Oxford Study Shows Waning Covid-19 Vaccine Efficacy Against Delta

Oxford University’s research found that Pfizer and AstraZeneca vaccine efficacy fell to 78% and 61% respectively within 90 days after the second shot, but the lead scientist says both Covid-19 vaccines are still very effective against Delta overall.

KUALA LUMPUR, August 20 — A new study by the University of Oxford has concluded that the effectiveness of the Pfizer-BioNTech and AstraZeneca-Oxford Covid-19 vaccines wanes with time, although they continue to offer protection against the Delta variant after two doses.

The research, based on a total 3,391,645 swab test results in Britain, found that the efficacy of the Pfizer and AstraZeneca vaccines fell to 78 per cent and 61 per cent, respectively, within 90 days after the second shot.

Pfizer’s performance was down from 92 per cent effective against a high viral load — a high concentration of the virus in a person’s test samples — 14 days after the second dose, while AstraZeneca fell from 69 per cent effective two weeks after a second dose.

The decline in the effectiveness of the Pfizer jab after three months seems more pronounced than the change in the protection provided by the AstraZeneca vaccine over the same timespan.

“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, as quoted by The Independent.

Sarah Walker, a medical statistician at the University of Oxford who led the study, said the drop in effectiveness should not be cause for alarm as two doses of both vaccines still do “really well” against the Delta variant.

The results, which have not yet been peer-reviewed, suggest that the effectiveness of both the Pfizer and AstraZeneca vaccines would be similar after five months, the researchers said.

The study also found that fully vaccinated people who do get infected tend to have a viral load similar to the unvaccinated with an infection. In comparison to when the Alpha variant was dominant in the United Kingdom, vaccinated people who became infected then had much lower peak viral loads.

“Anyone who thinks that if they get infected having been vaccinated, they can’t transmit — that isn’t likely to be true,” Walker said.

The Oxford findings are consistent with an analysis by the United States’ Centers for Disease Control and Prevention (CDC) reported last month that found vaccinated people infected with the Delta variant of Covid-19 may be able to transmit the virus as much as unvaccinated individuals due to similar viral loads.

The US is now planning to offer booster shots of the Pfizer and Moderna Covid-19 vaccines as early as mid-September for additional protection against the Delta variant, while recipients of the one-dose regimen Johnson & Johnson jab will also need an extra dose pending trial results.

US officials view Israel as a template as vaccinations started sooner. According to the New York Times, the latest data from Israel showed what experts describe as a continued efficacy erosion of the Pfizer vaccine, both against asymptomatic and mild Covid-19 infections in general and against severe disease among the elderly.

Israel, which has one of the world’s highest levels of vaccination for Covid-19, has administered over a million Covid-19 booster shots to date, as its public health chief said evidence points to waning Covid-19 vaccine immunity.

It was reported that almost 60 per cent of hospitalised patients with Covid-19 in Israel are fully vaccinated, with Pfizer being the primary jab.

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