Table 2.

Methodological characteristics of the included trials

Study	Domain of interest	Description
Anah et al. 1980 [27]	Design	Parallel-group trial.
	Randomization	Reported as a randomized trial, but the method of randomization was not described.
	Allocation concealment	Not described, but double-blinding implies that allocation must have been concealed.
	Blinding of participants and personnel	Reported as double-blind, which implies that participants and personnel were blind; however, the persons who were blind are not explicitly described.
	Blinding of outcome assessment	Reported as double-blind, which implies that outcome assessment was blind; however, the persons who were blind are not explicitly described.
	Drop-outs	No description of drop-outs.
Schertling et al. 1990 [30,31]	Design	Cross-over trial.
	Randomization	Reported as a randomized trial, but the method of randomization was not described.
	Allocation concealment	Not described, but double-blinding implies that allocation must have been concealed.
	Blinding of participants and personnel	Reported as double-blind, which implies that participants and personnel were blind; however, the persons who were blind are not explicitly described.
	Blinding of outcome assessment	Reported as double-blind, which implies that outcome assessment was blind; however, the persons who were blind are not explicitly described.
	Drop-outs	Total number of participants was 29, but histamine sensitivity is reported for 23 participants. The reasons for the 6 missing participants are not given.
Bucca et al. 1989 [25,26]	Design	Self-controlled trial. Two series of histamine challenge tests were done before and after vit C. The first series was carried out when the participants suffered from the common cold, and the second series was carried out 6 wk later after the participants had recovered. On both study days, vit C was administered after the baseline histamine challenge test and the second histamine challenge test was carried out 1 h later. No placebo.
	Randomization	Not a randomized trial.
	Allocation concealment	Not applicable.
	Blinding of participants and personnel	Not blinded.
	Blinding of outcome assessment	Not blinded.
	Drop-outs	One participant out of 10 was excluded from the statistical analysis because she had whooping cough and not the common cold.