Doctors say herd immunity is more complicated than we think.

Herd immunity happens when a large portion of a community gets sick, recovers and then develops antibodies that prevent reinfection.

Dr. Lamar Hasbrouck is a public health professional, a former senior medical epidemiologist at the Center for Disease Control and Prevention and health director for the state of Illinois.

He says there are two ways we can achieve herd immunity. One is through a vaccine and the other is if a large mass of people becomes infected and develops antibodies.

Dr. Torian Easterling is the newly appointed senior medical officer of the city's Health Department. He says at least more than half of a population would have to have antibodies for herd immunity to happen and even then, it's complicated.

"Because we do not know the ability of these antibodies to prevent infection, there's really no way for us to match this conversation to even talk about herd immunity," said Esterling.

Experts warn that allowing the disease to spread unchecked can lead to disastrous consequences - leaving thousands, or even millions, dead.

Doctors at ParCare Community Health Network in Williamsburg say they are seeing a hyperlocal incident of herd immunity in Brooklyn.

CEO Gary Schlesinger says they have been conducting up to 600 antibody tests a day. He says he is seeing a correlation across his five centers - the more Orthodox Jewish patients a location sees, the more positive antibody tests.

In April, the city saw spikes in coronavirus cases in areas with large populations of Orthodox Jews. For example, on April 21, testing sites in Williamsburg and Borough Park reported between 58% and 74% of people coming in to get tested were sick with the virus. Those numbers dropped by the beginning of May even though testing continued to increase.

Schlesinger says it is all about the nature of this community. Before the pandemic, members prayed together several times a day, big gatherings were a norm and families are typically large.

Medical director Dr. Paul Rosenstock says because of this, the virus spread quickly in its early days and its effect on this community has been especially devastating.

"There is a price to pay for herd immunity and I don't think it was by design. They didn't do this intentionally," said Rosenstock.

Rosenstock says they saw a large peak in patients in March and early April around the Jewish holidays of Purim and Passover.

"If you look at the term herd immunity, you would definitely look at the Orthodox Jewish community as a case study," said Rosenstock.

Easterling disagrees.

"There is no herd immunity," he said. "We cannot be having this conversation."

Easterling says most recent data on the city's infection rates contradicts the message that any community is at or near herd immunity.

Earlier this month, Gov. Andrew Cuomo announced a new plan to tackle new clusters. He pointed to mass gatherings in Orthodox communities - calling them super spreader events.

Anger erupted from protesters in Borough Park over Cuomo's plan to impose stronger restrictions in these clusters. Protesters said community leaders were not consulted. They say many yeshivas and synagogues in Brooklyn closed voluntarily, even before the city shut down. After being at the forefront of plasma donations, they say they were being blamed for the cluster.

Most medical professionals agree - the best way to get through the pandemic is to develop an effective vaccine.

Medical professionals say the most responsible thing anyone can do is act like they can get the virus, even if they already tested positive for antibodies.