Can people vaccinated against COVID-19 still spread the coronavirus?

When the U.S. Centers for Disease Control and Prevention changed its guidelines about mask-wearing on May 13, 2021, plenty of Americans were left a little confused. Now anyone who is fully vaccinated can participate in indoor and outdoor activities, large or small, without wearing a mask or physical distancing.

But some people cannot be vaccinated because of underlying conditions. Others with weakened immune systems, from cancer or medical treatments, may not be fully protected by their vaccinations. Children aged 12 to 15 became eligible for the Pfizer-BioNTech vaccine only on May 10, 2021. And no COVID-19 vaccines are yet authorized for the nearly 50 million children in the U.S. younger than 12.

Vaccines don't always prevent infection. Researchers had hoped to design safe COVID-19 vaccines that would prevent at least half of the people vaccinated from getting COVID-19 symptoms.

Fortunately, the vaccines have vastly outperformed expectations. For example, in 6.5 million residents of Israel, aged 16 years and older, the Pfizer-BioNTech mRNA COVID-19 vaccine was found to be 95.3% effective after both shots. Within two months, among the 4.7 million fully vaccinated, the detectable infections fell by 30-fold. Similarly in California and Texas, only 0.05% of fully vaccinated health care workers tested positive for COVID-19.

Immunologists expect vaccines that protect against viral illnesses to also reduce transmission of the virus after vaccination. But it's actually tricky to figure out for sure if vaccinated people are not spreading the germ.


COVID-19 poses a particular challenge because people with asymptomatic and pre-symptomatic infections can spread the disease - and insufficient contact tracing and testing mean those without symptoms are rarely detected.