Britain's panel of vaccine advisers on Friday said that people under 40 should be offered an alternative to Oxford/AstraZeneca's COVID-19 shot where possible, due to a small risk from rare blood clots.
The Joint Committee on Vaccination and Immunisation (JCVI) broadened the age range of people who should be offered alternatives to include people aged 30-39.
Previously, advice was only for people under 30 to be offered an alternative vaccine.
AstraZeneca's COVID-19 shot, developed by Oxford, has resulted in reports of rare blood clots with low platelet levels that occur more commonly in younger adults, with several countries advising the shot is given only to older people.
The JCVI said that the advice reflected low levels of COVID-19 infection in Britain and the availability of other vaccines made by Pfizer and Moderna.
"As COVID-19 rates continue to come under control, we are advising that adults aged 18–39 years with no underlying health conditions are offered an alternative to the Oxford/AstraZeneca vaccine, if available and if it does not cause delays in having the vaccine," said Wei Shen Lim, COVID-19 Chair for JCVI.
"The advice is specific to circumstances in the UK at this time and maximises use of the wide portfolio of vaccines available."