Vaccines have been in use for many years with rare or no long-term adverse side effects. We anticipate the same for COVID-19 vaccines, based on understanding how vaccines work in general. As with all vaccines or medical treatments, any side effects are closely monitored regularly. Canada has a strong track record of rigorously monitoring for Adverse Events Following Immunization (AEFIs), which is a key part of tracking both vaccine safety and effectiveness. Vaccine safety is built on extensive lab trials and real-world human trials over time. Normally experts require a critical mass of millions of doses to be delivered, in order to have the data necessary to make certain claims about vaccine safety and effectiveness. What is unique about the COVID-19 vaccine is more than 1.36 Billion doses have been delivered to people in a matter of months. Such a milestone would typically take much longer. We have accumulated more data, from more places around the world, at a much faster rate than normal.
There have been some reports of concerning health outcomes (Ex. blood clots or bleeding events) following vaccination with some of the viral vector products, AstraZeneca and Johnson & Johnson vaccines. These reports have included a very small number of deaths within the 28 days after vaccination; in Canada, as of May 7, 2021, there have been 2 deaths associated with these reports, both in individuals who received one of the more than 2.1 million doses of the AstraZeneca vaccine administered.
While short-term side effects are not unusual and the majority of AEFIs resolve quickly, all side effects are investigated according to national standards. In Canada usually the most concerning AEFI is anaphylaxis. This remains very rare; the highest rate among the different COVID-19 vaccines is 30/1,000,000 doses. This is partly why all people who receive vaccines are required to be observed for at least 15 minutes following immunization as this is the most likely time for anaphylaxis to occur.