Abstract: We identified three prophylactic trials which recorded 37 cases of community-acquired pneumonia in 2335 people. Only one was satisfactorily randomised, double-blind and placebo-controlled. Two trials examined military recruits and the third studied boys from "lower wage-earning classes" attending a boarding school in the UK during World War II. Each of these three trials found a statistically significant (80% or greater) reduction in pneumonia incidence in the vitamin C group. We identified two therapeutic trials involving 197 community-acquired pneumonia patients. Only one was satisfactorily randomised, double-blind and placebo-controlled. That trial studied elderly patients in the UK and found lower mortality and reduced severity in the vitamin C group; however, the benefit was restricted to the most ill patients. The other therapeutic trial studied adults with a wide age range in the former Soviet Union and found a dose-dependent reduction in the duration of pneumonia with two vitamin C doses. We identified one prophylactic trial recording 13 cases of hospital-acquired pneumonia in 37 severely burned patients; one-day administration of vitamin C had no effect on pneumonia incidence. The identified studies are clinically heterogeneous which limits their comparability. The included studies did not find adverse effects of vitamin C. The prophylactic use of vitamin C to prevent pneumonia should be further investigated in populations who have a high incidence of pneumonia, especially if dietary vitamin C intake is low. Similarly, the therapeutic effects of vitamin C should be studied, especially in patients with low plasma vitamin C levels. The current evidence is too weak to advocate prophylactic use of vitamin C to prevent pneumonia in the general population. Nevertheless, therapeutic vitamin C supplementation may be reasonable for pneumonia patients who have low vitamin C plasma levels because its cost and risks are low.

Plain-language summary: Vitamin C was identified in the early 1900s and suggestions that one of its biological roles may be to resist infections are supported by numerous animal studies. We looked for studies in humans and found three trials with a total of 2335 participants that looked at whether vitamin C prevents pneumonia in the community. Two of these preventive trials studied soldiers while the third studied boys in a UK boarding school in the 1940s. Two therapeutic trials with a total of 197 pneumonia patients looked at whether vitamin C might be beneficial for pneumonia patients. One studied patients aged 66 to 94 years in the UK with pneumonia. The other therapeutic trial was conducted in the former Soviet Union but the social and nutritional backgrounds of the patients were not described. One study with 37 burns patients examined the effect of vitamin C on hospital-acquired pneumonia. Our searches were up-date-as of April 2013. Five of the identified trials found preventive or therapeutic benefits of vitamin C against pneumonia but the study on hospital-acquired pneumonia found no effect. The overall quality of the studies was good. However, the five trials with positive findings were carried out in such extraordinary conditions that the results should not be extrapolated to the general population. Therefore, more research is needed. In the meantime, supplementing pneumonia patients who have low plasma vitamin C levels may be reasonable because of its safety and low cost. None of the five trials reported noteworthy adverse effects of vitamin C.