Does the Moderna Vaccine Prevent Transmission?

It’s likely, but still unknown

Credit: SOPA Images / Contributor / Getty Images

An open question about the Covid-19 vaccines available — and soon to be available — is whether they prevent a vaccinated person from getting infected with the virus and spreading it to another person, even if they don’t have symptoms. Experts say the vaccines likely will reduce transmission, but it’s not confirmed yet.

Similar to Pfizer-BioNTech’s Covid-19 vaccine, the Moderna vaccine data presented to the U.S. Food and Drug Administration (FDA) expert advisory panel on Thursday revealed a vaccine with high effectiveness at protecting against mild, moderate, and severe Covid-19 disease. This means the vaccine appears to prevent symptomatic disease very well. But does it prevent transmission of the virus? Could someone who has been vaccinated still contract the virus and spread it to another person even if they don’t have symptoms?

While Moderna is not able to definitively answer the infection question yet, the company provided some data to suggest that the vaccine might prevent asymptomatic cases and transmission. Moderna gave Covid-19 tests to all trial participants between their first and second dose of either the company’s vaccine or a placebo. Among people who tested positive for Covid-19 without symptoms, there were 14 people in the vaccine group and 38 in the placebo group (there were over 14,000 people in each group). These are small numbers, but they suggest that the Moderna vaccine might prevent asymptomatic infections. Overall, the number of people to test positive for Covid-19 was higher in the placebo group than the vaccine group, especially after the second dose.

Knowing whether the vaccine prevents transmission is important. If it doesn’t, then people will likely need to continue prevention measures like masks and distancing around unvaccinated people. The good news is that Moderna and the other companies with vaccines are studying this, and will hopefully have more insights on the transmission question soon. Again, experts predict the vaccines “will likely reduce transmission, but we don’t yet know, so use caution for now.”



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Alexandra Sifferlin

Alexandra Sifferlin


Health and science journalist. Former editor of Medium’s Covid-19 Blog and deputy editor at Elemental. TIME Magazine writer before that