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08 April 2022

A third COVID-19 vaccine boosts waning immunity

A third dose of COVID-19 vaccine boosts vaccines effectiveness to levels higher than one month after the second dose in people over 55 and with one or more long-term illnesses, new research has found.


The study, published today in The Lancet Infectious Diseases journal, by King’s and scientists from ZOE, looks at vaccine effectiveness among those who have received two or more doses of a COVID-19 vaccine.

This large-scale community-based study of over 600,000 people from the ZOE COVID Study App between May 2021 and November 2021 finds there is substantial waning of vaccine effectiveness against COVID-19 infection 5-8 months after the second vaccine dose. Individuals who had two doses of the Pfizer vaccine had 82.1% protection after five months, 81.6% after six months and 75.7% after eight. Individuals who had two doses of the AstraZeneca vaccine saw vaccine effectiveness at 75.7% after five months and 75.2% at six months; Moderna vaccine effectiveness at five months was 84.3%.

Researchers noted a steeper decline of effectiveness in individuals aged over 55. At 5 months, Pfizer vaccine effectiveness was 76.3% in those over 55 years compared to 83% in those aged 55 or younger; AstraZeneca 67.8% in over 55 years compared to 76.7% in 55 or below.

Individuals with one or more illnesses who received two doses of the Pfizer or AstraZeneca vaccine had significantly lower vaccine effectiveness. For Pfizer, that effectiveness at five months stood at 77.5% compared to 82.8% in healthy individuals, and 70.8% compared to 76.1% for the AstraZeneca vaccine.

However, receiving a Moderna or Pfizer booster reverses a decline in vaccine effectiveness. Previous studies by King’s College London scientists found one month after the second dose, infection risk in the vaccinated group were significantly lower than the unvaccinated population. Vaccination effectiveness was 91.6% for those who received a second Pfizer dose, 83.1% for AstraZeneca and 94.1% for Moderna.

A similar level of protection was seen in people over 55, including co-morbidities, after receiving a booster with vaccine effectiveness varying depending on combination: 95.4% for three doses of Pfizer, 91% protection for those receiving a Pfizer booster after two AstraZeneca doses, 92.5% for those receiving a Moderna booster after two doses of Pfizer and 88.8% protection for those receiving a Moderna booster after two AstraZeneca doses. Due to the period of study, analysis was not performed on vaccine effectiveness in the under 55 age group.

The study also investigated side-effects after receiving a booster. A minority 15.9% of individuals reported at least one adverse effect that is felt in the body (systemic), such as fatigue, fevers and headache. However, most people (73.4%) only reported one or more local effects such as tenderness and pain around the site of injection. People who received a different booster vaccine from their first and second doses were more likely to have local and systemic side effects.

This latest study is reassuring as it shows that vaccine boosters are safe to use in the community. Indeed we find that the rate of side effects of boosters is lower than for second doses and most people will have a sore arm.

Dr Cristina Menni, from the School of Life Course & Population Sciences

Tim Spector, Lead Scientist on the ZOE COVID Study from the School of Life Course and Population Sciences said: The clear benefits of vaccines on infection and severity do start to wear off faster in the over 55s and those with other diseases. This paper shows vaccine boosters are effective in restoring protection against infection. This supports the decision to recommence the booster campaigns in the UK to those who are at greater risk. We will be asking all our contributors to make sure they log all their vaccine doses in the ZOE COVID Study to allow us to continue our vital research into COVID.”

In this story

Tim Spector

Professor of Genetic Epidemiology