Mostly against COVID spike (S) protein that facilitates host cell entry.
Pfizer vaccine is mRNA vaccine, completely in vitro derived, uses nanoparticles to aid absorption into host cells which then produce the S protein themselves from the mRNA.
AstraZeneca vaccine is chimp adenovirus vector for genetic sequence – mRNA produced once virus taken up by host cell.
For 2023, new programme for under 4s (US has offered routinely) – aged 6 months to 4 years in a clinical risk group (as defined in table 4 of the Green Book Chapter) should be offered two doses of the Pfizer-BioNTech vaccine (Comirnaty®). There should be an interval of at least 8 weeks between the first and second doses. The odds of admission to paediatric intensive care units with severe COVID-19 is over 7-fold greater for infants and young children with underlying medical conditions compared to those without underlying medical conditions.