Can a Vaccinated Person Still Spread the Coronavirus?

Ten vaccines have proved effective at protecting people from developing symptoms of Covid-19, the disease that can result from infection with SARS-CoV-2. Less is known about how well some of the various inoculations prevent people from getting an asymptomatic infection or passing the virus on to others. But preliminary evidence suggests that two vaccines, one from Moderna Inc. and another from Pfizer Inc. and BioNTech SE, are highly protective against the spread of the virus in addition to symptoms, and another from AstraZeneca Plc does a reasonably good job.

While getting vaccinated gives people considerable insurance against falling ill with Covid, which is sometimes fatal, if they can still get silently infected with SARS-CoV-2, they might pass it on, potentially sickening people who aren’t immune. Those who are infected but never develop symptoms are responsible for 24% of transmission, one study estimated. The more SARS-CoV-2 circulates, the more opportunity the virus has to mutate in ways that enhance its ability to spread, sicken and kill people, and evade the immunity provided by existing vaccines or a past infection. Already, variants of the virus have emerged that appear to be more dangerous. Also, using vaccination to achieve so-called herd immunity, when an entire community is protected though not everyone has been immunized, requires vaccines that prevent transmission.