While vaccines can lessen the severity of Covid symptoms, it does not prevent humans from catching Covid. A number of studies have suggested people with an unbalanced microbiota in their gut have a higher chance of contracting Covid and for some people to suffer from a more severe form of Covid.
There is conclusive evidence to show an association between gut microbiota composition and the levels of cytokines and inflammatory markers in patients with COVID which suggest that the gut microbiome is involved in the magnitude of COVID severity possibly via modulating host immune responses. Furthermore, the gut microbiota dysbiosis after disease resolution could contribute to persistent symptoms, highlighting a need to understand how gut microorganisms are involved in inflammation and COVID.
Source: https://gut.bmj.com/content/70/4/698: