When Will the U.S. Reach Herd Immunity With COVID?

The number of people to have received at least one vaccine dose against COVID in the U.S. is approaching 150 million—but it is still uncertain when the U.S. will reach herd immunity.

Herd immunity refers to the fact that having people immune to a virus in a given population will slow down the virus' spread through that population. The more people in that population who are immune, the slower the virus will spread.

Scientists refer to a herd immunity threshold, at which point the percentage of immune individuals within a population is high enough that the epidemic begins to come to a stop.

This percentage differs depending on the virus. More contagious viruses will have a higher herd immunity threshold than others. For COVID, many scientists think it is somewhere between 60 and 80 percent.

Vaccines and prior infections can both be a route to herd immunity, but a number of factors including new variants, vaccine hesitancy, and a lack of data about whether vaccines reduce transmission, make herd immunity difficult if not impossible to predict currently.

Lauren Meyers, Professor of Integrative Biology and Statistics & Data Sciences at The University of Texas at Austin, would not put a date on herd immunity in the U.S.

She told Newsweek: "There are several factors that can impede herd immunity, including lack of access to vaccines in resource-poor communities, vaccine hesitancy in communities where vaccines are widely available, and the continual evolution of the virus leading to variants that are capable of infecting people who have already been infected or vaccinated.

"If new variants can infect people who have already been infected or vaccinated or if a large fraction of people do not vaccinate, then we may not get to the point where most of the population is immune."

John Drake, director of the Center for the Ecology of Infectious Diseases at the University of Georgia, said the threshold for herd immunity against COVID in the U.S. could be reached by the summer—but stressed this would not necessarily mean the epidemic comes to an end.

He said variants were a concern but pointed to vaccine hesitancy as a bigger threat to herd immunity, as well as "the possibility of waning immunity."

He said: "I expect COVID transmission to go way down, but I do not expect it to go away altogether.

"We do not yet have enough information about the frequency or speed of waning immunity. Recent research suggests that there is a lot of variation among people in the persistence of their immune response to SARS-CoV-2.

"Thus, at least some (and maybe all) COVID survivors are expected to become susceptible again. This creates another route for the virus to maintain itself in the population."

Jennifer Dowd is an associate professor of demography and population health at the University of Oxford in the U.K. She said there were too many unknowns to take a guess at a herd immunity date but added: "If kids can get vaccinated during the fall and there are boosters for new variants, that should make a big difference by New Years 2022.

"The good news is the vaccines still seem effective against preventing severe disease even for new variants, and previous infection also likely lessens severity. Even if SARS-CoV-2 is shape-shifting a bit, our immune systems are very clever."

Vaccines and transmission

One issue is that it is still uncertain as to how well vaccines prevent transmission of COVID, even if they are effective at preventing serious illness and death.

Some data is emerging. A February study from Israel that has not been peer-reviewed looked at post-vaccination infections with the Pfizer jab and found viral load—the amount of virus in the body—was reduced four-fold 12 to 28 days after vaccination, which the study said hinted at lower infectiousness.

Another February study from the Harvard T.H. Chan School of Public Health, which was also not peer-reviewed, said evidence was "scarce" about vaccines preventing COVID transmission, but found the Moderna vaccine could potentially reduce transmission by at least 61 percent based on earlier randomized controlled trial data and simulations.

Meyers told Newsweek: "There is promising evidence that several of the major vaccines provide some degree of protection against asymptomatic infection, in addition to symptomatic infection, but we don't know yet exactly how effective they are at preventing transmission."

Drake said: "Yes, it is still unclear the extent to which vaccines confer protective immunity. But we do know they reduce symptom severity, which is correlated with transmission. So, even if vaccines don't eliminate transmission altogether, they should still reduce it."

Dowd said: "If the vaccine does not fully reduce transmission the herd immunity threshold will be higher. We are seeing promising data that vaccines significantly reduce transmission, but not fully."

The graph below, by Statista, shows the countries with the most comprehensive vaccine rollout. Israel's has been touted as the best, and scientists will hope to gather more data on how effective vaccines are as rollouts continue.

Statista graph
A Statista graph showing the eight highest rates of COVID vaccination by country. https://www.statista.com/chartoftheday/ Statista/OurWorldInData

Looking forward

Scientists will be looking to countries with high vaccination rates to study the data and get a better idea of how vaccines are preventing illness, death, and transmission rate, and this could clear up herd immunity predictions.

Meyers told Newsweek: "Scientists are scrambling to learn as much as possible about the impact of vaccination on transmission of the virus, including newly emerging variants. Data from countries that have successfully vaccinated large proportions of their population, like Israel, will be critical to navigating the challenges ahead.

Dowd said: "All eyes are indeed on Israel since they should be the first real test of vaccines at the population level. So far cases are falling much faster in the groups vaccinated first, but only time will tell how well the vaccines will eliminate hospitalizations and deaths in the real world setting. So far the data looks very hopeful."

People cycling with face masks on
People ride and walk on a path along beach amid the coronavirus pandemic on May 15, 2020 in Huntington Beach, California. There are a lot of factors that make an accurate herd immunity date prediction difficult. Michael Heiman/Getty