Why restrict medical effective altruism?

In a challenge trial, research subjects are purposefully exposed to some pathogen in a controlled setting, in order to test the efficacy of a vaccine or other experimental treatment. This is an example of medical effective altruism (MEA), where individuals volunteer to risk harms for the public good. Many bioethicists rejected challenge trials in the context of Covid-19 vaccine research on ethical grounds. After considering various grounds of this objection, I conclude that the crucial question is how much harm research subjects can permissibly risk. But we lack a satisfying way of making this judgment that does not appeal simply to the intuitions of doctors or bioethicists. I consider one recent and structurally plausible approach to critically evaluating the harm question. Alex London defends a social consistency test for research risks: we should compare the risks undertaken by research subjects to relevantly similar risks which are accepted in other spheres of society. I argue there is no good reason not to consider volunteer military service as a relevant social comparison. This implies there is essentially no cap on acceptable risks on the social consistency rationale. In short, if soldiers can be heroes, why can't research volunteers?