A handful of recent studies suggest that the coronavirus vaccines developed by Pfizer/BioNTech and Moderna not only prevent people who are exposed to the virus from contracting Covid-19, but also prevent them from spreading the virus to others—and some experts say public health messaging should start reflecting that benefit.

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A crucial question: How well do the vaccines prevent asymptomatic infections?

When federal health authorities authorized the first Covid-19 vaccines late last year, they acted based on evidence proving that vaccinated people were far less likely than unvaccinated people to develop Covid-19 symptoms.

But as public health officials emphasized at the time, there wasn't enough evidence to determine whether the vaccines also prevented people from developing asymptomatic coronavirus infections. That matters because, at least in theory, a vaccinated person with an asymptomatic infection could still transmit the coronavirus to others.

To gauge how likely post-vaccination transmission is in the real world, researchers are conducting mass coronavirus screenings of vaccinated people, hoping to detect any asymptomatic cases.

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For instance, according to Vox, a working paper—not yet peer-reviewed—released Friday in The Lancet assessed thousands of Covid-19 screenings among health care workers at a hospital in Cambridge, England, including both unvaccinated staff and staff who had received the Pfizer/BioNTech vaccine.

The researchers found that, among the unvaccinated staff, 0.80% tested positive for the coronavirus. In comparison, among staff who had received the vaccine, only 0.37% tested positive less than 12 days after their vaccination, and just 0.20% tested positive more than 12 days after vaccination.

According to Mike Weekes, an infectious disease specialist at Cambridge University and co-leader of the study, the results suggest the risk of developing asymptomatic Covid-19 is four times less among health care workers who have been vaccinated for at least 12 days.

Meanwhile, a press released on a pre-published, not-yet-peer-reviewed paper from the Israeli Health Ministry and Pfizer found that the vaccine appeared to reduce all coronavirus infections—including asymptomatic infections—by 89.4% and symptomatic infections by 93.7%.

While both studies focused specifically on Pfizer's vaccine, experts told Vox's Kelsey Piper that Moderna's vaccine would likely produce similar results, since both vaccines work similarly.

For its part, Moderna found in its supplemental research submitted to FDA—based on nasal swab test data—that only 14 of the 14,134 people given its vaccine had an asymptomatic case of Covid-19, compared with 38 of the 14,073 people in the control group.

Marc Lipsitch and Rebecca Kahn, both of whom are epidemiologists at the Harvard School of Public Health, in a preprint study estimated from the same data that "one dose of vaccine reduces the potential for transmission by at least 61%, possibly considerably more."

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How well the vaccines reduce viral load

But according to Vox, reduced infection rates are only part of the equation when it comes to curbing transmission. Another critical factor is just how infectious a vaccinated person is if they develop asymptomatic Covid-19. And the research here, although also preliminary, is good: Evidence suggests that Pfizer/BioNTech's vaccine is effective at reducing the viral load in people who do catch the coronavirus.

According to a preprint study in Israel, in the first 12 days after receiving Pfizer/BioNTech's vaccine, both vaccinated and unvaccinated people who tested positive for the coronavirus had the same average viral loads. But after 12 days, vaccinated people saw their average viral loads decrease, suggesting the vaccine is driving the reduction.

"We find that the viral load is reduced fourfold for infections occurring 12-28 days after the first dose of vaccine," the study concluded. "These reduced viral loads hint to lower infectiousness, further contributing to vaccine impact on virus spread."

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Why experts are optimistic—but still want more research

Even though experts say more evidence is needed, an increasing number of scientists feel confident the vaccines reduce viral transmission, Vox reports.

For instance, in an opinion piece for The Daily Beast, M. Kate Grabowski and Justin Lessler, both epidemiologists at Johns Hopkins, said they are "confident vaccination against Covid-19 reduces the chance of transmitting the virus."

Lipsitch agreed with them, saying in a tweet, "I have been very cautious due to limited evidence on transmission effects but agree with (Grabowski and Lessler) that a large transmission effect is the best explanation of the limited evidence to date."

In fact, Lipsitch argued it would be "beyond shocking" if the vaccines had no effect on coronavirus transmission, saying that the evidence suggests a transmission reduction of at least 50%.

And Virginia Pitzer, an infectious disease modeler at the Yale School of Public Health, said in Nature that the data on Covid-19 vaccines so far "is certainly intriguing and suggestive that vaccination may reduce the infectiousness of Covid-19 cases, even if it does not prevent infection altogether."

On the whole, there's an emerging consensus that "the data indicate[s] a reduction in total infections, as well as symptomatic infections," Kilpatrick said. "People disagree on whether we can accurately estimate how (large is) the reduction in total infections and infectiousness" (Piper, "Future Perfect," Vox, 3/3; Kelland, Reuters, 2/26).