From Washington’s Capitol Hill to Tokyo’s Olympic Village, positive COVID-19 tests among some people who’ve been fully vaccinated are raising concerns about so-called breakthrough infections. That in turn has people questioning whether certain vaccines are more effective against newer variants of the coronavirus, or whether the shots’ efficacy could be wearing off with time.

It’s impossible to say how many breakthrough illnesses are happening in the U.S. because of incomplete data and inconsistent tracking. But according to the Centers for Disease Control and Prevention, the rate of severe breakthroughs is exceedingly low: As of July 19, the CDC had received reports of 5,914 COVID-19-related hospitalizations and deaths out of more than 161 million fully vaccinated people in the United