When US health leaders announced the country’s COVID-19 booster programme this week, they pointed to “very clear” evidence of the need for a third dose of the vaccine.
They may have been referring to a real-world study from the UK, published in preprint, that shows protection from both the Pfizer/BioNTech’s Comirnaty and AstraZeneca’s Vaxzevria reduces over time.
The large community study saw researchers from the University of Oxford with the UK’s Office for National Statistics analyse a huge data set of nearly 3.5 million PCR tests to check for people infected with SARS-CoV-2. The study identified nearly 400,000 adults between December 2020 and May 2021, when the Alpha variant was dominant in the country, and just over 350,000 people between 17 May and 1 August this year when the Delta variant was dominant.
The results show that, while both vaccines were effective against the Delta variant, protection wanes over time. While Pfizer/BioNTech’s Comirnaty was 92% effective against high viral load 14 days after the second dose, it fell to 78% after 90 days. AZ’s Vaxzevria also saw a drop, from 69% effective against high viral load after 14 days to 61% after 90 days.
Sarah Walker, a medical statistician at the University of Oxford who led the study, says the drop in effectiveness should not be cause for alarm as for “both of these vaccines, two doses are still doing really well against the Delta variant”.
Data from the UK study also shows that the period of time between the two doses of the vaccines has no effect on vaccine effectiveness. However, people who have previously had COVID-19 and received two doses of a vaccine had the best protection against future infection.
Dvir Aran, a biomedical data scientist at Technion, Israel’s Institute of Technology in Haifa, pointed out that the study was focused on infection and not severe disease and confirm observational evidence from his country. “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.”
Against calls from the World Health Organization to wait until vulnerable people in poorer countries are protected, more rich nations are pushing ahead with booster shots. Following the announcement in the US, this week the UK’s Secretary of State for Health, Sajid Javid, said the country’s booster campaign would likely begin in September and focus on the most vulnerable, although he gave no information on who would be eligible.