We first wrote about respiratory tract infections (RTI) and vitamin D in 2006, in a paper that is now the most cited review paper in the history of the journal. While the article tried to zero in on influenza, it was really about respiratory tract infections in general. It is free to download in its entirety.
We next wrote about influenza in 2008 after I meticulously researched the epidemiology of influenza, including secondary attack rates, serial interval, and multiple studies from 1919 about the failure to demonstrate influenza’s person-to-person infectivity. In the 2008 paper, we put forward a radically different model for the infectivity of influenza, one that depended on vitamin D and a subset of “good infectors.” Using those two assumptions, we explained all nine of influenza’s conundrums. That 2008 paper is the most popular paper in the history of the journal. It is also free to download.
While I have subsequently left RTI and jumped into autism, I have been glad to see the RTI field continue to grow. Since our papers, at least eleven randomized controlled trials have looked at the issue.
Recently, Professor Jonathan Lindh’s group at the Karolinska Institute in Stockholm published a meta-analysis of the 11 randomized controlled trials of vitamin D in preventing respiratory tract infections, to see what all the trials had to say when combined.
They found a protective effect against RTI with an odds ratio (OR) of 0.64 for taking a vitamin D supplement rather than a placebo, which is a fairly strong effect. It means that taking a vitamin D supplement reduces risk of RTIs by 36%. The protective effect was larger in studies using once-daily dosing compared to bolus doses (OR = 0.51 vs OR = 0.86, p = 0.01).
These odds ratio were found despite the fact that most of the 11 studies used inadequate doses. For example, only one of the studies used daily doses of 4,000 IU per day and none tested higher daily doses. Also, none of the 11 studies limited themselves to a strictly vitamin D deficient population. Few of the studies had before and after 25(OH)D levels.
The 11 studies were heterogeneous (different doses, different populations, different end points, different methods of detecting infections and different lengths of the study) that comparison between studies was different. Also, there is always the possibility that negative trials will not be published, called publication bias.
Still, the authors concluded:
“Aggregated evidence from 11 randomized controlled trials indicates that supplementation with vitamin D could be an effective means of preventing respiratory tract infection.”