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.