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.

4th Dose COVID mRNA Vaccines’ Immunogenicity & Efficacy Against Omicron VOC

Key statement:

“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:

Activation or exhaustion of CD8+ T cells in patients with COVID-19

T-cell exhaustion in the tumor microenvironment