There have been a half dozen challenge studies[1] of influenza in the last 50 years. They're interesting in that there are threshold effects and a dose response

There have been a half dozen challenge studies[1] of influenza in the last 50 years. They're interesting in that there are threshold effects and a dose response relationship. None of the small numbers of test subjects get sick or seroconverted at low doses. Medium doses produce seroconversion without illness. Higher doses produce illness. Before covid I found a Thai study on the effect of masking nurses in healthcare clinics. Masks reduced infections with some respiratory viruses and not others. Can't be stated strongly enough is you cannot use the behavior of one virus as a proxy for the behavior of an unrelated one. [1] Lack of rigorous studies of respiratory infections in the last 70 years caught public health policy makers completely flat footed. Because they were working off out of date textbook knowledge that was low quality.