Herd immunity happens when over 60% of the population develops immunity—antibodies—to a disease, a phenomenon that usually occurs when a population is vaccinated against a virus.

In the new study cited by New York Governor Andrew Cuomo on 3,000 people across the state, 13.9% exhibited COVID-19 antibodies (21% in New York City), implying that 2.7 million people across the state had been exposed to COVID-19, according to Bloomberg, 10-times higher than the presumed infection rate.

Dr. Nate Favini, medical lead at preventative health clinic Forward, told Forbes he’s skeptical about the antibody tests, and cautions against opening up the country to reach herd immunity, saying that would require infecting four-times the amount of people who’ve had the virus—all over the country—leading to a much higher number of deaths by possibly overwhelming hospital resources.

It is unclear whether, and for how long, those with COVID-19 antibodies are immune to second-time infection, as the CDC says survivor immunity is “not yet understood.”

Further, the validity of antibody tests have been widely criticized, as many on the market are not approved by the FDA.

Favini also thinks that more information about how the study was conducted is needed to accept and understand these numbers.