Why ‘breakthrough’ COVID-19 cases happen

While research suggests that the available COVID-19 vaccines provide strong protection against the known variants, experts say that some “breakthrough” infections are to be expected because no vaccine is 100% effective.

In recent weeks, there have been high-profile breakthrough infections among vaccinated athletes and government staff members.

In a July 22, 2021, NBC News article, Harvard T.H. Chan School of Public Health’s Sarah Fortune said that the vaccines act like screens to block most virus particles from invading the body. “These variants are more transmissible, so they’re better at getting through the screens,” said Fortune, John LaPorte Given Professor of Immunology and Infectious Diseases and chair of the Department of Immunology and Infectious Diseases. “The other factor is how much virus is out there trying to get in, and that’s determined by vaccination rates in your local community. It’s how much virus you’re being exposed to.”

Fortunately, breakthrough infections are rare, according to the experts quoted in the article. Most vaccinated people who become infected will either be asymptomatic or will experience only mild symptoms. But it’s still important to control outbreaks, said Fortune. That’s because the more that the virus is left to circulate, the more chances it has to mutate in a way that makes it more transmissible, able to cause more severe disease, or more able to evade the protection of vaccines.

“Every pathogen arms race ends badly, because this is fundamentally evolution,” she said. “What we’re talking about is the virus trying to not go extinct, and evolution is going to favor transmission. Evolution is going to favor vaccine escape.”

Read the NBC News article: What breakthrough infections mean for the Covid vaccines