Prioritizing second-generation SARS-CoV-2 vaccines through low-dosage challenge studies.

The design of human challenge studies balances scientific validity, efficiency and study safety. This Perspective explores some advantages and disadvantages of 'low-dosage' challenge studies, in the setting of testing second-generation vaccines against COVID-19. Compared with a conventional vaccine challenge, a low-dosage vaccine challenge would be more likely to start, and start earlier. A low-dosage challenge would also be less likely to rule out a vaccine candidate that would have potentially been effective, particularly in certain target uses. A key ethical advantage of a low-dosage challenge over a conventional challenge is that both it and its dose escalation process are safer for each participant. Low-dosage studies would require larger numbers of participants than conventional challenges, but this and other potential disadvantages are less serious than they may initially appear. Overall, low-dosage challenges should be considered for certain roles such as prioritizing between second-generation vaccines against COVID-19.