A number of countries have restricted the use of the AstraZeneca vaccine after the European Union's drug regulator pointed to a "possible link" to rare cases of blood clots.
Britain's Joint Committee on Vaccination and Immunization said an alternative to the vaccine should be given for people aged under 30, where possible.
Italy recommends that the vaccine be used only for people over 60. But people who have taken the first dose can receive the second, even if they are below 60.
Spain is giving the vaccine only to people between 60 and 65.
Australia recommends the Pfizer vaccine over the AstraZeneca one for adults under 50. But those under 50 who have developed no serious effects after the first dose of the AstraZeneca vaccine can be given the second one.
The European Medicines Agency said on Wednesday that "unusual blood clots with low blood platelets should be listed as very rare side effects" of the vaccine. But the agency, as well as the British authorities, say the benefits of the vaccine continue to outweigh the risks for people who receive it.
The vaccine was jointly developed by AstraZeneca and the University of Oxford.