# Can herd immunity stop COVID-19?

Whether or not COVID-19 can be conquered through herd immunity is an open question, according to experts.

In an April 1, 2021, article in the Boston Globe, Marc Lipsitch, professor of epidemiology and director of the Center for Communicable Disease Dynamics at Harvard T.H. Chan School of Public Health, was among experts who said that a lack of clear data on the virus and vaccines makes it hard to determine the possibility of reaching herd immunity, which could occur if enough people are vaccinated or otherwise immune to the virus.

There’s a simple math formula for calculating herd immunity. But the problem is that one of the figures needed for the formula is a disease’s infectiousness rate—and scientists aren’t sure what that rate is.

Early in the pandemic, scientists thought that each infectious person with COVID-19 would infect 2.5 to 3 other people—meaning that reaching herd immunity would require roughly two-thirds of a population to be immune, according to the formula. But that estimate may be off. Many early cases may have been missed because of limited early testing for COVID-19. And because people have changed their behavior over time, it may have masked the disease’s true infectiousness. “We were sort of stuck with bad data,” said Lipsitch.

Other factors further complicate the herd immunity calculation. One is the presence of highly infectious variants. Another is that it’s not known whether, and to what extent, vaccinated people can still transmit the disease to others.

If COVID-19 is more infectious than previously thought, the herd immunity threshold would be higher too. So, for instance, if each infectious person could infect four people, 75% of the population would need to be immune to stop the virus from spreading. If each person could infect five others, 80% would need to be immune; if they could infect six others, 85% would need immunity.

Regardless of whether herd immunity can be achieved, it’s still important to vaccinate as many people as possible to slow COVID-19’s spread, according to the experts. “If we can turn this into a disease that is harmful, but to a smaller number of people, and on a smaller scale in terms of the hospital system, we will then decide as a society, as we do with the flu…to try to reduce it, but not to disrupt life on the same scale we’ve disrupted life so far to control it,” said Lipsitch. “We should not despair.”

Read the Boston Globe article: Experts say ‘herd immunity’ could conquer COVID-19. But is it even possible?