Outcomes related to immunogenicity (how well a vaccine works, or the ability to stimulate the development of antibodies), especially the presence or absence of antibodies in the blood of patients or an estimate for the amount of these antibodies, were the most frequently reported outcome in more than 170 studies each. In addition, adverse events were assessed often (N = 126 studies), whilst SARS-CoV-2 infection was reported in only 80 studies.