Will the United States achieve herd immunity for COVID-19 before a vaccine is ready?
The answer to that question depends on which epidemiologist one asks and the percentage of the population that one thinks needs to be infected to achieve herd immunity.
Herd immunity occurs when a large portion of the population, the “herd,” becomes immune to a virus — through a combination of people getting infected with the virus and people getting vaccinated. Once herd immunity has been achieved, it is harder for the virus to spread to the smaller part of the population that has not been infected or vaccinated.
“It will depend on how soon we get the vaccine. Right now, we are in a very fragile balance of suppressing the virus,” said Dr. Manoj Jain, an infectious disease physician at the Rollins School of Public Health. “By our efforts with masking, distancing, and testing, we are balanced with the force of the transmission of the virus in various subpopulations.”
Professor Gabriela Gomes, a statistician at the University of Strathclyde in Glasgow, Scotland, who constructs statistical models of virus infection, thinks we may be close to herd immunity.
“I think that herd immunity is being achieved in some heavily affected locations, and many others will follow before a vaccine is available,” she said. “Vaccines will still be very valuable, mainly to confer protection to the more vulnerable groups.”
Achieving herd immunity without a vaccine would exact a heavy toll.
“In the U.S., the disease continues to have a lot of opportunity to spread, so it’s possible that the U.S. will get to herd immunity before the vaccine is available,” said Joel Miller, a statistician at La Trobe University in Australia. “If so, then the U.S. will end up with a much higher fatality rate than the rest of the world.”
Epidemiologists have suggested that herd immunity is achieved when 60%-70% of the population has been infected or vaccinated. At issue among epidemiologists and statisticians is the role that subpopulations play in herd immunity. For example, COVID-19 may spread less rapidly in neighborhoods with many older residents who tend to be less socially active than in neighborhoods with many teenagers and young adults. The virus also tends to spread more rapidly in urban areas as opposed to suburban and rural areas, where people usually live farther apart.
A New York Times article reported that it is possible that some parts of New York City, London, and Mumbai have already achieved substantial immunity.
Jain suggested that the number of people who have been infected in the general population needed to attain herd immunity may be lower if certain subpopulations have higher rates of infection.
“I believe that the prevalence of past infection at 40% in the general population may reflect a prevalence of 60%-70% in the subpopulation of those who are intermingling in the community and possibly spreading the infection,” he said. “When that subpopulation number reaches that level of 60%-70% of infection, as is seen in some areas in New York or Mumbai, then the transmission declines dramatically — and we see a downward epidemic curve.”
Tom Britton, a dean of mathematics and physics at Stockholm University in Sweden, says that the 60%-70% assumption is flawed.
“That is based on old results [related to] vaccination,” said Britton. “It assumes that if you vaccinate, you have to vaccinate 60%-70% of the population to attain herd immunity.”
Britton constructed a model for COVID-19 that divided the general population into age groups since different ages have different rates of susceptibility and exposure to a virus. Younger people, for example, tend to be more social and thus have higher rates of exposure. His model also took into account the different rates at which age groups are socially active.
Britton is cautious with the interpretation of his model.
“Exactly what the number is, we don’t claim to know,” he said. “Our particular model, rather than 60% [herd immunity], was 43%. But we don’t claim 43% to be a magic number. I think the actual number is substantially lower than people actually believe. But I don’t think it is a controversial result.”
Gomes produced a model suggesting that it may be as low at 10%-20%. She said that traditional models often assume that everybody is susceptible to a virus based on the average susceptibility of the general population. She examined susceptibility on an individual level in the countries of Belgium, England, Portugal, and Spain.
“Why is this so crucial? In a heterogeneous population, individuals are not infected at random,” said Gomes. “Specifically, those who are more susceptible or exposed tend to be infected and become immune earlier.” The greater the susceptibility and exposure to the virus that some people have, the lower the threshold for herd immunity is for the general population.
Jain, though, offers the U.S. a warning.
“By definition, herd immunity means we can let our guard down, that we can go about life as usual,” he said. “That is not the case in our declining numbers. Our numbers are declining in part because we are masking and distancing. This lowers our transmission number, which in turn lowers the percent of population required to achieve ‘herd immunity.’ … If we stop that, then the population percent required for herd immunity will go up.”
