In this Tuesday, March 2, 2021 file photo, a pharmacy technician loads a syringe with Pfizer's COVID-19 vaccine at a mass vaccination site at the Portland Expo in Portland, Maine. Credit: Robert F. Bukaty / AP

The spread of the delta coronavirus variant has pushed the threshold for herd immunity to well over 80 percent and potentially approaching 90 percent, according to an Infectious Diseases Society of America briefing on Tuesday.

That represents a “much higher” bar than previous estimates of 60 percent to 70 percent, because delta is twice as transmissible, said Richard Franco, an assistant professor at the University of Alabama at Birmingham.

“It is becoming clear that this is a very dangerous, way more dangerous virus than the original one,” Franco said.

Herd immunity is based on the idea that when a certain percentage of the population has been vaccinated against the virus or gains immunity by a previous infection, it helps protect the broader population and reduce transmission.

Nearly 60 percent of Americans have received at least one dose of a coronavirus vaccine, and about 50 percent have been fully vaccinated, representing about 165 million individuals, according to CDC data. Some 35 million people in the U.S., meanwhile, have tested positive for the virus over the course of the pandemic.