Two COVID-19 vaccine doses provide the most effective protection from the Delta variant in the UK, a new study has shown.
Researchers from the University of Oxford, in partnership with the Office for National Statistics and the Department for Health and Social Care, observed that the Pfizer-BioNTech and Oxford-AstraZeneca vaccines still offer good protection against new infections of the Delta variant. However, effectiveness is lower against this variant than it is against the Alpha variant.
The study, based on data from the COVID-19 Infection Survey, is the largest to evaluate, and directly compare, the real-world effectiveness of the Pfizer-BioNTech and Oxford-AstraZeneca vaccines against all infections, including those without symptoms, after the Delta variant has dominated.
Two doses as effective as prior infection
Comparing protection from infections from COVID-19 vaccines before and after 17 May 2021, when Delta became the main variant in the UK, the researchers found that two doses of either vaccine still provided at least the same level of protection as having had COVID-19 previously through natural infection. The protection was even higher for people who had been vaccinated after already being infected with COVID-19, compared to 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.
Sarah Walker, Professor of Medical Statistics and Epidemiology at the University of Oxford and Chief Investigator and Academic Lead for the COVID-19 Infection Survey, said: “We don’t yet know how much transmission can happen from people who get COVID-19 after being vaccinated – for example, they may have high levels of virus for shorter periods of time.
“But the fact that they can have high levels of virus suggests that people who aren’t yet vaccinated may not be as protected from the Delta variant as we hoped. This means it is essential for as many people as possible to get vaccinated – both in the UK and worldwide.”
The study also found that a single dose of the Moderna vaccine had similar or greater effectiveness against the Delta variant as single doses of the other vaccines.
Greater initial effectiveness after Pfizer doses
Two doses of Pfizer-BioNTech showed greater initial effectiveness against new COVID-19 infections, but this declined 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 findings also showed that the time between doses did not impact effectiveness in preventing new infections, but that younger people have even more protection from vaccination than older people.
Dr Koen Pouwels, Senior Researcher in Oxford University’s Nuffield Department of Population Health, said: “The fact that we did not see any effect of the interval between first and second doses, and the greater effectiveness of having had two doses, rather than one dose, supports the decision to reduce this to eight weeks now Delta is the main variant of concern in the UK.
“However, whilst vaccinations reduce the chance of getting COVID-19, they do not eliminate it. More importantly, our data shows the potential for vaccinated individuals to still pass COVID-19 onto others, and the importance of testing and self-isolation to reduce transmission risk.”
Researchers analysed 2,580,021 test results from nose and throat swabs taken from 384,543 participants aged 18 years or older between 1 December 2020 and 16 May 2021, and 811,624 test results from 358,983 participants between 17 May 2021 and 1 August 2021.
The COVID-19 Infection Survey will continue monitoring the pandemic in the UK on a weekly basis to look for early warning signs of rising infection rates in different regions, sub-regions, and demographic groups, as well as continuing to compare the effectiveness of different vaccines and monitor the impact of immunity on protection against COVID-19.
Professor Walker said: “Without large community surveys 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. We are very grateful to all our participants for giving up their time to help us.”
