The pause was recommended by the National Advisory Committee on Immunization for safety reasons. The Canadian provinces, which administer health in the country, announced the suspension on Monday.
“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, vice-chair of the National Advisory Committee on Immunization.
Deeks said the updated recommendations came amid new data from Europe that suggests the risk of blood clots is now potentially as high as one in 100,000, much higher than the one in one million risk believed before.
She said most of the patients in Europe who developed a rare blood clot after vaccination with AstraZeneca were women under age 55, and the fatality rate among those who develop clots is as high as 40%.
Dr Joss Reimer of Manitoba’s vaccine implementation taskforce said despite the finding that there was no increase risk of blood clots overall related to AstraZeneca in Europe, a rare but very serious side-effect has been seen primarily in young women in Europe.
Reimer said the rare type of blood clot typically happens between four and 20 days after getting the shot and the symptoms can mirror a stroke or a heart attack.
“While we still believe the benefits for all ages outweigh the risks I’m not comfortable with ‘probably’. I want to see more data coming out of Europe so I know exactly what this risk-benefit analysis is,” Reimer said.
The AstraZeneca shot, which has been authorised in more than 70 countries, is a pillar of a UN-backed project known as Covax that aims to get Covid vaccines to poorer countries. It has also become a key tool in European countries’ efforts to boost their sluggish vaccine rollouts. That makes doubts about the shots especially worrying.
“This vaccine has had all the ups and downs. It looks like a rollercoaster,” said Dr Caroline Quach-Thanh, the chair of the National Advisory Committee on Immunization, when asked if the latest news will lead to further vaccine hesitancy.
Health Canada said it has not received any reports of blood clots in Canada, and the department’s chief medical adviser, Dr Supriya Sharma, said she still believed the vaccine’s benefits outweighed the risks.
Last week, the department changed its label on the vaccine to warn about the rare risk of blood clots.
Only those 60 and above have received AstraZeneca in Ontario, Canada’s most populous province.
“We have no concerns with those who have received it so far,” said Dr David Williams, Ontario’s chief medical officer.
Several European countries that had suspended using the vaccine over concerns it could cause blood clots have resumed administering it after the EU’s drug regulator said the vaccine was safe.
The vaccine is used widely in Britain, across the European continent and in other countries, but its rollout was troubled by inconsistent study reports about its effectiveness, and then more recently the scare about clots that had some countries temporarily pausing inoculations.
Canada is expected to receive 1.5m doses of AstraZeneca from the US this week.