Q: How and why do we update vaccines against viruses like SARS-CoV-2?
A: For seasonal viral infections such as influenza, yearly vaccines are recommended to “boost” waning immunity and provide improved protection against severe disease in advance of higher community circulation periods. Additionally, updated vaccines serve as an opportunity to better match vaccines to the anticipated common virus subtypes for the coming season by including updated vaccine antigens or sequences. Governmental and public health bodies are now applying this strategy to SARS-CoV-2 vaccination.
Specifically, FDA and CDC are in agreement that Omicron XBB sublineages, which dominate the global variant landscape as of fall 2023, should be the focus of the updated vaccine formulations. The authorized mRNA vaccines made by the companies Pfizer and Moderna include the XBB.1.5 spike sequence, which is very similar to that of several other widespread XBB sublineages such as EG.5 and FL.15.1. Preliminary data suggest that vaccination with this strain update leads to improved generation of neutralizing antibodies and other immune responses against these and other variants (such as the emerging sublineage BA.2.86); this antibody response would be in addition to recharging other aspects of naturally waning immunity (Pfizer ACIP presentation, September 2023; Moderna ACIP presentation, September 2023; Sheward, September 2023 – preprint, not peer-reviewed; Chalkias, September 2023 – preprint, not peer-reviewed).
In summary, deployment of updated COVID-19 vaccines is an important component of improving protection in fall-winter 2023. Like influenza, there is a goal to update these vaccines regularly, though timing and frequency of updates will depend upon whether SARS-CoV-2 achieves predictable seasonal circulation as well as how quickly and how substantially the virus changes over time.