Gili Regev-Yochay's Israeli study (I mentioned prior) is critical, for it showed 2nd booster (4th shot) failed to boost the immune response & only re-established the waning immune response (3rd shot)
by Paul Alexander
Immune exhaustion is a real concern; the 4th shot (2nd booster) yielded little extra protection against COVID-19 when compared to the initial three doses; it short, the 4th shot has failed.
“The fourth COVID-19 mRNA dose restores antibody titers to peak post-third dose titers. Low efficacy in preventing mild or asymptomatic Omicron infections and the infectious potential of breakthrough cases raise the urgency of next generation vaccine development.”
Immune exhaustion is a real concern if the immune system is constantly boosted and this was not studied, see these references to open the debate: