A booster dose of vaccine against COVID-19 continues to provide robust protection against hospitalization for older people nearly four months after getting the third dose, new data from the UK's Health Security Agency on Thursday showed.

Vaccine effectiveness against hospitalization for people aged over 65, 15 weeks after a booster, was 85%, down from 91% two weeks after getting the third dose, the latest vaccine surveillance report from the agency estimated.

The data is the first released by the UK on the longer term durability of boosters. The UK is administering fourth doses to vulnerable age groups, joining a number of other countries including Israel as the world fights the more infectious omicron variant of the coronavirus.

Fourth doses will be given six months after the third dose and a wider campaign is being considered for the autumn.

The report attempts to distinguish between people who were hospitalized because of COVID and those who were in hospital with an illness and test positive as part of routine checks.

The figures come from its calculations regarding people who are in hospital primarily because of respiratory problems, which suggests their admission is more likely to be COVID-related.

For people aged 18-64, vaccine effectiveness against hospitalization appears to show a steeper drop - to 67% after 15 weeks, compared with 88% two weeks after the dose.

But UKHSA said this was likely to be because younger people were more likely to be in hospital for another reason and also have COVID, skewing the numbers to make the vaccines seem less effective.

However, the report reiterated that protection against symptomatic disease falls much more dramatically for all groups: from around 60 to 75% effectiveness two to four weeks after a booster, to between 25-40% after 15 weeks.

The UK has mainly administered vaccines developed by BioNTech-Pfizer, Moderna and AstraZeneca. The booster program, which started in September, has used Pfizer and Moderna's shots.