Conducted in partnership with the Office of National Statistics (ONS) and the Department for Health and Social Care (DHSC), the study found that with Delta, Pfizer-BioNTech and Oxford-AstraZeneca vaccines still offer good protection against new infections, but effectiveness is reduced compared with Alpha.
Two doses of either vaccine still provided at least the same level of protection as having had COVID-19 before through natural infection; people who had been vaccinated after already being infected with COVID-19 had even more protection than vaccinated individuals who had not had COVID-19 before.
However, Delta infections after two vaccine doses had similar peak levels of virus to those in unvaccinated people; with the Alpha variant, peak virus levels in those infected post-vaccination were much lower.
Other key findings from the study:
- A single dose of the Moderna vaccine has similar or greater effectiveness against the Delta variant as single doses of the other vaccines.
- Two doses of Pfizer-BioNTech have greater initial effectiveness against new COVID-19 infections, but this declines faster compared with two doses of Oxford-AstraZeneca.
- Results suggest that after four to five months effectiveness of these two vaccines would be similar – however, researchers say long-term effects need to be studied.
- The time between doses does not affect effectiveness in preventing new infections, but younger people have even more protection from vaccination than older people.