Vitamin D and respiratory infections: the benefit shrank every rerun

The 2025 funnel plot leans left, the pattern seen when small trials that found nothing were never published. All three versions pool trials running from infancy to age 95 across many countries, so the pooled group is not one population. The 2017 version worked from individual participant records and the later two from summary numbers, which is a weaker footing for any subgroup.

Protection against respiratory infection shrank each time the same analysis was rerun with more trials. Whether the effect was never there, or the newer trials enrolled people with less to gain, these analyses cannot separate.

A subgroup result is a comparison the trials were never designed to make, which is why it is the kind of finding that most often fails to repeat. Those 255 deficient people produced an odds ratio of 0.60, an interval running from a 72 percent reduction to a 30 percent increase. Only 2.4 percent of that trial's participants were deficient at all.

Whether vitamin D protects severely deficient people from respiratory infection was never properly tested. A 2017 estimate of 70 percent lower odds did not reappear in larger analyses, and trials enrol too few deficient people to tell.