New research from the UK provides the clearest indication yet of how protected fully vaccinated people are from the Delta variant of the coronavirus. Based on ongoing surveillance data, researchers estimate that the vaccines developed by Pfizer/BioNTech and Oxford University/AstraZeneca are 50% to 60% effective at preventing infection from Delta, including asymptomatic cases. While the findings do indicate that vaccines can still limit the transmission of Delta, they also reflect a substantial drop in their ability to prevent any infection, compared to their strength against past strains of the virus.
The new data comes courtesy of the REACT-1 study, an ongoing project led by researchers from the Imperial College London to monitor the spread of the pandemic in the UK. As part of the study, volunteers are randomly enlisted from the general population and regularly take at-home swab tests for the virus. This latest round of testing involved about 100,000 people in England, who were tested between June 24 and July 12 this year. By then, Delta had become the dominant strain circulating in the UK.
During that time, the researchers said, about 0.63% of volunteers became infected, or one in 158—a level of community spread not seen by the study since the previous peaks in October and January. But those who were fully vaccinated were three times less likely to test positive than the unvaccinated. Based on that data, the REACT-1 scientists estimate that full vaccination with either of these two shots is somewhere between 50% and 60% effective at preventing infection (for whatever reason, the effectiveness is not broken down by vaccine type).
The results are different in a very important way from other studies in the UK and elsewhere, which have estimated that the Pfizer vaccine is around 88% effective against illness from Delta and the AstraZeneca vaccine around 67% effective. These studies only tracked cases where people become noticeably sick, while the REACT-1 study tries to account for any and all infections caused by the virus, including those that don’t lead to symptoms.
Some recent research has suggested that Delta is more likely to break through and cause infection in vaccinated people and that infected vaccinated people may then have a similar viral load initially as unvaccinated people, raising fears that they may also be as contagious as the unvaccinated. But this new research actually showed that vaccinated people had smaller viral loads on average, while other studies have found that vaccinated people clear their infection faster, limiting their window of transmission.
All together, it’s a good sign that vaccines still significantly reduce the odds of someone spreading the virus to others, even as Delta has taken over. At the same time, the estimates of infection prevention from Delta are substantially lower than those taken in the past. The Pfizer and Moderna mRNA vaccines, in particular, were previously found to prevent more than 90% of all infections, with infected vaccinated people having much lower viral loads. So while these vaccines are holding strong against Delta’s worst effects, the best-case scenario of vaccinated people having near complete protection from the virus is no longer realistic.
“These findings confirm our previous data showing that both doses of a vaccine offer good protection against getting infected. However we can also see that there is still a risk of infection, as no vaccine is 100% effective, and we know that some double vaccinated people can still become ill from the virus,” said Paul Elliott, director of the REACT program at Imperial’s School of Public Health, in a statement from the college.
Though the UK is coming down from its latest, Delta-fueled peak of the pandemic, the U.S. is currently in the middle of one. States such as Florida are now experiencing their highest levels of pandemic-related hospitalizations, while most states are reporting daily case increases. The country’s decent but not great vaccination rate will prevent much of the potential harm caused by Delta, but not all of it.