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One vaccine dose significantly reduces COVID-19 infections and boosts immunity

Published: 26 April 2021

A single dose of either the Oxford-AstraZeneca or Pfizer-BioNTech vaccines leads to a substantial drop in the spread of COVID-19, new research supported by the NIHR has shown.

The research, a collaboration between the University of Oxford, the Office of National Statistics and the Department for Health and Social Care (DHSC), is the first to show the impact of vaccination on new infections and antibody responses in a large group of adults from across the UK.

A study of test results taken from more than 370,000 participants showed that in the three weeks after the first dose of either vaccine, rates of new infections had fallen by 65%. Symptomatic infections were down by 74% while infections without symptoms decreased 57%. 

Data for the study, which examined more than 1.6 million results from nose and throat swabs taken between 1 December 2020 and 13 March 2021, came from the National COVID-19 Infection Survey, supported by the NIHR Oxford Biomedical Research Centre (BRC). The study, which tracks the coronavirus in the UK, has been designated as Urgent Public Health research by the NIHR. 

The research also showed reductions in infections were even greater among participants who had received their second vaccine dose, with two jabs shown to be as effective in preventing the spread of the virus as the level of immunity offered by a previous COVID-19 infection.

The Oxford-AstraZeneca and Pfizer-BioNTech vaccines were shown to be equally effective across a range of variants, including the Kent strain, which was responsible for a surge of infections over winter.

Lead author of the study Dr Koen Pouwels, a senior researcher in Oxford University’s Nuffield Department of Population Health, said: “The protection from new infections gained from a single dose supports the decision to extend the time between the first and second doses to 12 weeks to maximise initial vaccination coverage and reduce hospitalisations or 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. This emphasises the need for everyone to continue to follow guidelines to reduce transmission risk, for example through social distancing and masks.”

A second study, looking at blood samples, compared how antibody levels changed after one dose of either vaccine, or two doses of the Pfizer-BioNTech vaccine. 

Among all groups who had not contracted the virus, a single vaccine dose boosted levels of protection, although fewer antibodies were present in those over 60. However, older people had a significant boost in their antibody levels on receipt of a second jab of the Pfizer BioNTech vaccine, which led to high levels of protection for all groups.

Antibody numbers rose quicker and reached higher levels after one dose of Pfizer-BioNTech compared with one dose of Oxford-AstraZeneca, but dropped at a faster rate to leave a similar amount of protection. 

Participants who became infected despite either having taken the vaccine or having previously contracted COVID-19, carried a lower amount of the virus than those yet to be vaccinated who had caught it for the first time. 

Chief investigator of the COVID-19 Infection Survey is Sarah Walker, Professor of Medical Statistics and Epidemiology at the University of Oxford and the NIHR Oxford Biomedical Research Centre’s Co-theme Lead for Antimicrobial Resistance and Modernising Microbiology.

She said: “Without large community studies such as ours, it is impossible to estimate the impact of vaccination on infections without symptoms – these have the potential to keep the epidemic going, particularly if people who have been vaccinated mistakenly think they cannot catch COVID-19. However, these studies show that vaccination and previous infection both protect against getting infected again.”  

Health Secretary Matt Hancock said: “Vaccines work and today’s findings from the ONS and Oxford University provide further evidence that both the Pfizer and AstraZeneca vaccines are having a significant impact on reducing infections across the UK.

“With over 33 million first jabs already in arms, saving lives and cutting the risk of infection, it’s vital everyone gets their second dose when invited, to protect you and your loved ones against this disease. The vaccine programme has shown what our country can achieve when working as one, it is our way out of the pandemic. When you get the call, get the jab.”

The ongoing survey will monitor the pandemic weekly to look for early warning signs of rising infection rates in different regions and demographic groups.


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