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People vaccinated against Covid-19 are less likely to spread the virus even if they are infected, according to a new study, adding to a growing body of evidence that vaccines can reduce transmission of the delta variant.
British scientists at the University of Oxford examined national records of nearly 150,000 contacts that were traced from around 100,000 original cases. The samples included people fully or partially vaccinated with Pfizer-BioNTech or AstraZeneca vaccines, as well as unvaccinated people. The researchers then looked at how vaccines affected the spread of the virus if a person had a breakthrough infection with the highly contagious alpha variant or delta variant.
Both vaccines reduced transmission, although they were more effective against the alpha variant compared to the delta variant. When infected with the delta variant, a given contact was 65% less likely to be positive if the person to whom the exposure had occurred was fully vaccinated with two doses of Pfizer vaccine. With AstraZeneca, a given contact was 36% less likely to be positive if the person to whom the exposure had occurred was fully vaccinated.
The risk of transmitting a breakthrough infection was much higher if someone had received a single dose of either vaccine.
The study was published online Thursday and has yet to be peer reviewed. But scientists not associated with the research said the results were credible.
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“This is the highest quality study we have to date on the issue of infectivity of vaccinated people infected with the delta,” said Dr. Aaron Richterman, infectious disease physician at the University. from Pennsylvania, who was not involved in the research.
Susan Butler-Wu, a clinical microbiologist at the University of Southern California, called the study “well done”, especially because it reflects real-world transmission as it followed the spread among contacts narrow.
Using cycle cutoff (Ct) values, researchers found a similar level of viral load in unvaccinated and vaccinated people infected with the delta variant, confirming previous studies. Even so, people who were fully vaccinated were still less likely to infect others.
“The transmission is much more complex,” said Butler-Wu, which means the Ct value is only a snapshot in time and does not reflect the entire course of the disease.
People who have been vaccinated are likely to clear the infectious virus from the body more quickly. A previous study from Singapore found that although virus levels were initially the same in people infected with the delta variant, regardless of vaccination status, by day 7 the virus levels declined rapidly in those who were vaccinated. , which may reduce the ability to spread the disease.
There is new evidence that even though cycle cutoffs may be the same regardless of vaccination status, vaccinated people may have less infectious virus in their bodies, potentially reducing transmission. Richterman pointed to a recent pre-print from China that found a large reduction in transmission in those who had received two doses of the vaccine, compared to those who had not been vaccinated.
Vaccines have the ability to prevent transmission of the virus in two ways, he said. The first is to completely prevent infection. The other is to reduce the amount of infectious virus if someone gets sick.
“People who have been vaccinated will have an immune system ready to coat the virus with antibodies much faster than unvaccinated people who must develop an immune response,” Richterman said.
Transmission protection decreases
The new study, however, found that protection against transmission appeared to wear off over time. After three months, people who had peak infections after being vaccinated with AstraZeneca were just as likely to spread the delta variant as those who were unvaccinated. Although protection against transmission decreased in people who had received the Pfizer vaccine, there was still an advantage over those who had not been vaccinated.
Since antibody levels also tend to decline over time after vaccination, a reduction in protection against transmission is to be expected, Richterman said.
“We know that the amount of antibodies circulating in the blood decreases over time after vaccination, even though immune memory remains durably strong and is still able to prevent infections, especially serious infections,” he said. . “These circulating antibodies that are immediately available probably play a role in preventing transmission in the event of infection, so I don’t think it’s surprising to see reduced protection against transmission over time.”
Richterman and Butler-Wu agreed that while community transmission remains high, masks and testing remain important.
“We need to combine our vaccines with other measures to reduce the amount of virus we are exposed to through things like masking and testing,” Butler-Wu said. “Additive measures are the name of the game here.”
Richterman agreed.
“While there may be a sufficiently high level of immunization coverage where other interventions like masking and distancing are not needed, we are certainly not there yet in many parts of the United States.” , did he declare.
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