Very early clinical studies show an association between low vitamin D levels and more severe disease and death from the virus. For vitamin B12, early computer modelling and laboratory-based research indicate that vitamin B12 may bind to at least one of the viral proteins and thereby slow down viral replication. Neurological features of COVID-19 and vitamin B12 deficiency overlap.
https://www.hra.nhs.uk/planning-and-improving-research/application-summaries/research-summaries/vitamin-d-and-b12-levels-a-clue-to-severity-of-respiratory-covid-19-covid-19/