People who have been vaccinated against covid-19 can still catch and transmit the virus, but are significantly less likely to do so than unvaccinated people, the latest data suggests.
The question of whether vaccines halt transmission is one of the biggest and most important unknowns of the pandemic. If they do, vaccine-induced herd immunity may be possible. If not, the virus will still be able to circulate even in a fully vaccinated population and continue to pose a deadly threat to people who cannot be vaccinated or do not mount an immune response after receiving a vaccine. Circulating virus could also mutate and escape our defences, reigniting the pandemic.
The latest news is mixed. 鈥淭here have been several bits of data just in the last couple of week that suggest that vaccines do not block transmission but are very likely to significantly reduce transmission,鈥� immunologist Eleanor Riley at the University of Edinburgh said on a Royal Society of Medicine webinar on 28 January.
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One bit of data is from a carried out by vaccine manufacturer Moderna. It found that people given the vaccine were a third as likely to test positive for the virus when they returned for their second jab, compared with people who got the placebo. In other words, the first shot cuts infection rates and hence transmission by about 66 per cent. The second jab is given 28 days after the first; its effect on transmission is still unknown, because trials generally monitored people after their second shot only if they felt ill, not whether they were asymptomatically infected.
AstraZeneca has from one of its clinical trials. Volunteers who received the half dose/full dose regime 鈥� which the company discovered by accident 鈥� were 60 per cent less likely to be asymptomatically infected than people who got the placebo. However, there was a much smaller difference in people given the planned full doses, just 4 per cent.
A found that infected people without symptoms are much less likely than those with symptoms to transmit the virus to others in their household, which suggests that even if vaccines do not prevent asymptomatic infections they can still significantly cut the transmission rate.
Finally, a by the Clalit Research Institute in Ramat Gan, Israel, found that the Pfizer/BioNTech vaccine reduced asymptomatic infections in people over 60 by about a third. The effect did not kick in until 14 days after the first dose. Pfizer and BioNTech have not released any data on transmissibility but their on the phase 3 trials says they plan to do so.
Taken together these data strongly suggest that vaccines can significantly but not completely halt the spread of the virus, Riley says.
鈥淭here are no data per se of infectiousness of vaccine recipients 鈥� the link is indirect,鈥� she told 快猫短视频. 鈥淏oth symptomatic and asymptomatic infection are less likely after vaccination and asymptomatic people less likely to transmit.鈥�
This means that we will have to continue measures such as social distancing to retard transmission until everybody has been vaccinated and boosted, she said. The Clalit Research Institute warned that its results 鈥渃learly indicate that there is no complete protection against corona infection鈥� Therefore, even those who have been vaccinated must wear masks in public space and observe the rules of social distance.鈥�
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