Abstract
Most Australian adults now have hybrid immunity to the SARS-CoV-2 virus, referring to a combination of protection from previous vaccine doses and past infection. Protection from both vaccination and past infection wanes over time. Booster doses are recommended to ensure that those who are at increased risk of severe COVID-19 remain protected. The optimal timing of future booster doses to maintain adequate protection against severe illness is not yet known. Older age remains the most important risk factor for severe COVID-19, including in the current Omicron variant era. The original COVID-19 vaccines are monovalent vaccines based on the ancestral strain of the SARS-CoV-2 virus. Bivalent vaccines have been developed based on earlier Omicron subvariants (BA.1 or BA.4-5) and the ancestral strain. These provide enhanced protection against severe illness from Omicron compared with the original monovalent vaccines. Updated monovalent vaccines based on a more recent Omicron subvariant (XBB.1.5) have been developed. COVID-19 vaccines have an excellent safety record, and serious adverse events are extremely rare.