Scientists look for safety issues during the testing phase and continue their monitoring as shots roll out around the world. So far, the only serious warning to emerge is a rare risk of severe allergic reactions.
Different types of Covid-19 vaccines have been authorized and it's possible side effects will differ for each - although there's more public data on the vaccines being rolled out in Western countries than elsewhere. Countries also vary in their vaccine standards, with some allowing the use of shots before final-stage testing involving large numbers of volunteers.
But in the US, Britain and European Union, regulators required any vaccine to be tested in tens of thousands of people before distribution. So far, the US is using shots from Pfizer and Moderna, while Britain and Europe have cleared those plus the AstraZeneca vaccine.
Officials expected to receive reports of health problems, even deaths, that occur just by chance in the days or weeks after vaccination, given the huge numbers of people, including the frail elderly, getting inoculated.Authorities consider the person's overall health and how often the reported condition occurs without vaccination.
With more than 52 million vaccine doses administered in the US by mid-February, the CDC said it hasn't detected any patterns in deaths that signal a safety problem.
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