The Pfizer-BioNTech vaccine is initially more effective against the Delta coronavirus variant than the Oxford-AstraZeneca jab, but this protection then declines at a quicker rate, new research has shown.
Scientists from the University of Oxford have confirmed that the general performance of the two jabs is diminished by Delta, compared to the previously dominant Alpha variant, with vaccinated people likely to pass the virus on to others.
However, two doses of either jab still provides at least the same level of protection acquired through natural infection, and there is not yet clear evidence to suggest that the vaccines are failing to keep people infected with Delta out of hospital.
There appears to be little change in the effectiveness of the AstraZeneca vaccine three months after a second dose, according to the study. In contrast, there is a clear decline in protection provided by the Pfizer jab over this same timeframe.
The results, which have not yet been peer reviewed, suggest that after five months the effectiveness of these two vaccines would be similar, the researchers said.
“Even with these slight declines in protection against all infections and infections with high viral burden, it’s important to note that overall effectiveness is still very high because we were starting at such a high level of protection,” said Dr Koen Pouwels, a senior researcher at the University of Oxford’s Nuffield Department of Population Health.
The study, conducted in partnership with the Office of National Statistics (ONS) and the Department for Health and Social Care (DHSC), looked at data between December 2020 and August 2021 from the Covid-19 Infection Survey.
Swab tests from more than 700,000 participants were analysed from before and after 17 May 2021, when Delta became the main variant in the UK.
Analysis revealed that for infections with a high viral load, protection a month after the second Pfizer dose was 90 per cent greater than an unvaccinated individual, reducing to 85 per cent after two months and 78 per cent after three.
For AstraZeneca, the equivalent protection was 67, 65 and 61 per cent, the researchers said.
Dr Pouwels said that the team “can be confident” that the numbers “really represent a decline” for the Pfizer vaccine, whereas for AstraZeneca “the differences are compatible with chance, that is there could be no change at all in the protection from AZ”.
The findings also suggest that those infected with the Delta variant after their second jab had similar peak levels of virus to unvaccinated people.
Sarah Walker, a professor of medical statistics at Oxford and chief study investigator, said it was unclear “how much transmission can happen from people” who are infected with Delta after being fully vaccinated.
“But the fact that they can have high levels of virus suggests that people who aren’t yet vaccinated may not be as protected from the Delta variant as we hoped. This means it is essential for as many people as possible to get vaccinated – both in the UK and worldwide.”
Both scientists stressed that the study’s results do not offer any indication on vaccine protection levels against severe disease and hospitalisation.
It did suggest, though, that the time between doses did not affect effectiveness in preventing new infections, and that younger people (aged 18-34) had more protection from vaccination than older age groups (35 to 64-year-olds).
The research also found that a single dose of the Moderna vaccine had similar or greater effectiveness against the Delta variant as single doses of the other vaccines, but the scientists added that they did not yet have any data on second doses of the US-made jab.
Dr Alexander Edwards, associate professor in Biomedical Technology at the University of Reading, who was not involved in the study, said: “Overall this study is excellent as it shows that although Delta is better at infecting vaccinated people than previous variants, the vaccines still work remarkably well.
“There are subtle differences - between different vaccine types, and some changes over time - but they all work brilliantly.”
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