Disclaimer: not an immunologist. > So what happens if the second dose arrives very (eg. many months) late? Can you start again and give 3? WHO guidance on inc

Disclaimer: not an immunologist. > So what happens if the second dose arrives very (eg. many months) late? Can you start again and give 3? WHO guidance on incomplete vaccine courses (pre-COVID): https://www.who.int/immunization/policy/Immunization_routine_table3.pdf https://www.who.int/immunization/policy/Immunization_routine... In almost all cases, the suggestion is "resume without repeating the previous dose". The only exceptions are typhoid and cholera vaccinations. I don't know what these suggestions are based upon, but the source is as close to expert consensus as it can go (the WHO has gotten a lot of flak on COVID, but I have never heard anyone seriously doubt its guidance on standard vaccinations). > And is there any chance all the partial immunities will catalyze resistant mutations? Apparently this doesn't even happen much for antibiotics ( https://jamanetwork.com/journals/jamainternalmedicine/article-abstract/2536180 https://jamanetwork.com/journals/jamainternalmedicine/articl... ), so I wouldn't worry about it happening for vaccines. See https://www.pnas.org/content/115/51/12878 https://www.pnas.org/content/115/51/12878 for a comparison. If it were a major issue, it would just as well be an issue with fully delivered vaccination courses, as they don't offer 100% immunity either. The amount of mutations arising should still be proportional to the sick population (or, rather, to its total prevalence over time and population), and anything that reduces the latter should reduce the former. Any vaccine-immune mutation can just as easily come up before the vaccine, unless it hampers the virus so strongly that it is unviable until the evolutionary landscape has shifted. At that point, however, we're looking at a much simpler problem.