Four randomised placebo-controlled double-blind trials were included. There was a total of 475 participants in three of the studies, and at least 1,200 participants in the fourth.
When the three smaller studies were pooled, this gave a pooled RR of common cold infections of 0.50 (95% CI: 0.35, 0.69) in favour of vitamin C groups. The results of a further study, which was originally excluded on the grounds of having no placebo control, were reported post-hoc as having a similar result.
The fourth and largest study, excluded post-hoc, showed no difference in common cold incidence between the vitamin C and placebo groups. However, it did show a significant decrease (p=0.04) in the incidence of pneumonia in the vitamin C group, compared with the placebo group.