Can Johnson & Johnson’s Vaccine Prevent the Spread of the Virus?

An early analysis shows that it helps prevent asymptomatic cases

Photo: iStock / Getty Images Plus

We know that Moderna, Pfizer, and now Johnson & Johnson’s Covid-19 vaccines are highly effective against preventing severe disease and hospitalizations. That’s a major win for public health and our strained health care system. But one of the biggest lingering questions about these vaccines is whether they also prevent people from spreading the virus to others.

You might be surprised to know that vaccines can prevent people from getting sick but may not actually stop them from getting infected. This is because some viral particles may still slip past our immune defenses.

It’s likely that Covid-19 vaccines will reduce the risk of transmitting the virus to others but not completely — and not in everyone. Right now, the spotlight is on Johnson & Johnson’s vaccine as the Food and Drug Administration considers whether to authorize it for emergency use. Here’s what we know about how well Johnson & Johnson’s vaccine prevents infection.

According to an FDA briefing document released this week, the company’s vaccine reduced asymptomatic infections by 74%. That number, however, is based on fewer than 3,000 people out of around 40,000 who took part in Johnson & Johnson’s vaccine trial. Researchers identified asymptomatic infections in its trials by testing participants for coronavirus antibodies 71 days after they received a vaccine or placebo.

“These findings are preliminary, and while further follow-up is needed to assess whether or not they are confirmed in the larger data set, they do suggest a protective effect of the vaccine on asymptomatic SARS-CoV-2 infection,” said Macaya Douoguih, MD, head of clinical development for the vaccines group at Janssen, Johnson & Johnson’s pharmaceutical arm, during an FDA advisory committee meeting on February 26.

In the briefing document, the FDA cautioned, “There is uncertainty about the interpretation of these data and definitive conclusions cannot be drawn at this time.”

Whether other vaccines protect against infection remains to be seen. In preliminary findings from the U.K., Pfizer’s Covid-19 vaccine reduced asymptomatic infections among health care workers by 75% after one dose. The study, which is based on nearly 9,000 participants, hasn’t yet been peer-reviewed. A leaked report from Israel also suggests that Pfizer’s vaccine is sharply curtailing transmission. Moderna is also conducting studies now to determine how well its vaccine prevents infection.

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