By all existing measures of safety they're by far the least safe vaccines on the market, and even their apparent effectiveness may have just been the result of their immune suppressive effect: https://www.frontiersin.org/journals/immunology/articles/10.... .
>In support of this hypothesis, Dr. Netea’s group reported dampened transcriptional reactivity of the immune cells and decreased type I interferon responses in vaccinated individuals to secondary viral stimulation (97), while our group described inhibition of adaptive immune responses and alteration in innate immune fitness in mice with this platform (99). The immune-tolerant environment induced by these vaccines is further supported by recent studies that have discovered a correlation between an increased number of prior mRNA vaccine doses and a higher risk of catching COVID-19 (100–102). Thus, these data suggest that these vaccines’ efficacy in decreasing disease severity and death might lie with their previously undiscovered immune suppressive characteristics.