How effective are the COVID-19 vaccines against the new delta variant?

While the majority of adults in the UK have now been double-vaccinated against COVID-19, many of us are concerned about the rise of the new delta variant, and how effective our vaccinations will be against this new, more transmissible strain. This question was the topic of a feature article by Chris Baraniuk published last month in The BMJ:1

The cause for concern is certainly understandable since the delta variant is now the most common form of SARS-CoV-2 (COVID-19) in many countries, including the UK. Adding to this worry is the fact that almost half of those who have sadly died within 28 days of testing positive for the new delta variant in the UK were double-vaccinated. According to Public Health England (PHE), 49% of these 457 deaths between 1 February and 19 July were double-jabbed, and 98% of these were at least 50 years old.1

As worrying as this sounds, there is reason to be optimistic regarding the effectiveness of the vaccines. Analysing data right up to 04 August from the React study, researchers at Imperial College London estimated that people who were double-vaccinated were 50-60% less likely to be infected by the delta variant than those who were not double-vaccinated.1 However, if you are unlucky enough to contract the delta variant, data show that you are more likely to show symptoms than if you contract the original alpha variant or other earlier variants. This has been shown provisionally in several countries, with the Pfizer BioNTech vaccine and the Oxford AstraZeneca vaccine.1

A drop in symptomatic illness is easier to accept if the vaccines still prevent many hospitalisations and deaths. Fortunately, this seems to be the case. PHE data show that the Pfizer BioNtech, AstraZeneca, and Moderna vaccines used in the UK all reduce the risk of death by more than 85%, irrespective of variant.

Exactly why the current vaccines have reduced efficacy against the delta variant is not fully understood, but it is thought that the production of neutralising antibodies to virus spike protein is reduced. However, additional help from immune B-cells producing antibodies and killer T-cells destroying virus-infected cells means that even with a ten-fold drop in protection, antibodies resulting from two vaccinations are still sufficient to keep us safe.1

Despite this, vaccine manufacturers including Pfizer are currently working on an updated vaccine which will specifically target the delta variant. To read Chris Baraniuk’s fascinating article in full, click on the link:



  1. Baraniuk C. Covid-19: How effective are vaccines against the delta variant? BMJ 2021;374:n1960. doi: