People fully vaccinated against covid-19 are much less likely to infect others, despite the arrival of the delta variant, according to several studies. The results refute the idea, which has become common in some circles, that vaccines do little to prevent the spread of the coronavirus.
“They absolutely reduce transmission,” says Christopher Byron Brooke of the University of Illinois at Urbana-Champaign. “People who have been vaccinated transmit the virus in some cases, but the data is very clear that the risk of transmission for a vaccinated individual is much, much lower than for an unvaccinated individual. “
A recent study found that vaccinated people infected with the delta variant are 63% less likely to infect unvaccinated people.
This is only slightly lower than that of the alpha variant, says Brechje de Gier of the Center for Infectious Disease Epidemiology and Surveillance in the Netherlands, who led the study. His team had previously found that vaccinated people infected with alpha were 73% less likely to infect unvaccinated people.
What’s important to understand, says de Gier, is that the full effect of vaccines on reducing transmission is still greater than 63%, because most people vaccinated do not get infected in the first place.
De Gier and his team used data from the Dutch contact tracing system to determine the so-called secondary attack rate – the proportion of contacts infected with positive cases. They then determined to what extent this was reduced by the vaccination, adjusting for factors such as age.
De Gier says they can’t calculate the complete reduction in transmission due to vaccination because they don’t know exactly how much vaccination reduces the risk of infection. But even assuming that vaccination only halves the risk of infection, that would still imply that vaccines reduce transmission by more than 80% overall.
Others have developed the full effect. Earlier this year, Ottavia Prunas of Yale University applied two different models to data from Israel, where the Pfizer vaccine was used. His team’s conclusion was that the vaccine’s overall effectiveness against transmission was 89%.
However, the data used only dates back to March 24, before the delta became dominant. The team is now using more recent data to determine the impact of the delta, explains Prunas.
The idea that vaccines are no longer as effective against transmission may come from reports in July claiming that vaccinated people who are infected “can carry as many viruses as others.” Even if this were true, vaccines would still significantly reduce transmission by reducing infections in the first place.
In fact, the study that triggered the reports didn’t directly measure someone’s virus count, but relied on so-called Ct scores, a measure of viral RNA. However, this RNA can come from viruses destroyed by the immune system. “You can measure RNA, but it’s made useless,” explains Timothy Peto at the University of Oxford.
There are now several sources of evidence that Ct scores are not a good measure of how much virus a person has. First, the fact that infected vaccinated people are much less likely to infect others. Peto performed a similar study to de Gier’s using contact tracing data from England and found similar results.
Second, Peto’s team has specifically shown that there is little association between Ct scores and infectivity. “It turned out that people who were positive after vaccination had the same viral load as those who were not vaccinated. We thought they were just as contagious. But it turns out you’re less contagious, ”says Peto. “It’s pretty important. People were too pessimistic.
Another evidence comes from a study by Brooke. His team took samples from 23 people every day after their first positive test until the infection cleared and performed tests, including trying to infect cells in a box with the samples.
With five of the six people fully vaccinated, none of the samples were infectious, unlike most unvaccinated people. The study shows that vaccinated people shed less virus and also stop shedding earlier than unvaccinated people, says Brooke.
The only bad news is that Peto’s study shows that the protection offered by a vaccine against an infected person who infects others declines over time, by about a quarter in the three months after a second dose of the vaccine. . “It made me believe in boosters,” he says. “They should continue, given that we are in the middle of a major epidemic [in the UK]. »
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