A man receives a COVID-19 vaccine at a health services center in Rehovot, Israel, on January 14, 2021. Xinhua/Gil Cohen Magen via Getty Images

Now that effective coronavirus vaccines are authorized and being distributed, the crucial question is: Do they stop transmission?

In clinical trials, Pfizer and Moderna showed that their shots prevent severe COVID-19, but they didn’t test whether their vaccines prevent asymptomatic cases. Without curtailing these symptom-less infections, it’s difficult to stop the coronavirus’ transmission from person to person. But evidence is coalescing around the idea that people who get these vaccines don’t spread the virus after all.

“There have been some studies that are pointing into a very favorable direction,” Dr. Anthony Fauci, director of the National Institute for Allergy and Infectious Diseases, said in a briefing last week.

A preliminary study from Israel, for example, found that starting 12 days after vaccination, the people who got COVID-19 despite getting Pfizer’s shots had four times less virus in their bodies. Reduced viral loads are linked to lower transmission rates.

“We are confident vaccination against COVID-19 reduces the chances of transmitting the virus,” M. Kate Grabowski and Justin Lessler, two epidemiologists from Johns Hopkins, wrote in the Daily Beast last week, adding, “it may be that protection against transmission is appreciably less than protection against severe disease, but at this point it would be beyond shocking if no impact was there.”

Johnson & Johnson’s COVID-19 single-dose vaccine, though not authorized in the US yet, also seems effective in preventing asymptomatic infections, according to data released Wednesday from the Food and Drug Administration.

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Vaccinated people may be less contagious if they get infected

People on the London Underground on September 25, 2020. Aaron Chown/PA Images via Getty Images

Research shows the more viral particles a person has in their mouth and nose, the more likely they are to pass the coronavirus to others.

“In other words, higher viral load, good transmissibility; low viral load, very poor transmissibility,” Fauci said.

So a vaccine should reduce transmission if it can ensure that even those who still get the coronavirus after their shots – whether a symptomatic or asymptomatic case – have a lower viral load than they would have otherwise.

The recent Israeli study, which has yet to be peer-reviewed, suggests that is the case for the Pfizer’s vaccine. The researchers looked at more than 1,000 people who’d tested positive for the virus after being fully vaccinated in Tel Aviv. Those people’s viral loads in the period from 12 to 28 days after their second dose were four times lower than their viral loads in the first 11 days after their vaccinations.

“These reduced viral loads hint to lower infectiousness, further contributing to vaccine impact on virus spread,” the study authors wrote.

Another study from Israel, also not yet peer-reviewed, suggested the Pfizer vaccine reduced viral loads by a factor of up to 20.

Some research suggests viral loads are linked to disease severity, so a patient with a lower viral load is also less likely to have severe COVID-19. That may in part explain why Pfizer’s vaccine significantly reduces the chance of symptomatic infection.

Vaccinated people are less likely to develop asymptomatic infections

Dr. Jason Smith shows off his bandage after getting vaccinated at the University of Louisville Hospital in Kentucky. Jon Cherry/Getty Images

To pinpoint whether vaccines truly reduce spread, it’s critical to determine whether the shots prevent asymptomatic COVID-19 cases in addition to symptomatic infections.

Pfizer and Moderna’s clinical trials only tested volunteers for COVID-19 if they felt ill. Otherwise, the companies would have had to require regular COVID-19 testing for all tens of thousands of volunteers. So at first, neither company could say whether their vaccines prevent asymptomatic cases.

But Moderna did test trial volunteers on the day they got their second shots. And the findings suggested that there were fewer asymptomatic infections among participants who’d received the real vaccine than among those who got a placebo. Just 14 people of the 14,000-plus in the trial’s vaccine group had asymptomatic cases that day, compared to 38 of the similarly sized placebo group.

That’s a 61.5% drop, according to Marm Kilpatrick, a disease ecologist at the University of California, Santa Cruz. He wrote on Twitter that the data suggests Moderna’s vaccine blocks about 91% of transmission.

A vial of Moderna’s COVID-19 vaccine. Radek Mica /AFP via Getty Images

Animal studies offer similar findings: An October paper found that the Moderna vaccine prevented the coronavirus from replicating in the nose, throat, and lungs of rhesus macaques four weeks after they’d been vaccinated. If the viral particles can’t copy themselves, it’s unlikely an infected host will pass on particles to others.

When it comes to Pfizer’s vaccine, meanwhile, new research out of Israel (though not peer-reviewed), suggests the shot reduces asymptomatic cases by 89%, Reuters reported. Similarly, a preliminary study published in The Lancet found Pfizer’s vaccine to be at least 85% effective at preventing any type of infection – symptomatic or asymptomatic. The study looked at more than 23,000 healthcare workers across hospitals in the UK.

“We provide strong evidence that vaccinating working-age adults will substantially reduce asymptomatic and symptomatic SARS-CoV-2 infection and therefore reduce transmission of infection in the population,” the study authors wrote. (SARS-CoV-2 is the clinical name of the coronavirus.)

Johnson & Johnson’s clinical trial data on asymptomatic infections also seems promising. The company tested blood samples from almost 3,000 participants for a type of coronavirus antibody 71 days after they’d been vaccinated. (The presence of this antibody suggests participants had been infected even if they didn’t show symptoms.) Only two vaccinated people tested positive, whereas 16 people who’d received a placebo did.

That suggests J&J’s vaccine may be 74% effective against asymptomatic infections, though the FDA noted that more data is needed to be sure.

“There is uncertainty about the interpretation of these data and definitive conclusions cannot be drawn at this time,” the agency said.

Vials of the Pfizer-BioNTech COVID-19 vaccines at Renown Health in Reno, Nevada on December 17, 2020. Patrick Fallon/AFP via Getty Images

Even the Oxford-AstraZeneca vaccine, which is still in clinical trials in the US, may reduce asymptomatic infections.

An Oxford study, which has yet to be peer reviewed, found that among people who received just one dose, the number of positive COVID-19 tests – among both symptomatic and asymptomatic study participants – fell by 67%.

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