Does 100% of the population have to be vaccinated for herd immunity?

Throughout the COVID-19 pandemic, the term "herd immunity" has been tossed around as a future benchmark of progress in the fight against the virus. For over a year, health officials and news organizations have analyzed what herd immunity means for COVID-19, and whether and when the United States might achieve it.

But we recently came across a post on Instagram that makes a series of problematic claims about the effectiveness of vaccines and the concept of herd immunity.

Notably, it asserts that the benchmark revolves only around vaccines and can’t realistically be achieved because, "100 percent of the population would have to be vaccinated for that to work."

In arguing that herd immunity can’t be achieved with vaccines, the Instagram post points out that vaccines don’t guarantee immunity. But the new COVID-19 vaccines’ high efficacy numbers undercut this point. 

Clinical trials and follow-on studies show that the vast majority of people who get the COVID-19 vaccines are protected.And all three vaccines approved for emergency use in the U.S. have been proven to be highly effective at protecting against serious COVID-19 disease and death among those who do get infected.

The post bases its claim on the premise that "today we take ‘herd immunity’ to mean that no one gets the disease if we all vaccinate."

That’s not the prevailing definition of herd immunity.

Herd immunity occurs when enough people are immune — either through vaccination or previous infection — to make its spread from person to person unlikely. That’s because an infected person is less likely to encounter a non-immune person to pass it on to, breaking that link in the chain of transmission.