NEW YORK — COVID-19 spread in city schools has stayed relatively low even as students returned full-time to in-person class this year and the more infectious Delta variant surged, new city data suggest — though questions about the numbers remain.
Out of roughly 22,000 kids and staffers who were exposed to a positive COVID-19 case in school between Oct. 10 and 31, city officials identified a total of 130 who went on to get infected — an estimated “secondary attack rate” of roughly .6%, according to data from the Situation Room task force that monitors coronavirus in schools.
That rate is slightly higher than the .5% that city scientists estimated for COVID spread in schools last fall, when only a fraction of kids were in classrooms and the Delta variant hadn’t yet emerged, but still far below the average 15-20% transmission rate in households.
“We have pretty high confidence that these secondary attack rates demonstrate low levels of transmission in the schools,” said Jay Varma, a senior public health adviser to Mayor Bill de Blasio.
There are caveats to the data, however.
City schools don’t require students or staff who are exposed to a COVID-19 case at school to get tested for the virus after the exposure, and analysts didn’t track how many of the 22,000 people who were likely exposed went on to get a test.
That means the city data could have missed some kids or adults who caught the virus at school but never got tested — a weakness Varma acknowledged. But he countered that even if the real secondary attack rate is twice as high as the .6% officials estimated, it would still mean one out of roughly every 83 people exposed to COVID-19 in school goes on to get infected — a number he called reassuring.
Anna Bershteyn, a professor of population health at New York University who has closely tracked coronavirus data in city schools, agreed that it’s “reassuring to see this low secondary attack rate despite Delta.”
She said the number city scientists found generally fits with what researchers around the world have seen studying COVID-19 spread in schools that use mitigation measures.
But Bershteyn added that city officials could strengthen their data, and help prevent any outbreaks, by requiring anyone exposed to the disease at school to get tested, rather than just suggesting it.
Also complicating the data is the fact that city officials recently changed how they define who has been “exposed” to COVID-19 in schools.
For all of last year and the first month of this school year, anyone who shared a classroom with an infected person was considered a close contact.
But starting Oct. 10, city officials narrowed the definition of close contact to include only kids who are unmasked or within 3 feet of an infected peer for a sustained time. That change could make this year’s school transmission rate look higher than last year’s, Varma said. But it’s also unclear how closely schools are adhering to the new guidelines, which can be difficult to follow.
Another surprising finding in the city data was that vaccinated students and staff were slightly more likely to catch the virus in school than people classified as “unvaccinated.”
Studies have consistently shown that vaccinated people are less likely to catch COVID-19, in addition to being much less likely to get seriously sick or die.
Varma said there are several possible reasons for the discrepancy. Some vaccinated kids may have never reported getting the shot and been misclassified as unvaccinated, he said. It’s also possible that vaccinated people were more likely than unvaccinated people to get tested after an exposure, boosting the chances their cases were identified.
There are other signs in the data that vaccine is having an effect.
Last year, researchers found 78% of COVID-19 cases transmitted in city schools started with an adult. But this year — following the vaccine mandate for school staffers — only 35% of school transmissions started with an adult.
Varma said it’s “likely” there is more transmission in schools this year than last because of the more infectious Delta variant, but said “we’re counteracting that with lots of vaccination.”
“To me what’s been reassuring is that we haven’t seen any marked increase in transmission rate even though we have a substantial increase in the number of kids in school,” he said.