The report by the Centers for Disease Control and Prevention (CDC) came from more than 22,000 people who signed up to a vaccine safety smartphone app and who received a booster shot between August 12 and September 19.

During this time, third doses were authorized for people who are immunocompromised, but not the wider population.

"The frequency and type of side effects were similar to those seen after the second vaccine doses, and were mostly mild or moderate and short lived," CDC director Rochelle Walensky said at a press briefing.

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