WHO: COVID-19 Herd Immunity Unlikely This Year

Scientists from the World Health Organization (WHO) warned this week that it will not be possible to achieve herd immunity from COVID-19 through vaccination this year.

WHO's chief scientist Soumya Swaminathan, MD, said during a press briefing on Monday that it will take more time to create and distribute enough vaccines around the world to stop the spread of COVID-19. "We are not going to achieve any levels of population immunity or herd immunity in 2021," she said. However, Swaminathan said, there may be a few “pockets” of herd immunity achieved in certain countries but “it's not going to protect people across the world.”

Swaminathan added that people will need to continue to focus on social distancing, hand hygiene, and mask-wearing to prevent the spread of the virus until herd immunity is achieved. “We have to be a little bit patient,” Swaminathan said. “The vaccines are going to come. They're going to go to all countries, but meanwhile, we mustn't forget that there are measures that work.”

Herd Immunity Basics

Herd immunity, also known as population immunity or community immunity, is when a significant portion of a population develops immunity to an infectious disease, either through vaccination or having a prior illness, according to the Centers for Disease Control and Prevention (CDC). Once herd immunity is achieved, the spread of the infectious disease from person to person is unlikely.

Even those who aren’t vaccinated, like newborns, are given some protection in this case because the disease has little opportunity to spread in the community, the CDC says. 

The WHO stresses the importance of achieving herd immunity to COVID-19 through vaccination, and not by exposing people to the virus. “Vaccines train our immune systems to create proteins that fight disease, known as ‘antibodies,’ just as would happen when we are exposed to a disease but—crucially—vaccines work without making us sick,” WHO states.  

Achieving Herd Immunity

To safely achieve herd immunity against COVID-19, the WHO says that a “substantial portion” of the population would need to be vaccinated.

“The percentage of the population that needs to be immunized in order to achieve herd immunity varies by disease,” Thomas Russo, MD, professor and chief of infectious disease at the University at Buffalo in New York, tells Verywell.

It’s calculated using equations that require estimates of the population at risk and the infectiousness of the virus, i.e., its R0 (pronounced “R naught”), Thomas Giordano, MD, MPH, professor of medicine and section chief of infectious diseases at Baylor College of Medicine in Texas, tells Verywell.

“The calculations are only estimates and they can be affected by population density, behavior change—masks, distancing—and other factors,” he says. “But in the purest sense, herd immunity for a population should be a number that is calculable.”

It’s been difficult to make that calculation with COVID-19. “We’re not absolutely sure what it is because of public health measures that were instituted when the pandemic began,” Russo says. “And, if new variants of the virus prove to be more infectious, that’s going to move the R0 upward as well.”

Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases, has changed the number he cited for herd immunity over time. While he used to say 60% to 70% of the population needed to be vaccinated to achieve herd immunity, he later told The New York Times that it may be closer to 90%. “We need to have some humility here,” he said. “We really don’t know what the real number is. I think the real range is somewhere between 70 to 90%.”

Herd Immunity Takes Time

As for whether immunity will differ between those who are vaccinated against the virus and those who have actually had the virus, Giordano says it shouldn’t. “Immunity is immunity, whether from vaccines or from natural infection,” he says. “If one sort of immunity lasts longer than another or is more protective against asymptomatic infection and transmission than another, then the composition of the immunity might need to be a factor in estimating at any time whether there is currently enough immunity in the population to limit transmission. But that so far is speculation.” 

While vaccinations in the U.S. and across the world have moved at a slower-than-expected pace, Russo says he’s “optimistic” that will change. “I do think we’re going to get enough people to get vaccinated to achieve our goals,” he says. “It just may take some time.”

Giordano stresses that, even once it’s achieved, herd immunity requires work. “It is a state that has to be achieved and maintained unless the virus is completely eliminated, which is unlikely anytime soon,” he says. “Look at measles. We had herd immunity for decades, but then enough people in the U.S. were not vaccinated that we saw spread in the population...If it turns out the vaccine immunity is not long-lasting, we will need to re-vaccinate periodically. That would still be better and likely less expensive for the worldwide economy than a pandemic.”

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