Two doses of either the Pfizer/BioNTech or the Oxford/AstraZeneca vaccine provide good protection against symptomatic infections by the delta coronavirus variant, but both vaccines are around 15 per cent less effective against delta than against the alpha variant, . The findings also show a waning of protection over time, and imply that vaccinated people who do get infected might be just as infectious聽as unvaccinated people.
The key message is that it is important to get both vaccine doses, says study leader at the University of Oxford, UK. 鈥淭wo doses are always better than one.鈥�
It has previously been established that vaccines provide less protection against delta than against alpha, but by how much has been greatly debated. Some early estimates were based on very small case numbers and therefore aren鈥檛 reliable.
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The UK study is based on a weekly survey that started in April 2020. Since delta became dominant in the UK, the team has received results from 800,000 PCR tests done on 360,000 individuals.
Overall, after two doses, the Pfizer/BioNTech vaccine provides 84 per cent protection against symptomatic infections by delta, compared with 97 per cent for alpha, the study found. For the Oxford/AstraZeneca one, it is 71 per cent compared with 87 per cent. Both vaccines are less effective in older age groups against both variants.
The findings also show that the effectiveness of the Pfizer/BioNTech vaccine wanes much faster than that of the Oxford/AstraZeneca one, by around a fifth every month after the second dose.
The team thinks the protection provided by both vaccines will start to become similar after four or five months. That is an extrapolation, Walker cautions, as so far the team only has data going up to 80 days after two doses.
The results of the PCR tests also suggest that people who get infected by delta despite being vaccinated produce just as much of the virus as people who are unvaccinated. By contrast, vaccinated people infected with the alpha variant usually had much lower virus levels than unvaccinated people.
The implication is that vaccinated people who get infected might be just as infectious as unvaccinated people. But it isn鈥檛 possible to be certain about this, says team member , also at the University of Oxford. This is because the PCR tests might be detecting dead viruses rather than infectious ones, he says. Some other studies have also suggested that vaccinated people clear infections faster, meaning there would be less time for people to infect others.
Studies in the US also suggest that vaccinated people can be highly infectious. Last month, these findings prompted the US Centers for Disease Control to change its guidance for fully vaccinated people, where there is a high risk of transmission.
The UK study didn鈥檛 look at hospitalisations or deaths, but other studies suggest that the vaccines still provide excellent protection against both. 鈥淭here are a lot of reasons why vaccines may reduce the consequences of infections even if you have lots of the virus in your nose and throat,鈥� says Walker.
The Moderna vaccine has only recently begun to be used in the UK and mainly in young people, so the team cannot yet calculate its effectiveness after two doses.
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