According to national data, some 2,209 Canadians have reported adverse events following immunization with COVID-19 vaccines. But only 287 of those—or 1 in 9,900 doses administered in Canada so far—were considered serious, such as a severe allergic reaction.

In a statement released Sunday (March 21), Canada’s chief public health officer Dr. Theresa Tam says the numbers on adverse side effects include “medical events that occurred following immunization, but not necessarily related to the vaccine or the immunization process”.

More than 670,000 doses of COVID-19 vaccines have been administered this past week and more than 3.68 million doses in total in Canada since March 19.

After B.C. provincial health officer Dr. Bonnie Henry announced on March 1 that B.C. would be extending the dose interval for the AstraZeneca vaccine, Canada’s National Advisory Committee on Immunization (NACI) recommended expanding the AstraZeneca vaccine after a dozen European countries paused their rollout of the vaccine over blood clots in about 30 recipients.

On that front, Tam says: “Based on the assessment of the evidence to date, it has been determined that vaccine is not associated with an increase in the overall risk of blood clots. However, it is possible that the vaccine may be associated with very rare but serious cases of blood clots associated with thrombocytopenia (i.e. low levels of blood platelets).”

Tam says that while vaccines are proving effective in containing the spread of the coronavirus in high-risk populations, young adults and variants of the virus represent an increasingly high proportion of cases. As of March 18, a total of 4,499 variants of concern have been reported in Canada. And overall, infection rates nationally are highest among young adults 20 to 39 years of age.

Tam warns the growth of the virus in “more mobile and socially-connected adults presents an ongoing risk for spread into high-risk populations and settings”.

The USA, where the AstraZeneca vaccine is manufactured, will reportedly send 1.5 million doses of the AstraZeneca vaccine to Canada as part of a loan deal.