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Canada's 13 provinces and territories, on advice from the National Advisory Committee on Immunization, suspended use of AstraZeneca's vaccine on people 55 and younger Monday. The advisory committee said its recommendation was a "precautionary measure" due to "rare" cases of blood clots in Europe, mostly affecting women under 55. "There is substantial uncertainty about the benefit of providing AstraZeneca COVID-19 vaccines to adults under 55 given the potential risks," said Dr. Shelley Deeks, the committee's vice chair.

AstraZeneca is a relatively small part of Canada's COVID-19 vaccine strategy, but it is hugely important for the global vaccination effort and is widely used in Britain. Europe's drug regulator said there is no evidence of an overall increase in blood clots and the vaccine is safe and effective to use. Canadian officials said they want more information about the blood clots, especially in young women. There have been no reports of blood clots in Canada, and the country is still using the vaccine for those over 55.

AstraZeneca's "vaccine has had all the ups and downs," Dr. Caroline Quach-Thanh, chair of the National Advisory Committee on Immunization, told reporters Monday. "It looks like a roller coaster." Canada approved the vaccine, developed by Oxford University, in late February, but only for people under 65. When British data came in showing its effectiveness on older people, Canada reversed itself. Now it has changed its advice again.

"The messaging has been brutal overall," Dr. Andrew Morris, a professor of infectious diseases at the University of Toronto, tells The Associated Press. "I am fearful it is toast. It shouldn't be."