Study shows decline in Covid vaccine efficacy over time
A BRITISH public health study has found that protection from either of the two most commonly used Covid-19 vaccines against the now prevalent delta variant of the coronavirus weakens within three months.
It also found that those who get infected after getting two shots of either the Pfizer-BioNTech or the AstraZeneca vaccine may have a higher viral load than under previous variants of the coronavirus.
The Oxford University study, based on more than three million nose and throat swabs taken across Britain, showed 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, two weeks after a second dose. The decline in efficacy was more pronounced among those aged 35 years and older.
“Both 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.
Walker was not involved in work on AstraZeneca’s vaccine, which was initially developed by immunology experts at 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-five months after the second shot.
Highlighting the increased risk of contagion from the delta variant, the study also showed that those who get infected despite being fully vaccinated tend to have a viral load similar to the unvaccinated with an infection.
The Oxford findings are in line with an analysis by the US Centers for Disease Control and Prevention (CDC). They 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 fact that we do see ... more viral load hints (...) that indeed herd immunity might become more challenging,” said the study’s co-author Koen Pouwels, also from Oxford University.
Herd immunity is when a large enough portion of the population is immune to a pathogen, either by vaccination or prior infection, which prevents 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 the severity of symptoms and that they had no new data on the duration of infections.
The study was conducted in partnership with the Office for National Statistics (ONS) and the Department for Health and Social Care (DHSC).