With herd immunity elusive, vaccination best defense against COVID-19, Stanford epidemiologist says

COVID-19 vaccinations are now readily available to all Americans, so herd immunity should be attainable, right?

Probably not, says Julie Parsonnet, MD, professor of medicine and of epidemiology and population health. Paradoxically, it may be the very concept of herd immunity that is thwarting the uptake of vaccinations in the United States.

“We need to stop pushing herd immunity to the public,” Parsonnet said, as it may discourage some people from getting vaccinated in the mistaken belief that, if other people get vaccinated, they can just wait for herd immunity. “Public health departments don’t talk about herd immunity because it’s not helpful for the immediate protection of individuals and the overall response to the pandemic. What’s important is getting as many people vaccinated as you possibly can.”

Herd immunity is reached when the number of people in a population who are susceptible to disease drops to such a low level, usually due to vaccination, that any new cases cannot spread. Parsonnet, the George DeForest Barnett Professor in Medicine, said the concept of herd immunity is best used to model disease and figure out a public health strategy. Herd immunity is a nice idea, she said, but in reality, it’s a concept best applied to cow herds — or perhaps to nursing homes, ships, boarding schools or islands — but not to an entire country or the world.

Nevertheless, the concept caught the public’s attention last year as cases skyrocketed. Some have latched onto the idea, thinking that once the population reaches a certain threshold, the coronavirus will dissipate. But, while almost half the population of the United States has received at least one dose of a COVID-19 vaccine, hesitancy is high — about 30% — and a vaccine rate of 70% won’t bring us close to herd immunity, Parsonnet said.

COVID-19 is not measles

Diseases such as measles and smallpox have been nearly eradicated, or at least heavily tamped down, thanks to widespread and effective vaccination. It’s unlikely the United States has actually reached herd immunity for measles, as many children are now unvaccinated, Parsonnet said. “Measles cases are currently quite rare, and when they do occur, they’re always symptomatic. This allows for those who’ve been exposed to be isolated, and those at risk can be protected through something called ‘ring vaccination,’ in which people who may encounter a sick individual are vaccinated,” she said.

But COVID-19 is not measles. Unlike measles, not all cases of COVID-19 are symptomatic, so sick individuals can’t all be isolated; the COVID-19 vaccine is not 100% effective, and it’s unknown how long it bestows protection; there are many variants of the virus; and the virus can infect animals. “It took 40 years to control measles; for COVID, it is likely to take a lot longer to control,” Parsonnet said.

“The other important thing to remember is that herd immunity is not this ah-ha moment where suddenly there’s no more disease and we don’t have to worry about it anymore,” Parsonnet said. “It’s something that must be maintained, mostly through vaccinations, once transmission has slowed down.”

The need for immunological upkeep is due to new “susceptibles,” or individuals who have no immunity to a given disease, in a population. And with regard to COVID-19, there are a lot of susceptibles, such as newborns or immunosuppressed individuals. “We don’t live in isolation, and to get herd immunity in such an interconnected world is extremely challenging,” Parsonnet said.

It’s true that susceptible individuals who acquire COVID-19 will have natural immunity, but it’s not enough to protect the herd. Parsonnet uses measles to illustrate her point. The disease was introduced in the Americas in the 1500s, but even after hundreds of years, the U.S. population never developed natural herd immunity, likely due to newborns’ natural susceptibility, among other reasons. Only after 1963, when a measles vaccine was developed, did the United States start seeing large-scale immunity. For SARS-CoV-2, it’s even trickier, as variants could evade natural or vaccine-derived immunity, and natural immunity doesn’t seem to be as potent, Parsonnet said.

Stop planning on herd immunity

Parsonnet also is concerned that vaccine hesitancy remains high in the United States. Using an equation that estimates the transmissibility COVID-19 and the effectiveness of the available vaccines, her latest calculations show that about 90% of the United States would need to be fully vaccinated to reach herd immunity. There are other challenges as well.

 “What if vaccine immunity starts to wane? What if we all need booster shots for a different variant?” she said. Every time booster shots are required, she noted, there’s likely to be a dip in the number of people showing up to receive them.

So, what are we to do?

“The best way to handle this is to vaccinate the people who are most likely to transmit the virus — young adults, people who have a lot of contacts, people who live with a lot of other people in their households,” Parsonnet said.

People who are reluctant to get the vaccine and people who face obstacles to receiving it, such as immigrants who experience language barriers, are most likely to transmit the virus. Vaccination campaigns should focus on those groups, Parsonnet said.

“If we want to approach this in a realistic way, the focus shouldn’t be on herd immunity. It should be on vaccinating as many people as possible — especially the people who will have the biggest impact on a population level,” Parsonnet said. “That’s what will make the biggest impact on significantly diminishing the rates of COVID-19.”