COVID-19 vaccines are highly effective at reducing the risk of severe disease and death. Bivalent vaccines were developed in order to counter the more contagious Omicron variant, which was first detected in November 2021. As the protection provided by vaccination wanes over time, as does naturally acquired immunity, booster doses have been recommended by various public health authorities, particularly for those at high risk of severe COVID-19, as a way to maintain protection against the virus.