I had in mind PEG-coated-mRNA and adenovirus vaccines which cause the body's own cells to express a close analog to the SARS-CoV-2 spike protein antigen, a thoroughly novel approach which was rolled out to the general public roughly nine months after the very first tests were administered in humans, and in the case of mRNA vaccines more broadly, was described in articles as late as 2017 as potentially having problems with toxicity especially in repeat dosing.