Less than half a population needs COVID-19 infection for herd immunity, study says

June 23 (UPI) -- Less than half of the population would need to be immune to COVID-19 to develop "herd immunity" to the virus, according to an analysis published Tuesday by the journal Science.

The modeling study found that herd immunity potentially could be achieved with about 43 percent of the population being immune, as opposed to the 60 percent estimate derived from previous models.

Researchers say, however, that the findings are intended to serve as an interpretation of how population differences might influence herd immunity to the new coronavirus, SARS-CoV-2, rather than as a precise estimation, the authors said.

"These findings highlight that a lower percentage of people need to be immune through infection to achieve herd immunity, and that this is dependent on many things, including people's activities and exposure, and if immunity is sustained," public health specialist Brandon Brown, told UPI.

Brown, who is an associate professor of social medicine population and public health at the University of California-Riverside, was not part of the study team for the Science study, which is based at Stockholm University in Sweden.

The term herd immunity refers to a scenario in which most of a population is immune to an infectious disease, according to researchers at Johns Hopkins University. It provides indirect protection to those who are not immune to the disease, they said.

Research has indicated that people diagnosed with COVID-19, the disease caused by SARS-CoV-2, and recover develop antibodies capable of fighting off the virus, although it is unclear how long these antibodies last.

Measles, mumps, polio and chickenpox are among the infectious diseases that are now rare in the United States because vaccines have been developed to help establish herd immunity. However, no vaccine exists for COVID-19.

"The quicker immunity wanes, the less positive and relevant is our result," Tom Britton, co-author of the Science study, told UPI.

For their analysis, Britton, a professor of mathematics at Stockholm University, and his colleagues simulated herd immunity using an epidemiological model, taking into account the influence of age and social activity on a person's susceptibility to COVID-19.

Assuming that an infected person transmits the virus to an average of 2.5 other people, the model developed by the team predicted that a herd immunity level of 43 percent is sufficient to prevent a second major outbreak after lifting restrictions.

Lifting social restrictions gradually, rather than simultaneously, across populations could help prevent a resurgence of infections, they said.

"We need to keep up our strong prevention practices rather than rely on people becoming immune through infection, since herd immunity does not necessarily stop the spread of infection, particularly to people with weakened immune systems," said the University of California-Riverside's Brown.

"We have to utilize the prevention measures that we know work, including physical distancing, wearing masks, vigorously washing hands, avoiding touching our face, staying home if possible when we are sick, COVID testing, contact tracing and cleaning surfaces rather than focusing on herd immunity," he said.

Britton was part of the team that helped develop the COVID-19 outbreak response in Sweden, which chose not to widely implement social distancing measures or shut down its economy.

He had predicted that half of the country's population would become infected -- and thus create herd immunity -- by the end of April. However, a study released last month by the Public Health Agency of Sweden found that just 7.3 percent of 1,100 people tested had antibodies against the virus.

"I think that places who have seen major outbreaks -- Lombardia, Madrid, London and Stockholm -- might see small second waves, but very unlikely that they will experience big second waves due to immunity," Britton said.