Two vaccine doses are as effective at preventing Covid in those with high-risk health conditions as they are in everyone else, Public Health England (PHE) research suggests.
However, the study found that those who are immunosuppressed had almost no protection after just one dose.
Health officials said the data from a study of one million people in at-risk groups, such as those undergoing chemotherapy and sufferers of chronic heart disease, showed how vital it was that the vulnerable had both doses.
The figures showed that one dose of either the AstraZeneca or Pfizer/BioNTech vaccines gave protection of around 60 per cent in most vulnerable groups – similar to that in the wider population.
Two doses of Astra Zeneca offered 81 per cent protection for those in such groups aged 16 to 64, with too little data in this group to properly assess Pfizer.
For those 65 and over, vaccine effectiveness with Pfizer stands at 89 per cent, while it is 80 per cent with AstraZeneca.
Most of the protection levels were found to be similar to those of the wider population, with no statistical differences.
But for people who are immunosuppressed, one dose brought just four per cent protection against symptomatic disease. Experts said that showed how necessary it was to have both doses. A second jab brought effectiveness to 74 per cent, in line with the rest of the population.
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Some health conditions are linked to an increased risk of hospital admission and death from Covid, including diabetes, severe asthma, chronic heart, kidney or liver disease, neurological disease, and illnesses or therapies that weaken the immune system such as blood cancer, HIV or chemotherapy.
PHE said protection for these people against hospital admission and death was expected to be even higher than for symptomatic Covid illness.
Dr Mary Ramsay, the head of immunisation at PHE, 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."
Those living with immunosuppressed adults are also prioritised for vaccination to reduce the chance of infection. Ministers have promised new guidance for immunosuppressed patients as restrictions ease.
Sajid Javid, the Health Secretary, hailed a "phenomenal achievement" after more than 80 million vaccines were administered across the UK, with 87 per cent of people now having had one dose and 65 per cent two.