“…The finding that younger individuals are more hesitant to receive the booster dose is consistent with previous literature from various countries [ 21 , 22 , 23 , 24 , 25 ], as well as studies of age patterns in primary vaccination against COVID [ 26 , 27 , 28 , 29 , 30 , 31 ]. Only a few studies do not replicate this age pattern when estimating the willingness to receive a booster dose [ 32 , 33 ]. Our subgroup analyses, wherein we reran the analyses while splitting the sample into young, medium-aged, and old, provide an explanation for the observed age pattern: the old are willing to take boosters at a high level, irrespective of their protection motivation, whereas the willingness of the young depends on their sense of society being threatened, their sense of self-efficacy, their sense that the proposed behavior makes a difference (i.e., response efficacy), and their perceptions of cost related to complying with the recommended response (i.e., response cost).…”