People who have been vaccinated against COVID-19 are less likely to spread Sars-Cov-2 if they have a breakthrough infection (becoming infected with COVID-19 despite C19 vaccination) than unvaccinated people, a new study finds, adding to the growing evidence that vaccines are reduce the transmission of the delta variant.
British scientists from the University of Oxford examined national data from nearly 150,000 contacts traced from roughly 100,000 initial COVID-19 cases. The samples included people who had been fully or partially vaccinated with the vaccines from Pfizer-BioNTech or AstraZeneca, as well as people who had not been vaccinated. The researchers then looked at whether the vaccines affected the spread of the virus if a person had a breakthrough infection with either the alpha variant or the highly contagious delta variant.
Both vaccines reduced the transmission of Sars-Cov-2, although they were more effective against the alpha variant than against the delta variant. When infected with the delta variant, the chance of a positive test after contact with a COVID-19 patient was 65 percent lower if this person was fully vaccinated with two doses of the Pfizer vaccine. The chance of a positive test on contact with a COVID-19 patient was 36 percent smaller if this person was fully vaccinated with Astra-Zeneca.
The risk of transmission of Sars-Cov-2 after a breakthrough infection was much higher if someone had received only one dose of either vaccine.
The study has been posted online and has not yet been subject to peer reviews. But scientists not involved in the study believe the conclusions are credible.
“It is the best quality study we have to date on the infectivity of vaccinated people after a breakthrough infection with delta,” said Dr. Aaron Richterman, an infectious diseases physician at the University of Pennsylvania, who was not involved in the study.
Susan Butler-Wu, a clinical microbiologist at the University of Southern California, called the study “well executed”, mainly because it has mapped the transmission in practice, as it has monitored the spread among close contacts.
Using the Ct (cycle threshold) values, the researchers found a similar level of viral load in unvaccinated and vaccinated people infected with the delta variant, as confirmed by previous studies. Yet fully vaccinated people are less likely to infect others.
“Transmission is much more complex,” said Butler-Wu, meaning the Ct value is just a snapshot and does not reflect the full course of the disease.
It is likely that people who have been vaccinated ‘clear’ the Sars-Cov-2 virus from the body more quickly. An earlier study from Singapore found that while levels of ‘viral load’ were initially similar in those infected with the delta variant regardless of vaccination status, levels of the virus declined rapidly by day seven in those vaccinated. , potentially decreasing the ability to spread disease.
There is growing evidence that even though cycle thresholds are the same regardless of vaccination status, people who have been vaccinated may have less contagious virus in their bodies, potentially limiting transmission. Richterman pointed to a recent pre-publication from China that found a large reduction in transmission in those who received two doses of C19 vaccine, compared to the unvaccinated.
Vaccines have the ability to prevent transmission of the virus in two ways, he said. The first is completely preventable by infection. The other is by reducing the amount of contagious virus if someone does get sick.
“People who are vaccinated have immune systems that can make antibodies to the virus much faster than unvaccinated people who have yet to mount an immune response,” said Richterman.
Protection against transmission is declining
However, the new study found that protection against transmission appears to wane over time. After three months, people who developed a breakthrough infection after vaccination with AstraZeneca were just as likely to spread the delta variant as the unvaccinated. Although protection against transmission decreased in people who had received the Pfizer vaccine, there was still a benefit compared to people 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, although the immune memory remains powerful and long-acting and able to prevent infections, especially severe infections,” he said. “These circulating antibodies that are readily available probably play a role in preventing transmission in a breakthrough infection, so I don’t think it’s surprising to see some diminished protection against transmission over time.”
Richterman and Butler-Wu agreed that if transmission levels remain high in the community, masks and testing will 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,” Zei Butler-Wu. “Taking additive measures is very important in this case.” Richterman agreed.
“While a high enough vaccination rate can be achieved to make other measures such as wearing masks and keeping distance from each other superfluous, we are far from that in many parts of the US,” he said.
Vaccinated people are less likely to spread Covid, new research finds https://t.co/KYB05pQx6p via @nbcnews
— Kimmyann (@kimmyann1111) October 20, 2021
