The “reinfections” may just have been not-full recoveries, and the stress of returning to a more energetically demanding routine may have given the immune system of those people a challenge, and thus any lingering virus may have surged in their bodies?

Anyway, there are 2425 different genomes of the virus sequenced, non-functional changes in the genomes have been mapped. For now they all appear to be the same virus, as far as functionality is concerned. Whenther a single nucleotide change in a non-amino-acid-encoding part of the RNA somehow fools a human’s immune system I do not know.