When Will PA Reach Herd Immunity? Experts Weigh In

PENNSYLVANIA — As more and more of the population across Pennsylvania receives one of the COVID-19 vaccines, herd immunity, once an idealistic glimmer of the future, is suddenly a reality on the near horizon.

But just how near is it, and what must happen for the region to achieve it?

Experts from around the state and the country generally agree that herd immunity sets in once 70 to 80 percent of a population is protected against infection. Protection occurs in two ways: through vaccination, and through antibodies from a recent infection. An increasing number of individuals immunized in this way makes it harder and harder for the virus to find new hosts, thus slowing the pandemic's engine to a trickle and eventually a halt.

"Herd immunity refers to the proportion of a population required for the rate of disease spread to equal zero," says UC Davis distinguished professor James R. Carey, of the Department of Entomology and Nematology.

In Pennsylvania, there have been more than 986,000 cases of coronavirus. Another 4,043,398 individuals have received at least a partial vaccination, as of Thursday afternoon. With a total population just shy of 13 million, the state is shy of the 70 to 80 percent mark.

However, with vaccinations expected to proceed at a rapid rate — President Joe Biden has said there will be enough doses available for every American by the end of May — herd immunity may be nearer than it seems.

RELATED: Here's How Soon Everyone In PA Could Get Vaccinated

Precise estimates vary. Dr. Suzanne Judd, an epidemiologist in the School of Public Health at the University of Alabama at Birmingham, estimates the nation may reach herd immunity by May. Philadelphia Health Commissioner Dr. Thomas Farley said the city could reach that point by June, but added "please don't hold me to that."

"We're not there yet," Farley said earlier this week. "We may be at a point where it's reducing the number of infections, but we're not at that point where we can really cycle down to zero [cases]."

Montgomery County Commissioner Dr. Val Arkoosh concurred that herd immunity would be seen at around the 75 to 80 percent mark, but said it was too early to know exactly when the county and region might reach that point.

"We are working on trying to determine when we might get there," she said Tuesday, noting a team in the Office of Public Health was working on projections.

Even once herd immunity is reached, that does not mean the virus will be eradicated, Judd said.

"I think that COVID-19 is going to be endemic in the United States," Judd said. "It is going to be like the seasonal flu, something we have to tolerate regularly. There will be isolated outbreaks, and they will likely occur in populations with lower immunity. This means that, just like the flu, a strong vaccination campaign will be needed to keep people out of the hospital."

In the short term, the quickest way to reach to herd immunity will be an increase in the supply of vaccine doses. The national shortage remains an issue, but shipments are expected to continue to grow. This week's shipments of Moderna and Pfizer were expected to rise from 14.5 million to 15.2 million. In Pennsylvania, 4 million doses of the single-shot Johnson & Johnson vaccine are expected by the end of March.

With reporting from Patch correspondents Max Bennett and Michael Seale

For full information about getting a coronavirus vaccine in Pennsylvania, visit Patch's information hub.