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Impact of Delta on viral burden and vaccine effectiveness against new SARS-CoV-2 infections in the UK

Key findings from the study: 

  • Obtaining two vaccine doses remains the most effective way to ensure protection against the COVID-19 Delta variant of concern dominant in the UK today.
  • 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 findings:

  • 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, 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

Monitoring populations at increased risk for SARS-CoV-2 infection in the community

  • Using 4,091,537 RT-PCR results from 482,677 individuals, we show substantial variation in which of 60 demographic and behavioural characteristics were associated with swab positivity over July 2020 to July 2021.
  • Population-level demographic and behavioural surveillance can be a valuable tool in identifying the varying characteristics driving current SARS-CoV-2 positivity, allowing monitoring to inform public health policy.

Symptoms and SARS-CoV-2 positivity in the general population in the UK

  • Within the COVID-19 Infection Survey, recruiting representative households across the UK general population, SARS-CoV-2-associated symptoms varied by viral variant, vaccination status and demographics. However, differences are modest and do not currently warrant large-scale changes to targeted testing approaches