The vaccines showed similar efficacy in the prevention of symptomatic Covid-19 when given to people with underlying health conditions in the UK as the rest of the population.
The study involved more than one million individuals in at-risk groups, where one dose of either of these vaccines had 60% overall effectiveness against symptomatic infection, with low variation based on age.
Following two doses, the AstraZeneca vaccine showed 81% effectiveness in individuals in risk groups aged 16 to 64 years. Data for the Pfizer-BioNTech vaccine is not available, PHE said.
In individuals in risk groups aged 65 years and above, the effectiveness of the AstraZeneca and Pfizer-BioNTech vaccines was respectively 80% and 89%.
Among the immunosuppressed group, vaccine effectiveness following a second dose was 74% and similar protection was observed in those who are not in a risk group. This represents an increase from 4% post-first dose.
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While additional data is required, the researchers expect the protection against hospitalisation and death in risk groups to be higher than that against symptomatic disease, in line with studies in the general population.
PHE immunisation head Dr Mary Ramsay said: “This real-world data shows for the first time that most people who are clinically vulnerable to Covid-19 still receive high levels of protection after two doses of vaccine.
“It is vital that anyone with an underlying condition gets both doses, especially people with weakened immune systems as they gain so much more benefit from the second dose.”
Age is considered the primary risk factor for adverse outcomes related to Covid-19 but some health conditions also raise the risk of severe infection.
Diabetes, severe asthma, chronic heart, kidney and liver diseases, treatments that deteriorate the immune system, neurological diseases, HIV or chemotherapy are known to increase the risk of hospitalisation or death in Covid-19 patients.
According to modelling analysis by PHE and Cambridge University’s MRC Biostatistics Unit, vaccination prevented 30,300 deaths and 8,151,000 infections, as of 25 June 2021.
In May 2021, a PHE study found that Pfizer-BioNTech and AstraZeneca Covid-19 vaccines were effective against the B.1.617.2 variant of coronavirus first identified in India.