Scientists analysed swabs from May, when the Delta variant became dominant in the UK, so this is the main focus of the research. Although all three vaccines offer a lower level of protection against the Delta variant compared to the original Alpha variant, overall protection still remains high.
For infections with a high viral load, analysis found that protection a month after the second Pfizer dose was 90% greater than an unvaccinated individual, reducing to 85% after two months and 78% after three.
For AstraZeneca, the equivalent protection was 67%, 65% and 61%, researchers said. Between four and five months after vaccination, the protection from both vaccines was comparable.
Dr Koen Pouwels, senior researcher at the University of Oxford’s Nuffield Department of Population Health, 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 scientists 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 researchers added that they did not yet have any data on second doses of the US-produced jab.
The study also suggested 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).
Is it anything to worry about?
The short answer is no. Even with these slight declines in protection against infection, it’s important to note that overall vaccine effectiveness is still very high because we’re starting at such a high level of protection. Getting two doses of the vaccine is still the best way to protect yourself and others from coronavirus.
“It is also worth highlighting that these data here do not tell us about protection levels against severe disease and hospitalisation, which are two very important factors when looking at how well the vaccines are working,” added Dr Pouwels.
Is one vaccine ‘better’ than the others?
“Better” is perhaps too simplistic a term, but the vaccines are certainly “different”.
“All three vaccines currently in use in the UK are effective at reducing the risk of infection and the risk of symptomatic infection even from the Delta variant, especially after the second dose,” commented Paul Hunter, a professor in medicine from the University of East Anglia.
“It does look like AZ is less effective than Pfizer, though the long term difference is a bit more complex as the initial high effectiveness of Pfizer seems to diminish more quickly, which is not what would have been expected. Interestingly, the Moderna vaccine seems to be more effective against the Delta after a single dose than either Pfizer or AZ as single dose, or indeed after two doses of AZ.”
Dr Alexander Edwards, associate professor in Biomedical Technology at the University of Reading, added: “There are subtle differences – between different vaccine types, and some changes over time – but they all work brilliantly.”
What might this mean for booster jabs?
There’s been no suggestion that the study results will impact the
autumn booster jab programme. Booster jabs will be prioritised for those in high-risk groups, rather than determined by whether you’ve had the AstraZeneca, Pfizer or Moderna jab.
“We need to learn to live with this virus,” Savid Javid previously said of the programme. “Our first Covid-19 vaccination programme is restoring freedom in this country, and our booster programme will protect this freedom.”
However, the study results could influence decisions to offer immune-suppressed individuals – for whom the vaccines are less effective to start with –
a third jab ahead of the regular autumn booster programme.
“On this evidence, it certainly supports the case for third booster jabs for vulnerable individuals, as is now happening in Israel,” said Dr Simon Clarke, associate professor in cellular microbiology at the University of Reading.
Can double-jabbed people still spread Covid?
Absolutely. While the vaccines all offer protection to the individual against the Delta variant, the findings suggest they’re less effective at preventing transmission.
Although the chance of infection after two jabs is much lower compared to being unvaccinated, it doesn’t eradicate the risk completely. The researchers found that those who do become infected with the Delta variant after their second jab have similar peak levels of virus to unvaccinated people.
“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,” said Sarah Walker, professor of medical statistics and epidemiology at the University of Oxford. “This means it is essential for as many people as possible to get vaccinated – both in the UK and worldwide.”