Telescoping testing timelines and approvals may expose all of us to unnecessary dangers related to the vaccine. While preclinical trials to evaluate the potential safety and efficacy of vaccine candidates are likely to include tens of thousands of patients, it is still unclear whether that number will be large enough and a trial will last long enough to evaluate safety for a drug that would be administered to so many. The US alone plans to vaccinate hundreds of millions of people with the first successful candidate. One serious adverse event per thousand of a vaccine given to 100 million people means harm to 100,000 otherwise healthy people.
Aside from questions of safety that attend any vaccine, there are good reasons to be especially cautious for COVID-19. Some vaccines worsen the consequences of infection rather than protect, a phenomenon called antibody-dependent enhancement (ADE). ADE has been observed in previous attempts to develop coronavirus vaccines. To add to the concern, antibodies typical of ADE are present in the blood of some COVID-19 patients. Such concerns are real. As recently as 2016, Dengavxia, intended to protect children from the dengue virus, increased hospitalizations for children who received the vaccine.