Professor Sir Munir Pirmohamed, chairman of the Commission on Human Medicines, says pregnant women should speak to a healthcare professional before getting the AstraZeneca jab.
He added that people with a history of blood disorders should only have the vaccine if the benefits outweigh the risks.
The advice is being given after 79 people in the UK had blood clots following their first Oxford-AstraZeneca jab up to the 31 March.
Of those people, 19 have died – three under the age of 30.
SUBSCRIBE to our YouTube channel for more videos: http://www.youtube.com/skynews
Follow us on Twitter: https://twitter.com/skynews
Like us on Facebook: https://www.facebook.com/skynews
Follow us on Instagram: https://www.instagram.com/skynews
For more content go to http://news.sky.com and download our apps:
Sky News videos are now available in Spanish here/Los video de Sky News están disponibles en español aquí https://www.youtube.com/channel/UCzG5BnqHO8oNlrPDW9CYJog