is impacted by how many people are getting tested overall, which fluctuates—changing the numerator and denominator day-to-day.
“What may be happening here is that the number of people who are seeking testing overall is declining,” Piltch-Loeb said in an email. “Or the number of asymptomatic people who were previously getting tested to then justify seeing friends, family, or some other activity are now no longer doing so.”
That could be either because they are vaccinated, or they no longer see the need. There’s no simple answer, but the contrast between cases and positivity speaks to the importance of judging an outbreak by more than one disease metric.
Positivity, for example, tells you whether health officials are conducting enough tests to keep tabs on the outbreak, according to the Johns Hopkins Coronavirus Resource Center. That’s helpful during a surge. When the rate rises rapidly, it can mean that a region only has the capacity to test the sickest patients while less severe cases are missed, leading to more community spread.
The case density—the number of infections per 100,000 residents—is an easy way to gauge your likelihood of encountering someone with COVID-19 in your city or county. COVIDActNow shows that the New York-area ranks sixth among the nation’s largest urban centers in case density—at 39 per 100,000. This high mark seems troublesome but doesn’t tell the whole story.
“I don’t think we can effectively compare to another large city unless we know how they are measuring the data points that New York is measuring,” Piltch-Loeb said. “We need to know that both cities are taking the same approach to testing and contact tracing and that the population there is behaving relatively similarly.”
U.S. metro areas ranked COVID-19 case density
The transmission rate—Rt—can provide more clarity on whether an outbreak is growing. When it rises above 1.0, the spread is getting worse. Below 1.0, and the outbreak is shrinking. As of March 17th, the infection rates in five of the seven most populated counties in the city’s metro area—Manhattan, Queens, Bronx, Nassau, and Westchester—are below zero. The pandemic is currently shrinking in those places, and the region's infection rate ranks 74th among U.S. urban centers—between Oshkosh, Wisconsin, and Kokomo, Indiana. It’s a positive sign but one that needs to be sustained to mean the most.
“The Rt describes the average number of new cases produced per infectious person at a particular point in time,” Piltch-Loeb said. “Unless Rt is sustained below 1.0 long enough to drive infection to a low level, based on trend over a, say, two-week period, decisions to lift restrictions may be premature.”
U.S. metropolitan areas ranked coronavirus infection rate.
Indicators like hospitalizations, intensive care unit beds filled, and deaths are also among the “most informative,” David Vlahov, a nursing and epidemiology professor at Yale University, said via an email. The seven-day average of hospitalizations has dropped sharply in recent weeks—from 413 on February 8th to 177 on March 14th. Deaths saw a small reduction across this period, going from 86 to 66.
“There is also another consideration when looking at testing and results,” Vlahov said. “The city is made up on many neighborhoods, and it may be more meaningful to separate data to look at trends by neighborhood than averaging the City overall.“
While reopening continues—drawing criticism from public health experts—it can be confusing to see a rollback in restrictions when the positivity was low throughout the summer and autumn, hovering around 1% to 3% when the lockdown was tighter.
During a press briefing on Tuesday, NYC Health Commissioner Dr. Dave Chokshi acknowledged the positivity rate isn’t dropping as quickly as he would like to see, potentially due to more contagious variants spreading in New York City. Mayor Bill de Blasio said the city has administered 2.9 million doses of vaccines and remains hopeful the positivity levels would drop as the boroughs move toward his goal of inoculating 5 million New Yorkers by June.
Read More: 80% Of Adults In NY State To Be Eligible For COVID-19 Vaccines This Week
“We just have to stay ahead of these variants,” de Blasio told reporters Tuesday. According to Chokshi, getting tested, signing up for a vaccine when eligible, and continuing to take measures like social distancing and wearing a mask are critical to drive the numbers down further. But he noted the drop off in hospitalizations and deaths revealed “promising signals.”
The seven-day average of COVID-19 tests conducted has been falling since late February as the positivity rate leveled, but overall, it has cycled between 45,000 and 70,000 since early November. Health Commissioner Chokshi said on Wednesday his department isn’t seeing a major downtick in people getting tested. The stagnant positivity may be reflecting people’s decisions to get tested or not, rather than viral transmission alone, Piltch-Loeb added.
Read More: Why New York’s Last COVID Surge Was Far Less Deadly Than Its First
“People who are symptomatic or who have not been vaccinated yet may still be seeking a test relatively consistently,” she added. Vlahov echoed Piltch-Loeb, noting the measure is dependent on who shows up to get tested.
“The city is made up [of] many neighborhoods, and it may be more meaningful to separate data to look at trends by neighborhood than averaging the City overall,” he said.
City officials have been urging New Yorkers to get vaccinated when they’re eligible. More people receiving doses, particularly young adults, could help drive down the positivity level, too, the head of NYC Health + Hospitals, Dr. Mitchell Katz, said during the Tuesday briefing. The rollout has mostly focused on seniors and people with pre-existing conditions so far, and young people are more active, creating more chances for the coronavirus to infect more people.
“That’s why you’re seeing those dramatic reductions in hospitalizations and deaths,” Katz said. “Getting everybody vaccinated will make a huge difference.”
