Just planning and getting clinical trials of that magnitude underway for a potential COVID-19 vaccine is a major undertaking. First, researchers would need to recruit medical practitioners and research institutions and obtain permission from local Institutional Review Boards—to say nothing of actually producing sufficient amounts of vaccines for the trials that meet the FDA’s Current Good Manufacturing Practices standards. Then comes the accumulation, organization, and analysis of the data, first by the sponsors of the vaccine, then by regulators themselves.

Moreover, to demonstrate a vaccine’s efficacy—the ability to actually prevent the coronavirus infection—the trials would need to be done in places where the disease occurs in relatively large numbers so as to attain sufficient statistical power and sample size to show a difference between vaccine-treated and placebo groups.

One might think that ascertaining efficacy is simple—you give the vaccine to one group of subjects, a placebo to a second, and then count how many people develop COVID-19 in each group, right?

It’s actually much more complicated. How much of the vaccine should be in each dose? Does one dose suffice to elicit immunity, or do you need two (as is the case for Shingrix)? How well does it work in the elderly, who are highly vulnerable to COVID-19 infection but who tend to mount a poor immune response? Does immunity last long enough to make immunizing billions of people worthwhile?