Pfizer vaccine effectiveness declines more quickly than AstraZeneca, finds study

The study found that both Pfizer and AstraZeneca have similar effectiveness within 4-5 months.

A new study from the University of Oxford has found that two doses of the Pfizer vaccine has greater initial efficacy against the Delta variant than the AstraZeneca jab.

The study found that after four to five months, the level of protection offered by both vaccines is similar, with the AstraZeneca jab maintaining its effectiveness throughout the duration.

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At a glance: 5 key points

  • The findings, which have not yet been peer reviewed, also suggested that those infected with the Delta variant after their second jab carried similar peak levels of the virus to those who are unvaccinated.
  • Researchers said that although jabs don’t completely eliminate the chances of getting coronavirus, they do reduce risk and remain the most effective way to protect against the Delta variant. 
  • The study was conducted in collaboration with the Office of National Statistics (ONS) and the Department for Health and Social Care (DHSC), looking at data between December 2020 and August 2021 from the Covid-19 Infection Survey.
  • Their analysis showed that for infections with a high viral load, protection a month after the second Pfizer dose was 90% greater than an unvaccinated individual, reducing to 85% after two months and 78% after three.
  • For AstraZeneca, the equivalent protection was 67%, 65% and 61%, the researchers said.

What’s been said

Sarah Walker, professor of medical statistics and epidemiology at the University of Oxford, said:

“We don’t yet know how much transmission can happen from people who get Covid-19 after being vaccinated – for example, they may have high levels of virus for shorter periods of time.

“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 findings come as NHS officials are gearing up to start offering booster jabs from September, though they are yet to be given the green light from the Joint Committee on Vaccination and Immunisation (JCVI).

The academics involved in the study wouldn’t comment on what it might mean for a potential autumn booster campaign.

Additional reporting by PA.

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