Pfizer vaccine’s efficacy against Delta drops within 90 days of second dose, study finds

University of Oxford professor says two vaccines ‘are doing really well’ against variant

A British public health study has found that protection from two Covid-19 vaccines against being infected with the now prevalent Delta variant of the novel coronavirus weakens within three months.

It also found that those who get infected with the Delta variant after receiving two shots of either the Pfizer-BioNTech or the AstraZeneca vaccine may be of greater risk to others than under previous variants.

Based on more than 3 million nose and throat swabs taken across Britain, the University of Oxford study found that 90 days after a second shot of the Pfizer or AstraZeneca vaccine, their efficacy in preventing infections had slipped to 75 per cent and 61 per cent respectively.

That was down from 85 per cent and 68 per cent, respectively, seen two weeks after a second dose. The decline in efficacy was more pronounced among those aged 35 years and older than those below that age.


"Both of these vaccines, at two doses, are still doing really well against Delta . . . When you start very, very high, you got a long way to go," said Sarah Walker, an Oxford professor of medical statistics and chief investigator for the survey.

Prof Walker was not involved in work on AstraZeneca’s vaccine, which was initially developed by immunology experts at the University of Oxford.

The researchers would not project how much more the protection would drop over time, but suggested that the efficacy of the two vaccines studied would converge within four to five months after the second shot.

Viral load

Highlighting the increased risk of contagion from the Delta variant, the study also showed that those who do get infected now despite being fully vaccinated tend to have a viral load similar to the unvaccinated with an infection, a clear deterioration from when the Alpha variant was still dominant in Britain.

The Oxford findings are in line with an analysis by the US Centres for Disease Control and Prevention (CDC) and come as the US government outlines plans to make Covid-19 vaccine booster shots widely available next month amid a rise in Delta variant infections. It has cited data indicating diminishing protection from the vaccines over time.

Israel began administering third Pfizer doses last month to confront a surge in local infections driven by Delta. Several European countries are also expected to begin offering boosters to the elderly and people with weak immune systems.

Pfizer has said its vaccine’s efficacy drops over time. Last month AstraZeneca said it was still looking into how long its vaccine’s protection lasts and whether a booster dose would be needed to keep up immunity.

"The fact that we do see . . . more viral load hints . . . that indeed herd immunity might become more challenging," said survey co-author Koen Pouwels, also of the University of Oxford.

Herd immunity is when a large enough portion of the population is immune to a pathogen, either by vaccination or prior infection, stopping infection numbers from growing.

“Vaccines are probably best at preventing severe disease and slightly less at preventing transmission,” said Mr Pouwels.


The authors cautioned that they had no new data on the duration of infections.

The survey, which has yet to be peer-reviewed before publication in a scientific journal, underscores concerns among scientists that the Delta variant, first identified in India, can infect fully vaccinated people at a greater rate than previous lineages, and that the vaccinated could more easily transmit it.

To contrast periods before and after Delta became prevalent, the Oxford researchers analysed about 2.58 million swabs taken from 380,000 randomly picked adults between December 1st, 2020, and May 16th, 2021, and 810,000 test results from 360,000 participants between May 17th and August 1st.

The study was conducted in partnership with Britain’s Office of National Statistics and its Department for Health and Social Care. – Reuters