The number of days of missed work due to colds was the same in both groups, and the duration of symptoms averaged 12 days per cold in both groups. Severity scores were likewise similar between groups.
Nasal swabs showed no differences in the proportion that tested positive for respiratory viruses and no impact on the type of virus found either.
The trial avoided the shortcomings of the prior, inconclusive trials of vitamin D for cold prevention with its relatively long duration, larger sample size, and high dose of vitamin D, the researchers noted.
But it couldn't determine whether daily dosing would have been any better than monthly doses as used in the trial, they added. Prior studies have suggested different effects of vitamin D when given yearly versus every 4 months.
"However, it is purely speculative at this stage as to whether some conditions (e.g., infections) require a smaller steady dose of vitamin D supplementation for benefit," they wrote.