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, which has become common in some circles, that vaccines no longer do much
to prevent the spread of the coronavirus.
“They absolutely do reduce transmission,” says
Christopher Byron Brooke at the University of Illinois at Urbana-Champaign. “Vaccinated people do transmit the virus in some cases, but the data are super crystal-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 per cent less likely to infect people who are unvaccinated.
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.
De Gier and her team used data from the Netherlands’ contact tracing system to work out the so-called secondary attack rate – the proportion of contacts infected by positive cases. They then worked out how much this was reduced by vaccination, adjusting for factors such as age.
De Gier says they cannot calculate the full reduction in transmission due to vaccination, because they don’t know exactly how much vaccination reduces the risk of infection. But even assuming vaccination only halves the risk of infection, this would still imply that vaccines reduce transmission by more than 80 per cent overall.
Others have worked out the full effect. Earlier this year, Ottavia Prunas at Yale University applied two different models to data from Israel, where the Pfizer 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.
Vaccination still greatly reduces the risk of infecting others despite the arrival of the delta variant, recent studies show
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