Certain COVID-19 vaccines such as the Pfizer BioNTech and Moderna are specified to be administered in two doses separated by a 21 and 28 day interval, respectively. My questions are:
- Why administered in two doses versus (perhaps) one larger dose? Some vaccines are "single dose" but some are not. What is going on biologically/biochemically that requires or favors a two-dose regimen and why does this not apply in the case of single-dose vaccines? Does it relate to the "platform" (mRNA vs adenovirus, etc.)?
- Why the specified interval? How was it determined that 21 or 28 days would be the optimum interval between doses? What is going on in the body at or about 21 or 28 days that makes it optimal to administer a second dose at that time but not sooner or later?
- Why is there a difference in interval between the Pfizer at 21 days and Moderna at 28 days?