The Pfizer vaccine loses its effectiveness faster than AstraZeneca ( AZ ) vaccine over a period of months.
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 possible 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 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.
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.
This means it is essential for as many people as possible to get vaccinated – both in the UK and worldwide.”