The efficacy of two widely used coronavirus vaccines against the Delta variant weakens within three months of inoculation, but the jabs remain the most effective way to ensure protection against the strain, Oxford University researchers have found.
Two doses of the Pfizer and AstraZeneca vaccines provide protection levels of up to 75 and 61 percent respectively, 90 days after inoculation, they said in the largest study of its kind, published on Thursday.
That is down from 85 and 68 percent, respectively, seen two weeks after the second jab is administered.
The study was based on three million nose and throat swabs.
“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 University professor of medical statistics and the survey’s chief investigator.
Walker was not involved in work on AstraZeneca’s vaccine, which was initially developed by immunology experts at Oxford.
The researchers did 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.
Highlighting the increased risk of contagion from the Delta variant, the study also showed that those who do get infected 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 analysis by the US Centers 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 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 higher viral load hints that herd immunity might become more challenging, said co-author Koen Pouwels, also of Oxford University.
Herd immunity is when a large enough section 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 Pouwels.
The authors cautioned that the viral concentration in the throat was only a rough proxy for severity of symptoms and 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 over “breakthrough infections” – 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.
Earlier this month, England reported that a minority of fully vaccinated people were hospitalised with the Delta variant, while most were unvaccinated.
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 1, 2020, and May 16, 2021, and 810,000 test results from 360,000 participants between May 17 and August 1.
The study was carried out in partnership with Britain’s Office for National Statistics (ONS) and the Department of Health and Social Care (DHSC).