Vaccinated people less like to spread Delta variant

“It’s the highest quality study we have so far on the question of infectiousness of vaccinated people infected with delta,”

Dr. Aaron Richterman, infectious disease physician at the University of Pennsylvania

(CMR) British scientists at the University of Oxford now have data demonstrating that people vaccinated against COVID-19 are less likely to spread the virus even if they become infected. They examined national records of 150,000 contacts that were traced from 100,000 initial cases and the samples included both fully and partially vaccinated persons who took either the AstraZeneca or Pfizer-BioNTech vaccines.

The researchers were able to look at how the vaccines affected the spread of the virus if a vaccinated person has a breakthrough infection, particularly with the highly contagious delta variant. The news was carried by NBC News recently.

Research showed that both vaccines reduced transmission and were effective against both the delta and alpha variants. A contact was 65 percent less likely to test positive if the person was fully vaccinated with two doses of the Pfizer vaccine. With AstraZeneca the reduction in transmission rate was reduced to 36 percent with fully vaccinated persons.

The risk of transmission from a breakthrough infection was much higher if someone had received just a single dose of either vaccine. The study was published on Thursday but has not yet been peer-reviewed. Despite that many not connected to the study said that it was well performed and of the highest quality. The study used cycle threshold (Ct) values where the researchers found a similar level of viral load in unvaccinated and vaccinated people. Despite that,  those people who were fully vaccinated were still less likely to infect others.

“Transmission is much more complex,” Butler-Wu said, meaning the Ct value is just a snapshot in time and doesn’t reflect the entire course of illness. There are several theories including the fact that those who are vaccinated clear the infectious virus from their body faster.

Another study out of Singapore found that although levels of the virus were initially the same in those infected with the delta variant regardless of vaccine status by day seven levels in vaccinated people dropped quickly and likely reduced their ability to spread the illness.

There is emerging evidence that even though cycle threshold values may be the same regardless of vaccination status, people who are vaccinated may have less infectious virus in their bodies, potentially reducing transmission.

Over time the protection against transmission did drop off. Pfizer appears to offer more protection longer than AstraZeneca the research has shown. This is why experts continue to say that masks and testing remain important tools against fighting community spread against the virus.

“People who have been vaccinated will have immune systems at the ready that can coat the virus in antibodies much more quickly than unvaccinated people who have to build up an immune response,”