How many Americans must get a COVID-19 vaccine to achieve herd immunity?

While there’s a lot of hope that researchers can produce a COVID-19 vaccine in record time, people are also skeptical and afraid. Some polls show that about half of all Americans plan to get a vaccine as soon as one is available.

“There are some understandable fears people have that scientists may be rushing the vaccine, that perhaps the administration in the White House might be tempted to proclaim that they have a vaccine that works before the real scientific data is in,” says Yascha Mounk, contributing editor at The Atlantic.

After speaking with leading health experts, he tells KCRW that phase three vaccine trials are in progress.

“What’s happened so far is that doctors have found that a vaccine does seem to be effective against COVID-19, and they have shown that in trials with fewer people, there haven’t been significant adverse effects that should worry us. So that’s why now they are doing trials with much, much greater numbers of participants,” he says.

One goal with a vaccine is to achieve herd immunity. Mounk explains, “The idea is that when a sufficiently large number of people in a population is protected against a particular disease, either because they’ve been exposed to the disease before or they’ve had a vaccine, then the disease will stop to spread. There simply aren’t enough uninfected people around for the disease to be able to hop from one host to another in sufficiently large numbers.”

He says another benefit of herd immunity is that it protects people who can’t safely get vaccinated because they have compromised immune systems.

He notes that COVID-19 is far less infectious than measles. “On average, somebody who has COVID only passes the disease to two or three other people. And that means the threshold for herd immunity is much lower. … If there’s … perhaps one in three Americans who refuse to take a vaccine, that would not allow us to get to herd immunity against measles. But it likely would allow us to get to herd immunity against COVID-19.”

— Written by Amy Ta, produced by Rosalie Atkinson