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Academic papers

Effectiveness of vaccines in preventing new infections

  • **NEW** Impact of Delta on viral burden and vaccine effectiveness against new SARS-CoV-2 infections in the UK [MedrXiv, 24 August 2021; preliminary data for SAGE, 22 July 2021]
    • Key findings: With Delta, two doses of Pfizer-BioNTech and Oxford-AstraZeneca vaccines still offer good protection against new infections, but effectiveness is reduced compared with Alpha.
    • 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 and those who had had COVID-19 but not been vaccinated. 
    • 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.
    • Although vaccination reduced the chance of getting COVID-19, the smaller number of 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.
  • Impact of vaccination on new SARS-CoV-2 infections in the United Kingdom [Nature Medicine, 9 June 2021] [Original MedrXiv preprint, 23 April 2021]

Changes in antibody levels after vaccination and natural infection

  • **NEW** SARS-CoV-2 anti-spike IgG antibody responses after second dose of ChAdOx1 or BNT162b2 in the UK general population [MedrXiv, 16 September 2021; preliminary data for SAGE, 22 July 2021]
    • Key findings: In 186,527 individuals, we found significant boosting of anti-spike IgG by second doses of both vaccines in all ages and using different dosing intervals, including the 3-week interval for BNT162b2.
    • After second vaccination, BNT162b2 generated higher peak levels than ChAdOX1. Antibody levels declined faster at older ages than younger ages with BNT162b2, but were similar across ages with ChAdOX1. With both vaccines, prior infection significantly increased antibody peak level and half-life. Protection was estimated to last for 0.5-1 year after ChAdOx1 and >1 year after BNT162b2
  • Anti-spike antibody response to natural SARS-CoV-2 infection in the general population [MedrXiv, 5 July 2021]

    • Key findings24% of 7,256 participants who had positive swab SARS-CoV-2 PCR tests were ‘non-responders’, not developing anti-spike antibodies. These seronegative non-responders were older, had higher SARS-CoV-2 cycle threshold values during infection (i.e. lower viral burden), and less frequently reported any symptoms.
    • Among those who seroconverted, the estimated anti-spike IgG peak level was 7.3-fold higher than the level previously associated with 50% protection against reinfection, with higher peak levels in older participants and those of non-white ethnicity. The estimated anti-spike IgG half-life was 184 days, being longer in females and those of white ethnicity. Antibody levels associated with protection against reinfection likely last 1.5-2 years on average, with levels associated with protection from severe infection present for several years. 
  • Antibody responses to SARS-CoV-2 vaccines in 45,965 adults from the general population of the United Kingdom [Nature Microbiology, 22 July 2021] [Original MedrXiv preprint, 23 April 2021]

Characteristics of those testing positive and symptoms

Variants and trends over calendar time