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★★★Well replicated — this finding’s sources average out to meta-analyses, systematic reviews or high-powered trials.
The benefit landed on severe symptoms; mild ones did not shift
In the five comparisons reporting symptom types separately, vitamin C shortened severe symptoms and left mild ones unchanged. All of them dosed daily, before the cold.
What the study found
Vitamin C's severity benefit sits on severe symptoms, not mild ones
Vitamin C did not make colds uniformly milder. In the five comparisons that reported symptom types separately, the benefit fell on the duration of severe symptoms and mild symptoms did not shift at all. Every one of those trials dosed daily, before the cold had started.
The caveat · Some counted days spent indoors
Only five comparisons split the symptoms this way, out of ten trials in the pooling. Severity was measured inconsistently, in some trials by counting days spent indoors, which mixes how ill someone felt with whether they could take time off work.