Vaccines Reduce Coronavirus Transmission, Preliminary Research Shows

Now that effective coronavirus vaccines are licensed and distributed, the crucial question is: do they stop transmission?

In clinical trials, Pfizer and Moderna showed that their vaccines prevent severe COVID-19, but they did not test whether their vaccines prevent asymptomatic cases. Without reducing these infections without symptoms, it is difficult to stop person-to-person transmission of the coronavirus. But the evidence is coalescing around the idea that people who get these vaccines don’t transmit the virus after all.

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

A preliminary study from Israel, for example, found that starting 12 days after vaccination, people who received COVID-19 despite receiving Pfizer injections had four times less virus in their bodies. The reduced viral loads are linked to lower transmission rates.

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

Johnson & Johnson’s single-dose COVID-19 vaccine, while not yet licensed in the United States, also appears effective in preventing asymptomatic infections, according to data released Wednesday by the Food and Drug Administration (FDA).

Vaccinated people may be less contagious if they become infected

Research shows that the more virus particles a person has in their mouth and nose, the more likely they are to transmit the coronavirus to other people.

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

Therefore, a vaccine should reduce transmission if it can guarantee that even those who still contract the coronavirus after their injections, whether symptomatic or asymptomatic, have a lower viral load than they would otherwise.

The recent Israeli study, which has not yet been peer-reviewed, suggests that is the case for Pfizer’s vaccine. Researchers looked at more than 1,000 people who tested positive for the virus after being fully vaccinated in Tel Aviv. Those people’s viral loads in the 12 to 28 day period after their second dose were four times lower than their viral loads in the first 11 days after their vaccinations.

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

Another study from Israel, which has not yet been peer-reviewed, suggested that the Pfizer vaccine reduced viral load by a factor of up to 20.

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

Vaccinated people are less likely to develop asymptomatic infections

To determine whether vaccines actually reduce the spread, it is critical to determine whether injections prevent asymptomatic COVID-19 cases in addition to symptomatic infections.

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

But Moderna tested the volunteers the day they received their second injections. And the findings suggested that there were fewer asymptomatic infections among participants who had received the actual vaccine than among those who received a placebo. Only 14 people out of more than 14,000 in the trial’s vaccine group had asymptomatic cases that day, compared with 38 in the similarly sized placebo group.

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

Animal studies offer similar findings: An October article found that the Moderna vaccine prevented the coronavirus from replicating in the nose, throat and lungs of rhesus macaques four weeks after they were vaccinated. If the viral particles cannot copy themselves, it is unlikely that an infected host will transmit particles to others.

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

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

Johnson & Johnson clinical trial data on asymptomatic infections also looks promising. The company analyzed blood samples from nearly 3,000 participants for a type of antibody to coronavirus 71 days after they were vaccinated. (The presence of this antibody suggests that the participants had been infected even if they showed no symptoms.) Only two vaccinated people tested positive, while 16 people who had received a placebo did.

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

“There is uncertainty about the interpretation of these data and no definitive conclusions can be drawn at this time,” the agency said.

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

An Oxford study, which has not yet been peer-reviewed, found that among people who received just one dose, the number of positive tests for COVID-19, among both symptomatic and asymptomatic study participants, was reduced by 67 %.