The 70 percent figure for deficient people never turned up again
In 2017 the odds of infection fell 70 percent in people starting below 25 nanomoles per litre. Two later analyses by the same team found no effect that depended on level.
What the study found
The big benefit for severely deficient people did not replicate
In 2017 the effect looked concentrated in people starting below 25 nanomoles per litre, whose odds of infection fell by 70 percent. Neither later analysis by the same team found any effect that depended on starting level, and the largest trial to look had only 255 deficient participants.
The caveat · Trials never designed for subgroups
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.
What we don't know
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.