The Pfizer–BioNTech and Oxford–AstraZeneca COVID-19 vaccines are effective against the highly infectious Delta variant of SARS-CoV-2 — but their protection drops away over time, a study of infections in the United Kingdom has concluded.
Researchers at the University of Oxford, UK, and the country’s Office for National Statistics analysed a vast data set comprising the results of 2,580,021 PCR tests to check for SARS-CoV-2 from 384,543 UK adults between 1 December 2020 and 16 May 2021 — when the Alpha variant was dominant — and 811,624 test results from 358,983 people between 17 May and 1 August 2021, when the Delta variant was more prevalent.
The results suggest that both vaccines are effective against Delta after two doses, but that the protection they offer wanes with time. The vaccine made by Pfizer in New York City and BioNTech in Mainz, Germany, was 92% effective at keeping people from developing a high viral load — a high concentration of the virus in their test samples — 14 days after the second dose. But the vaccine’s effectiveness fell to 90%, 85% and 78% after 30, 60 and 90 days, respectively.
The vaccine developed by Oxford and the pharmaceutical company AstraZeneca in Cambridge, UK, was 69% effective against a high viral load 14 days after the second dose, falling to 61% by 90 days.
The drop in effectiveness shouldn’t be cause for alarm, says Sarah Walker, a medical statistician at the University of Oxford who led the study. For “both of these vaccines, two doses are still doing really well against Delta”, she says.
The study shows that vaccinated people who become infected with the Delta variant carry high peak levels of virus. When the Alpha variant was dominant in the United Kingdom, vaccinated people who became infected had much lower peak viral loads.
The implications of this aren’t clear, Walker says. “Most of our tests are monthly; we can’t really say very much at all about how long people are infectious for and particularly whether that’s different with Delta,” she says. “Anyone who thinks that if they get infected having been vaccinated, they can’t transmit — that isn’t likely to be true.”
The data also suggest that the time between doses of vaccine doesn’t affect vaccine effectiveness, and that people who have previously tested positive for COVID-19 as well as receiving two vaccine doses have the best protection against future infection.
The analysis focused on the 18–64 age group and didn’t look at hospitalizations or fatalities, points out Dvir Aran, a biomedical data scientist at Technion — Israel Institute of Technology in Haifa. “This study is about infection, not severe disease,” he says. The results back up observations from Israel, which vaccinated its population very early in the pandemic, he says. “We are seeing high levels of breakthrough [infections] in the population that was vaccinated early, and on the other hand, we are seeing robust protection in those vaccinated recently — especially in 12–15-year-olds.”
The results raise questions about whether it could be more effective to have doses of different vaccines, rather than multiple doses of the same one, especially if a third, booster dose is to be considered. Georg Behrens, an immunologist at Hanover Medical School in Germany, says that mixing vaccines could increase their effectiveness. The immune system reacts differently to different types of vaccine — and this could be exploited to trigger a better overall response. “Using a vector-based one first and then something that has no vector, but the same antigen, absolutely makes sense,” says Behrens.