A new study has found that fully jabbed adults can shelter virus levels as high as unvaccinated people if infected with the Delta variant, supporting the theory that hitting the threshold for herd immunity is unlikely.
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, according to a study from the University of Oxford.
Conducted in partnership with the Office of National Statistics (ONS) and the Department for Health and Social Care (DHSC), the study found that 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.
Professor 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.’
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.’