Indeed, in some seronegative participants with high humoral and cellular responses before the fourth dose, the increase in immunogenicity following the second booster was limited
とある。そんな効果ないのに希望してうたねえよ
努力義務ついたらうつけどね

https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(22)00282-1/fulltext