Vaccine efficacy against Delta wanes in months: study

Major study into the effectiveness of Pfizer and AstraZeneca finds both are effective against Delta, but effectiveness wanes over time.

The biggest real-world study to date into the effectiveness of the Pfizer and AstraZeneca vaccines against the Delta variant has found that both vaccines are effective against the now-dominant strain, but effectiveness wanes over time.

The study by researchers at the University of Oxford using data from the UK’s Office for National Statistics Covid-19 Infection Survey, found that the Pfizer vaccine was initially 92 per cent effective at keeping people from developing a high viral load after infection with SARS-Cov-2, but the vaccine’s effectiveness reduced to 85 per cent 60 days after inoculation and 78 per cent 90 days after a second shot.

The effectiveness of the AstraZeneca vaccine was lower immediately after vaccination, but its effectiveness did not reduce as rapidly as Pfizer. The study found that the AstraZeneca vaccine was 69 per cent effective against a high viral load 14 days after the second dose and effectiveness had declined to 61 per cent 90 days after a second shot.

The decline in efficacy was more pronounced among those aged 35 years and older than those below that age.

The researchers said that four to five months after vaccination, the effectiveness of both vaccines against Delta would be similar.

Immunity after vaccination was markedly higher in those who had been infected with SARS-CoV-2, the study found.

The data was based on more than three million nose and throat swabs taken across Britain, including swabs from 358,983 individuals taken between mid-May and the beginning of August after Delta became the dominant strain.

“Even with these slight declines in protection against all infections and infections with high viral burden, it’s important to note that overall effectiveness is still very high because we were starting at such a high level of protection,” said Dr Koen Pouwels, a senior researcher at the University of Oxford’s Nuffield Department of Population Health.

The study analysed data between December 2020 and August 2021 from the UK’s Covid-19 Infection Survey. Swab tests from more than 700,000 participants were analysed from before and after 17 May 2021, when Delta became the main variant in the UK.

The study’s chief investigator Sarah Walker told The Independent it was unclear “how much transmission can happen from people” who are infected with Delta after being fully vaccinated.

“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.”

The study also found that the time between the first and second doses did not affect either vaccines’ effectiveness.

The researchers also demonstrated in the data that Delta is indeed much more infectious that previous strains of the virus, and concerningly, viral loads were similar between those who get infected despite being fully vaccinated who are unvaccinated and acquire the infection. This could have implications for how well vaccines protect against transmission of the Delta strain.

The study, which has yet to be peer-reviewed, adds to concerns by scientists that the Delta variant can infect fully vaccinated people at a greater rate than previous strains, and that the vaccinated could more easily transmit it.

The study was conducted in partnership with Britain’s Office of National Statistics (ONS) and the Department for Health and Social Care (DHSC).