How States Could Reach Herd Immunity As Delta Variant Continues To Dominate

Cautious optimism has been voiced from some states that they may be approaching herd immunity against COVID.

Last week, Joshua Green, lieutenant governor of Hawaii, said the state is "seeing the beginning of the end of the pandemic" after its health department announced that 70 percent of Hawaii residents had completed COVID vaccination.

Green added that "herd immunity appears to have begun to set in," but stressed that more people still need to be vaccinated.

And on Monday, Oregon Health & Science University (OHSU) forecasted that the state of Oregon could reach herd immunity by around December 26 this year once 85 percent of its population becomes immune to the virus either through vaccination or recent infection.

Several experts have explained to Newsweek what sort of factors must be considered when thinking about when herd immunity might be achieved—and how it's not quite as simple as looking at which states have the highest vaccination rates.

"In general I would be very cautious about using the term 'herd immunity,'" warned Bill Hanage, associate professor of epidemiology in the Department of Epidemiology at the Harvard T.H. Chan School of Public Health.

"People interpret it as a finish line, and think it means that the virus is then eradicated or eliminated. But that's not accurate.

"Transmission of delta is continuing in many places where seropositivity [meaning people have COVID antibodies] is at 90 percent or more, and while vaccines make the consequences much less severe, infections in at-risk individuals can still be serious."

But what is herd immunity? Essentially, it's a threshold that's reached when, on average, one person who is infected with a disease goes on to infect less than one other person, explained Jorge Alfaro-Murillo, associate research scientist at the Center for Infectious Disease Modeling and Analysis at the Yale School of Public Health.

He told Newsweek: "It basically depends on four factors multiplied together: how many days is an infected person infectious; how many people one gets in contact with per day; the probability that a contact transmits the virus; and the fraction of the population that are not immune yet."

But COVID-19 has been extremely difficult to model, and forecasts have limitations, said Ricardo Franco, associate professor of infectious diseases at the University of Alabama at Birmingham.

For one thing, herd immunity is not the same between viruses. According to Franco, a population would have to be 90 percent vaccinated in order to reach the herd immunity threshold against the COVID Delta variant, for instance.

And "immune" does not necessarily mean vaccinated. Natural immunity through past infections must also be taken into account. "The more people infected, the lower the vaccination target rate needed to reach herd immunity, which unfortunately has been the case because of vaccine hesitancy," Franco added.

This ties back in to the aforementioned 85 percent immunity prediction in Oregon this coming December. Peter Graven is an affiliate assistant professor at OHSU who was behind the forecast.

He told Newsweek that the state's current immunity level is due to a combination of infection and, to a much lesser extent, vaccination. By December 26, he said, "it will be much more difficult for the virus to create a surge of hospitalizations."

Prediction isn't a perfect science. His model is based on a number of assumptions, like the number of people who were infected but never tested positive, or whether people who were previously infected are equally likely to get vaccinated.

But he's still optimistic. "Indeed, we need to be cautious but it appears the worst is behind us," he said, as long as people get booster shots and assuming no new variants crop up.

As mentioned, predicting herd immunity is about more than looking at vaccination rates. If it was, most states appear to be a while off the estimated 90% threshold.

Vermont, the U.S. state with the most successful vaccine rollout, has fully vaccinated 71 percent of its population. In Oregon, just 62 percent of people are fully vaccinated, according to the New York Times. Other factors change the game.

covid vaccine herd immunity
A nurse draws a COVID vaccine doses from a vial on March 25, 2021 in Bowie, Maryland. Predicting herd immunity is about many different factors, not just vaccination. Win McNamee/Getty Images