| Study | Reason for exclusion |
|---|---|
| Audera 2001c | Vitamin C was administered with flavonoids. Thus the comparison was not on vitamin C specifically. There was no difference between placebo and 3 g/day vitamin C + flavonoid groups. 2 other arms are included in our analyses (Audera 2001a; Audera 2001b) |
| Baird 1979 | Low dose. 362 UK students aged 17 to 25 years were studied for 72 days in a double‐blind RCT of regular supplementation. A daily drink contained either synthetic orange juice without ascorbic acid, synthetic juice with 0.08 g/d of ascorbic acid added, or natural orange juice with 0.08 g/d of ascorbic acid added. There was a highly significant reduction in common cold incidence among males (RR 0.63; 95% CI 0.50 to 0.78) but not in females (RR 1.24; 95% CI 0.95 to 1.61) (Hemilä 1997a and Hemilä 2006a). The heterogeneity between sexes was highly significant (Hemilä 2008). The benefit of low‐dose vitamin C supplementation may be explained by low dietary vitamin C intake in the UK (Hemilä 1997a) |
| Barnes 1961 | No placebo comparison. A trial in the USA. A multivitamin preparation that included 0.2 g/d vitamin C was given to 23 members (10 boys, 13 girls) of a basketball team for 7 weeks; medication being received from the coaches. The cold outcomes were compared with those of 16 people (8 boys, 8 girls) of the same age and background. The controls reported to the coaches daily. Days sick from cold were counted in each group. The study took place over 8 weeks during which the basketball players took medication on an average of 43 days. The only usable outcome was "mean days per person" in the vitamin C group 1.48 (SD 2.65) and in the control group 6.87 (SD 8.57). However, there are serious doubts about the comparability of the controls who were apparently not basketball players |
| Bartley 1953 | Low dose. "The volunteers did not know to which group they belonged, nor did the physicians responsible for the clinical investigations. All the volunteers were given each day 7 supplementary tablets of identical taste and appearance, some containing vitamin C, others being dummies" (p 8). 3 participants received 0.07 g/d vitamin C and a total of 14 cold episodes were recorded among them in the follow up, 4 participants were administered 0.01 g/d vitamin C (18 colds), and 6 persons were administered no vitamin C (30 colds). The geometric mean length of colds in vitamin C deprived participants was 6.4 days, and in non‐deprived participants 3.3 days, and the authors concluded "such evidence as there is definitely confirms the hypothesis that the absence of vitamin C tended to cause colds to last longer" (p 43) |
| Bendel 1955 | No placebo comparison and the control group was not parallel. 120 children at a summer camp for 2 weeks were given 0.2 g/d vitamin C daily and their cold experience was compared with that of participants in an earlier camp |
| Bergquist 1943 | Low dose. A Swedish trial involving supplementation with only 0.03 g/d vitamin C |
| Bessel‐Lorck 1958 | No placebo comparison. Berlin school children in a skiing camp. Abridged summary: "26 subjects received 1 g of vitamin C daily during the first 9 days. Under this regimen only one student became sick. In 20 participants the regular supplementation did not begin until the 9th day. At this point in time 9 students were already sick with upper respiratory infections; and 3 others became infected within the first 3 days after the trial began. All of those who were sick were treated with 2 g of vitamin C per day. Within just 24 hours a rapid improvement in the general condition was evident so that elevated physical demands were met without particular difficulty. All participants displayed a significant increase in their capacity to perform physical activities while being treated with vitamin C." The Bessel‐Lorck paper is available as a translation. This trial motivated Ritzel 1961 to carry out his RCT (see Analysis 1.1.2) |
| Bibile 1966 | This was cited by Kleijnen 1989, but we have been unable to retrieve a copy through library orders |
| Boines 1956 | No placebo comparison. Study of poliomyelitis sufferers |
| Chavance 1993 | Low dose. Double‐blind RCT of 0.09 g/d vitamin C in elderly participants. No benefit was demonstrated |
| Cuendet 1946 | No placebo comparison. 200 children in 3 mountain parishes took vitamin C supplements up to 0.3 g/d |
| Dyllick 1967 | No placebo comparison. Cohort workplace study involving 200 recipients of 1 g/d of vitamin C whose respiratory experience was compared with those not receiving vitamin C |
| Fogelholm 1998 | Vitamin C in combination with other antioxidants. Finnish study involving 75 athletes. RCT of 1 g/d vitamin C with 0.3 g/d vitamin E and 0.09 g/d ubiquinone versus an undescribed placebo. Methodologically strong study but was excluded from the meta‐analyses because there were 3 antioxidants in the active preparation which were each hypothesised to be potentially beneficial |
| Glazebrook 1942 | Low dose. 1500 boys at a UK boarding school during World War II. The participants were allocated as administrative units and not on an individual basis. Vitamin C (0.05 to 0.3 g/d) was added to cocoa and milk in the kitchen to a group of 335 boys. Although ineffective powder was not added to the drinks of the control group, the control drinks served functionally as a placebo. The number of participants who had colds was 17% lower in the vitamin C group (72/335 versus 286/1100; P = 0.10, Hemilä 2004) and the number of participants admitted to hospital because of the common cold was 23% lower (59/335 versus 253/1100; P = 0.04, Hemilä 2011) |
| Gormly 1977 | No placebo comparison. 14 males of 29 members of a 1‐year Antarctic expedition took 1 g/d vitamin C throughout their stay. Their health outcomes were compared with the remaining group who did not take vitamin C, and no difference was observed between the 2 groups |
| Gorton 1999 | No placebo comparison and the control group not parallel. A technical training facility in Chile was the site of this cohort study with 250 trainees who were given 3 g/d vitamin C during their 10‐day course. The vitamin C group was compared with a control group of 463 students who had been monitored in a somewhat similar way during the previous year (sic) |
| Himmelstein 1998b | There was an extreme and divergent drop‐out rate in the Himmelstein 1998b trial. They started with 52 marathon runners in 2 groups, but 42% (22 of 52) of the vitamin C group, and 75% (38 of 52) of the placebo group dropped out during the trial (P = 0.003) |
| Hopfengärtner 1944 | Low dose. Long‐term hospital baby study in which supplementation of 0.05 g/d vitamin C was used |
| Hunt 1994 | Not focused on the common cold. Double‐blind RCT. 57 elderly UK patients suffering from acute bronchitis or pneumonia who were admitted to hospital for treatment were administered 0.2 g/d of vitamin C (see Hemilä 2007) |
| Kimbarowski 1967 | No placebo comparison. 216 Russian soldiers were hospitalised because of influenza A. 114 were administered 0.2 g/d vitamin C. There were 2 cases of pneumonia in the vitamin C group in comparison with 10 cases in the control group. Thus this trial found a lower incidence of complications of viral respiratory infection (Hemilä 2004; Hemilä 2007) |
| Koytchev 2003 | No placebo comparison. Double‐blind RCT involving 1167 participants. 4 arms, colds treated with 0.9 g/d vitamin C plus or minus antihistamine and antipyretics |
| Maggini 2012 | Vitamin C in combination with zinc. 1 g/d vitamin C and 10 mg/d zinc for 94 participants. The combination decreased the duration or rhinorrhoea |
| Masek 1974 | Low dose. Two large studies of Czech coal miners comparing 0.1 g/d vitamin C and placebo over a period of 4 or 8 weeks. Excluded both on the basis of low dose and inadequacy of data for inclusion in meta‐analyses. The trials were neither randomised nor blind. Authors claimed benefits to the active recipients |
| Niemi 1951 | Low dose and no placebo comparison. Finnish study with military recruits. 1036 people were observed during a 3‐month period. 516 were administered 0.1 g/d vitamin C. No benefits of vitamin C |
| Peters 1940 | No placebo comparison. Short‐term baby supplementation study |
| Schmidt 2011 | Vitamin C in combination with vitamin D, folic acid and selenium. Double‐blind, placebo‐controlled RCT with 192 patients with recurrent colds. Authors claimed benefits to the active recipients |
g/d: grams per day RCT: randomised controlled trial RR: risk ratio SD: standard deviation