U.S. Could Reach Herd Immunity with 47 Percent Infection Rate, Study Finds

The United States could reach herd immunity against coronavirus when only around 47 percent of the population has been infected, a team of scientists has estimated.

In a pre-print study—one that has not been peer-reviewed—scientists found that the level of herd immunity the nation requires to emerge from the pandemic could well be significantly lower than frequently cited "classical" estimates of 60 percent or higher.

Scientists from the research arm of UnitedHealth Group—an American health care company based in Minnesota—wanted to understand what levels of herd immunity are required during the COVID-19 pandemic given that populations do not mix homogeneously. This means people only interact within a small circle of close contacts.

To do this, scientists used data from counties across the U.S. to run computer simulations of how the pandemic will play out up until its end point. They found that necessary levels of herd immunity vary greatly from county to county, but a population weighted average for the United States suggests this threshold could be reached when 47.5 percent of the country's population has been infected.

"We investigated this issue to consider the idea that people don't mix uniformly and what impact that may have on herd immunity," Ethan Berke, Chief Public Health Officer for UnitedHealth Group and an author of the study, told Newsweek.

"The key finding is that the threshold for herd immunity is likely lower than the range of classical estimates, based on the idea that some locations get hit really hard with infections while others have minor outbreaks."

Berke said their research also suggests that the end state of the pandemic will be characterized by "bursts" or "spikes" of infections in different regions.

"Even after we've reached a population averaged 'herd immunity,' outbreaks may still erupt in different communities at different times," he said.

Classical infectious disease models assume that all people transmit and get with SARSCoV-2—the virus that causes COVID-19—at the same rate, or in other words, that the population mixes homogeneously.

But in reality, populations are "heterogenous" meaning people only mix with a limited number of other groups. Thus taking this into account may lead to lower estimates for herd immunity than usually assumed, which could have "significant implications" for public health, the scientists said.

"Groups of teenagers tend to not interact with groups of people in their 60s," the authors wrote in the study. "People in Montana do not tend to interact with people in New York. Only a few people move across and between groups.

"Looking forward toward vaccination strategies, these results suggest we should consider not just who is vaccinated but where those vaccinations will do the most good."

New York City
People wear protective face masks in New York City on September 6, 2020. The United States could reach herd immunity against coronavirus when only around 47 percent of the population has been infected, a study has found. Noam Galai/Getty Images

Berke told Newsweek that his team hopes herd immunity levels will be reached via mass vaccinations rather than through people being infected, given that COVID-19 can cause severe illness and death.

"We also hope that new treatments like those that utilize neutralizing antibodies will improve mortality rates until a vaccine or herd immunity is achieved," he said.

Kate Langwig, a researcher in the Department of Biological Sciences at Virginia Tech, who was not involved in the study, told Newsweek it was important to address the fact that populations do not mix homogeneously when estimating thresholds for herd immunity.

"These findings cement some of the concepts that have been previously reported, namely, that herd immunity in heterogeneous populations is going to be lower than models which assume homogeneity among individuals," she said.

She added that naturally acquired herd immunity should not be a public health strategy.

"While the acquisition of herd immunity could occur in some communities, it is an unfortunate artifact of not being able to effectively control a very contagious virus," she said.

"In addition, because we expect transmissibility of the virus to vary over time due to seasonality and increased contacts, we can't expect that approaching a herd immunity threshold at one time point will result in sustained protection for a community for a long time period. A vaccine is a safer and more effective way to reach herd immunity."

Henry Raymond, an associate professor and epidemiologist at the Rutgers School of Public Health, who was also not involved in the study, told Newsweek that the UnitedHealth Group team's model was "somewhat novel" in its approach while saying that "all models are only as good as the inputs."

"One big assumption here is how much and for how long do SARS-CoV-2 antibodies confer protection," he said.