“However, we are seeing steady increases and that is a concern,” he said. “And the other thing that is a huge concern is if the new strain gains a foothold here, it can really spread very, very rapidly.”
Epidemiologists are wary of calling for immediate new lockdowns. Instead, for now, they are calling for targeted measures to try to bring down infection rates, such as a push for more testing in the most affected ZIP codes and among the most vulnerable.
In an inverse effect, parts of the city with the lowest rates of the virus — wealthy ZIP codes in Manhattan and Brooklyn — tend to have the highest rates of testing, city data shows. That should change, Dr. Frieden said, with more testing surging into harder-hit areas.
Patrick Gallahue, a spokesman for the city health department, said the city was already focusing its testing resources in hard-hit areas. Additional restrictions on businesses or gatherings, he said, would have to be enacted by the state.
But to reverse course in New York City before a vaccine takes effect will likely take more drastic measures, such as a lockdown, several experts said.
City officials need the state’s permission before instituting any citywide shutdowns, except for schools. Mr. Cuomo has suggested that schools should close only if the infection rate inside school buildings was higher than that of the community. As of midweek, the citywide positive test rate in schools was far less, at 0.49 percent.
Dr. Nash said other red lines and parameters should be established to guide shutdown decisions, such as one around mortality: How many deaths per day will we accept before we lock down? He also called for more attention to local conditions, such as virus rates in neighborhoods with overwhelmed intensive care units, and to the speed of the virus’s transmission, a measure known as R0, which is not regularly published by the state. State officials say they track those numbers internally.