Canada’s National Advisory Committee on Immunization recommended Monday that the AstraZeneca Covid-19 vaccine should not be used in adults under age 55 while rare cases of serious blood clots following vaccination are being investigated, according to a release from the committee.Canada’s National Advisory Committee on Immunization recommended Monday that the AstraZeneca Covid-19 vaccine should not be used in adults under age 55 while rare cases of serious blood clots following vaccination are being investigated, according to a release from the committee.