Fig. 1.

Risk of bias summary of the trials that reported on the effect of vitamin C on the common cold. Review authors’ judgments are shown for each risk of bias item for each included trial. A green plus sign (+) indicates that there is no substantial concern for bias in the particular quality item. A question mark (?) indicates that conclusions are unable to be drawn regarding potential bias. The red minus sign (-) indicates that there are concerns with the trial. The reference numbers to the trials are shown in Table 1. Justifications for the quality assessments are described in Additional file 1.