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COVID-19 | School of Public Health Expert Insights


School of Public Health Expert Insights

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New Data on COVID-19 Transmission by Vaccinated Individuals

A Q&A with Amesh Adalja | APRIL 8, 2021

In a recent White House press briefing, CDC director Rochelle Walensky cited new data indicating that the two-dose regimen of COVID-19 vaccines can reduce the risk of asymptomatic or presymptomatic infections.

What does this mean, exactly? And what might data like this suggest for public health guidance going forward? Amesh Adalja, MD, of the Center for Health Security, answers a few questions about our evolving understanding of immunity and COVID vaccines.

Can we say with any degree of certainty that vaccinated people are unlikely to spread COVID to unvaccinated individuals?

The emerging data confirms what many of us thought would be the case—that not only do the vaccines stop symptomatic COVID, but they also make it highly unlikely that someone can even be infected at all.

I think the preponderance of the evidence supports the fact that vaccinated individuals are not able to spread the virus.

What do we know about one-dose regimens, either single-dose vaccines or a delayed second dose that some nations have implemented to increase the availability of vaccines?

There is significant immunity that is engendered two weeks after a single dose of the two-dose vaccines, and though the data is more robust for the approved two-dose schedules, countries that prioritized first doses took a reasonable approach.

What is known about the variants of concern and their ability to infect fully vaccinated individuals?

When it comes to variants, it is likely the case that it depends on the variant and depends on the vaccine.

The B.1.1.7 variant (first identified in the U.K.) is one that vaccines appear robust against. The more concerning B.1.351 variant (first identified in South Africa) appears to pose problems for the J&J vaccine but not where it counts in terms of presenting serious disease, hospitalization, and death. Data on the P.1 variant (first identified in Brazil) is forthcoming, but I suspect it will be similar to the B1.351 variant.

Current guidance states that even fully vaccinated individuals should continue to wear masks and social distance. What’s the logic behind that?

Operationally, it is very challenging to know who is vaccinated and who is not, so the guidance in public places likely will be slow to change until more people are vaccinated. You can’t expect a cashier to ask for proof of vaccination.

Amesh Adalja, MD, is a senior scholar at the Center for Health Security and an adjunct professor in Environmental Health and Engineering. He is also an affiliate of the Johns Hopkins Center for Global Health.

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