Obtaining two vaccine doses remains the most effective way to ensure protection against the COVID-19 Delta variant of concern (VOC), first identified in India and now dominant in the UK, one of Britain's largest studies of its kind concluded on Thursday.
The study by scientists at the University of Oxford, involving more than 700,000 randomly selected people from within the community, found that the Pfizer/BioNTech and Oxford/AstraZeneca vaccines still offer good protection against new infections. The Oxford/AstraZeneca vaccine is being produced and administered in India as Covishield. With Delta, Pfizer/BioNTech and Oxford/AstraZeneca vaccines still offer good protection against new infections, but effectiveness is reduced compared with Alpha (first identified in England last year and previously the dominant VOC in the UK), read the findings released by the university's Nuffield Department of Medicine. 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, it notes.
However, the higher infectiousness of the Delta variant was also confirmed as it was found that 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. "The vaccines are better at preventing severe disease and are less effective at preventing transmission, said Dr Koen Pouwels, one of the lead researchers of the study. "The fact that you see more viral load (with the Delta variant) hints towards herd immunity being more challenging," he said.
In other findings, the study reveals that a single dose of the Moderna vaccine has similar or greater effectiveness against the Delta variant as single doses of the other vaccines. Also, 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. Results suggest that after four to five months, effectiveness of these two vaccines would be similar. However, long-term effects need to be studied, it notes.
The Pfizer/BioNTech vaccine had 93 per cent effectiveness against symptomatic infection two weeks after the second dose, compared with Oxford/AstraZeneca's 71 per cent. Over time, however, Pfizer/BioNTech's effectiveness dropped while Oxford/AstraZeneca's remained largely the same. Experts point out that both vaccines are well within the World Health Organisation (WHO) bar of 50 per cent and therefore continue to offer good protection against COVID-19. It was also noted that the time between doses does not affect effectiveness in preventing new infections, but that younger people have even more protection from vaccination than older people. The researchers analysed two and a half million test results from 743,526 participants in the UK's COVID-19 household-infection survey led by Oxford University and the Office for National Statistics (ONS).
