RE: Stay safe everyone3 Nov 2021 19:07
Hi Boil, you might find this New Scientist article intersting:
People who are fully vaccinated against covid-19 are far less likely to infect others, despite the arrival of the delta variant, several studies show. The findings refute the idea that vaccines no longer do much to prevent the spread of the coronavirus.
A recent study found that vaccinated people infected with the delta variant are 63 per cent less likely to infect people who are unvaccinated (medRxiv, doi.org/g3p3). This is only slightly lower than with the alpha variant, says Brechje de Gier at the National Institute for Public Health and the Environment in the Netherlands, who led the study. Her team had previously found that vaccinated people infected with alpha were 73 per cent less likely to infect unvaccinated people.
What is important to realise, de Gier says, is that the full effect of vaccines on reducing transmission is even higher than 63 per cent, because most vaccinated people don’t become infected in the first place.
Others have looked into this full effect. Earlier this year, Ottavia Prunas at Yale University applied two different models to data from Israel, where the Pfizer/BioNtech vaccine was used. Her team’s conclusion was that the overall vaccine effectiveness against transmission was 89 per cent.
However, the data used only went up to 24 March, before delta became dominant. The team is now using more recent data to work out the impact of delta, says Prunas.
The idea that vaccines are no longer that effective against transmission has circulated in some groups and may derive from news reports in July claiming that vaccinated people who become infected “can carry as much virus as others”. But even if this were true, vaccines would still greatly cut transmission by reducing infections in the first place.
In fact, the study that sparked the news reports didn’t measure the number of viruses in someone directly but relied on so-called Ct scores, a measure of viral RNA. However, this RNA can derive from viruses that have been destroyed by the immune system. “You can measure the RNA, but it’s rendered useless,” says Timothy Peto at the University of Oxford.
Yet another line of evidence comes from a study by Brooke. His team took samples from 23 people every day after they first tested positive until the infection cleared and performed tests, including trying to infect cells in a dish with the samples.
With five out of the six fully vaccinated people in the study, none of the samples were infectious, unlike most from unvaccinated people. The study shows that vaccinated people shed fewer viruses and also stop shedding sooner than those who are unvaccinated, says Brooke.
The one bit of bad news is that Peto’s study shows that the protection a vaccine provides against an infected person infecting others does wane over time, by around a quarter over the three months after a second vaccine dose. “This has made me a believer in boosters,” says Peto.
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