The Spring 2023 Covid Booster program in England will use vaccines from Pfizer/Biontech, Moderna, and Sanifo.
Here is my layman's understanding as to what they may contain. Corrections are welcome.
The genetic sequence of the Covid strain circulating in Wuhan during December 2019 was published by China in January 2020. This was the strain used by Pfizer/Biontech and Moderna in the development of their “original”, first generation, Covid vaccines, which were introduced in 2021. They proved effective against later Covid versions which circulated in the UK (e.g. Alpha , Beta, Delta), but had limited success with the Omicron variant. Both Pfizer/Biontech and Moderna introduced bivalent vaccines for the Autumn 2022 boosters - and these contained the original vaccine plus a new one targeted on the Omicron BA.1 version. For Spring 2023 boosters they have both introduced an updated bivalent vaccine which contains the original vaccine plus one targeted on Omicron BA.4-5 variants. I assume these will be used in the current booster program. The Pfizer/Biontech and Moderna vaccines are Messenger RNA vaccines.
The third vaccine to be used in the Spring 2023 booster program is produced by Sanofi. It is a protein based vaccine which is produced in a different manner to the Messenger RNA vaccines. Its name is “VidPrevtyn Beta” which indicates that it is developed from the Covid Beta variant. This is a later variant than is used in the first generation “original” Pfizer/Biontech and Moderna vaccines, but it is earlier than the Omicron variants that are included in the Bivalent Pfizer/Biontech and Moderna vaccines. However It also contains GSK’s adjuvant that is claimed to boost your immune response. In comparison with the use of an “original” Covid vaccine as a booster, it triggered a higher production of Omicron BA.1 antibodies.
From the Lancet:-
https://www.thelancet.com/journals/lanepe/article/PIIS2666-7762(23)00042-X/fulltext Could Beta variant containing COVID-19 booster vaccines tackle Omicron variants?
Disappointingly it came to a conclusion
The relative performance of a Beta-booster vaccine compared to Omicron-containing booster vaccines will provide valuable data to inform vaccination policy decisions on variant-containing vaccines. As far as we know, a direct comparison of Beta-containing and Omicron-containing booster vaccines has not been done or published.