Is Vitamin C good for immune function?

The evidence is still emerging, so it's too early to say for certain. A well-known Cochrane review found that regular vitamin C supplementation does not reduce the incidence of the common cold in the general population, though it modestly shortens cold duration and severity — a nuance later confirmed by dedicated meta-analyses of supplementary and higher-dose regimens. That's reflected in an evidence score of 35/100 for its role in immune function, drawn from 9 studies, with an estimated impact on longevity rated limited (18/100).