How effective are COVID-19 vaccines over time?

A summary of research published in the New England Journal of Medicine

A new study has shown that the protection given by two of the COVID-19 vaccines lasts for at least 20 weeks. The research, published in the New England Journal of Medicine, assessed the impact of vaccines as they are rolled out in the UK.


What did the study look at?

Researchers collected information from people who received either the Pfizer/BioNTech or the Oxford/AstraZeneca vaccine in England between December 2020 and October 2021. They looked at a person’s age and medical history, and whether they had tested positive for COVID-19 previously. If they had tested positive, it looked at whether the infection was from the Alpha or Delta variant of COVID-19. This study looked at whether a person had received the first or second dose of the vaccine. At this stage of the pandemic, it was only possible to have two doses.

What do the results show?

20 weeks after receiving the second dose, both vaccines were still very effective at preventing hospital stays and deaths from COVID-19. The results showed that protection was highest immediately after receiving the second dose, but this reduced only slightly over time.

The vaccines’ ability to prevent catching a milder version of COVID-19 dropped more over time. There was also a bigger drop in effectiveness in the older age groups and for people who are classed as ‘clinically extremely vulnerable’. This group includes people who have received an organ transplant and people with some lung conditions, including cystic fibrosis, severe asthma and severe COPD.

The researchers also found that the vaccines were slightly more effective against the Alpha variant than the Delta variant. However, this was difficult to conclude as there were low numbers of Alpha circulating after June 2021.

Why is this important?

The research confirms that vaccines are very effective at preventing severe disease or death from COVID-19 for at least 5 months. We knew this was the case from clinical trials of the vaccine. But this latest evidence provides reassurance that these results are also seen in real populations.

The vaccines’ protection against milder disease appears to drop over time. But the vaccines remain effective at reducing hospital visits and deaths. This is the crucial factor in helping to keep the pandemic under control.

Decisions on further doses, known as boosters, should consider the risk of new variants and the level of the disease in the community. An alternative option for a booster would be a vaccine that targets specific variants. This is currently under investigation.

Read the original research paper

Title: Duration of Protection against Mild and Severe Disease by Covid-19 Vaccines