Some new studies concerning bivalent vaccine performance and related matters...
(NHS/Moderna) A randomised trial (>3000 participants, aged 17-89) comparing omicron boosters (monovalent and bivalent) with original monovalent mRNA-1273 vaccine, given after a primary series. What is really striking here, beyond the headline conclusion, is that they actually find a relatively small difference between immunological or epidemiological outcomes for original monovalent (WT), monovalent omicron (BA.1) and bivalent (WT+BA.1) boosters (note the overlap of the CIs and bearing in mind efficacy was to any symptomatic disease; there were no COVID-19 fatalities).
Furthermore, these analyses were performed only about 1 month after administering the boosters (phase of memory B cell expansion). At 6+ months there may be next to no measurable immunological or epidemiological difference at all.
DOI:10.1101/2023.01.24.23284869.
Indeed, this (Columbia) study (41 participants, aged 24-63) suggests no significant immunological difference between monovalent (WT) and bivalent (WT+BA.5) boosters at 1 or 3 months post-dose (whilst hybrid immunity arising from subsequent BA.5 infection exhibited better immunological performance at those timepoints).
DOI: 10.1101/2023.02.13.528341.
Also of note, in the first study, immunologically they were only looking at neutralising and binding antibody titres. There were no measurements of the titres or performance of various T cell types, Fc effector functions, etc. This (LJI) study (108 participants, aged 23-73), not only confirms how vaccination primes response to subsequent infection but also provides clear evidence that CD4 and CD8 T cells are actually the fast first responders in post-vaccination SARS-CoV-2 infection episodes, kicking in in the first few days. B cells, neutralising antibodies, etc don't ramp up until the second week.
DOI:10.1101/2023.02.05.527215.
Ultimately, it appears that given sufficient dosing interval (more widely, adequate maturation), and after any initial post-dose memory B cell expansion (<<3 months), that (at least thus far) bivalents expressing more recent variant spike do not offer any significant advantage over the original (WT) spike in the immunocompetent.