People who have been vaccinated with a single dose of either the Oxford-AstraZeneca or Pfizer-BioNTech vaccines were 65% less likely to get a new Covid infection, a UK study has found.

The chances of becoming infected by COVID (SARS-CoV-2) fell sharply after a first dose.

Reductions increased to 70% after a second dose of the Pfizer vaccine, data from the UK Covid-19 Infection Survey show. However, not enough people had yet received two doses of the AstraZeneca vaccine to assess this.

The survey, carried out by the University of Oxford in partnership with the Office for National Statistics and the Department of Health and Social Care for England, included data from virus tests taken from 370 000 UK adults.

Three weeks after their shots, given between December 2020 and early April 2021, infections with symptoms fell by 74%, while infections with no reported symptoms fell by 57%.

Koen Pouwels, senior researcher in Oxford University’s Nuffield Department of Population Health and co-lead author of the study, said, “The protection from new infections gained from a single dose supports the decision to extend the time between first and second doses to 12 weeks to maximise initial vaccination coverage and reduce hospitalisations and deaths.

“However, the fact that we saw smaller reductions in asymptomatic infections than infections with symptoms highlights the potential for vaccinated individuals to get covid-19 again, and for limited ongoing transmission from vaccinated individuals, even if this is at a lower rate.”

Antibody levels

The second study compared how antibody concentrations changed after a single dose of either the AstraZeneca or Pfizer vaccine, or after two doses of the Pfizer vaccine (generally given 21-42 days apart), among 45 965 people in the survey.

In those who had not previously had covid-19, antibody responses to a single dose of either vaccine were lower in older people, especially those aged over 60 years.

Two Pfizer doses achieved high antibody responses across all ages.

Antibody concentrations rose more slowly and to a lower level with a single dose of the AstraZeneca vaccine than with Pfizer’s. However, they then dropped more quickly after a single Pfizer dose, reaching similar levels to those with a single dose of AstraZeneca, particularly among people at older ages.

But while the size of the immune response differed, no groups of individuals were found not to have responded at all to either vaccine.

Sarah Walker, professor of medical statistics and epidemiology at Oxford University and chief investigator for the UK Covid-19 Infection Survey, said, “We don’t yet know exactly how much of an antibody response, and for how long, is needed to protect people against getting covid-19 in the long term—but over the next year information from the survey should help us to answer these questions.”

Links:

24 April 2021, BBC - One Covid vaccine cuts infection rate in all age groups [2]

23 April 2021, the bmj- Covid-19: Infections fell by 65% after first dose of AstraZeneca or Pfizer vaccine, data show [3]

6 March 2021, The Lancet - Early rate reductions of SARS-CoV-2 infection and COVID-19 in BNT162b2 vaccine recipients [4]