Now that effective coronavirus vaccines are being authorized and distributed, the important question is: Does it stop transmission?
In clinical trials, Pfizer and Moderna have shown that their shots prevent severe COVID-19, but they have not tested whether their vaccines prevent asymptomatic cases. Without limiting these asymptomatic infections, it is difficult to stop the transmission of the coronavirus from person to person. But evidence is coming together about the idea that people who get these vaccines do not spread the virus.
“There have been some studies that point in a very favorable direction,” said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in an information session last week.
A preliminary study from Israel, for example, found that people who received COVID-19 had four times as much virus in their bodies as early as twelve days after vaccination, despite receiving Pfizer shots. Reduced viral loads are linked to lower transmission rates.
“We are confident that vaccination against COVID-19 will reduce the chance of transmitting the virus,” M. Kate Grabowski and Justin Lessler, two Johns Hopkins epidemiologists, wrote in the Daily Beast last week, saying “it can the protection against transmission is considerably less than protection against serious diseases, but at this stage it would be shocking if there was no impact. ‘
Johnson & Johnson’s COVID-19 single-dose vaccine, although not yet authorized in the US, also appears to be effective in preventing asymptomatic infections, according to data released by the Food and Drug Administration on Wednesday.
Vaccines can be less contagious if they become infected
Research shows that the more viral particles a person has in their mouth and nose, the more likely they are to transmit the coronavirus to others.
“In other words, higher virus load, good transmissibility; low virus load, very poor transmittance,” Fauci said.
Thus, 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 virus load than they would otherwise have.
The recent Israeli study, which has yet to be reviewed by peers, suggests that this is the case with the Pfizer vaccine. The researchers looked at more than 1,000 people who tested positive for the virus after being fully vaccinated in Tel Aviv. Those virus load in the period from 12 to 28 days after their second dose was four times lower than their virus load in the first 11 days after their vaccination.
“This reduced virus load indicates lower infectivity, which further contributes to the impact of the vaccine on the spread of viruses,” the study authors wrote.
Another study from Israel, which has not yet been peer-reviewed, suggested that the Pfizer vaccine reduces viral loads by a factor of up to 20.
Some research suggests that viral loads are linked to the severity of diseases, so that a patient with a lower viral load is also less likely to have severe COVID-19. This may partly explain why Pfizer’s vaccine significantly reduces the chance of symptomatic infection.
Vaccinations are less likely to develop asymptomatic infections
To determine if vaccines actually reduce the spread, it is important to determine if the shots occur asymptomatic COVID-19 cases other than symptomatic infections.
Pfizer and Moderna’s clinical trials tested volunteers on COVID-19 only when they felt ill. Otherwise, the companies would have to require regular COVID-19 tests for all tens of thousands of volunteers. Initially, no company could say whether their vaccines prevented asymptomatic cases.
But Moderna did do trial volunteers on the day they got their second shots. And the findings suggest that there were fewer asymptomatic infections among participants who received the right vaccine than among those who received a placebo. Only 14 people of the 14,000-plus in the vaccine group of the trial had asymptomatic cases that day, compared with 38 of the similar placebo group.
According to this, it is a decrease of 61.5% Marm Kilpatrick, a disease ecologist at the University of California, Santa Cruz. He posted on Twitter that the data indicate that Moderna’s vaccine blocks approximately 91% of transmission.
Animal studies offer similar findings: An October article found that the Moderna vaccine prevented the coronavirus from recurring in the nose, throat and lungs of rhesus jaws four weeks after being vaccinated. If the viral particles cannot copy themselves, it is unlikely that an infected host will transmit particles to others.
As for the Pfizer vaccination, new research from Israel (although not judged by a peer) indicates that the shot reduces asymptomatic cases by 89%, Reuters reports. Similarly, a preliminary study published in The Lancet found that Pfizer’s vaccine is at least 85% effective in preventing any form of infection – symptomatic or asymptomatic. The study looked at more than 23,000 healthcare workers in hospitals in the UK.
“We provide strong evidence that vaccinating working-age adults will significantly reduce the asymptomatic and symptomatic SARS-CoV-2 infection and reduce the 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 look promising. The company tested blood samples from nearly 3,000 participants for a type of coronavirus antibody 71 days after it was vaccinated. (The presence of this antibody indicates that participants were infected, even though they did not show symptoms.) Only two vaccines tested positive, while 16 people who received a placebo did test.
This suggests that J & J’s vaccine may be 74% effective against asymptomatic infections, although the FDA has noted that more information is needed to be sure.
“There is uncertainty about the interpretation of this 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.
A Oxford study, which has yet to be peer-reviewed, found that the number of positive COVID-19 tests – among both symptomatic and asymptomatic participants in the study – dropped by 67% among people who received only one dose .
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