The original COVID-19 vaccines all targeted the spike protein of the original SARS-CoV-2 virus. The current vaccines target the spike protein of a more recently circulating strain, known as the Omicron XBB.1.5 sublineage. The update is intended to boost production of antibodies that protect more effectively against disease caused by currently circulating Omicron subvariants.
The process of updating the strain without changing anything else is similar to the seasonal strain changes made for influenza vaccinations each year; FDA does not require manufacturers to repeat the large-scale clinical trials necessary for the original products before authorizing updated vaccines. Vaccine safety, side effects, and risk of allergic reactions are expected to be comparable to the original vaccines of the same brand and dose. As with seasonal influenza vaccines, future COVID-19 vaccines can continue to be updated when needed, as the circulating viruses evolve.