Respiratory infections reduced by 63 percent with 4000 IU vitamin D daily - RCT
Highlights of Report in Swedish translated by Google
Peter Bergman and his colleagues are fortunate to have 140 patients in daily treatment for one year with either vitamin D (4000 IU) or a placebo.
The study was double-blind: neither the patients - all of which were susceptible to infections - nor caregivers knew who belonged to which group before the trial was over.
The results show that vitamin D reduced patient discomfort.
The probability of requiring treatment with antibiotics during the study period was63 % lower in the vitamin D group compared with the placebo group, and the incidence of fungal infections, including respiratory, decreased.
Chart from 2015 publication

Note: if takes ~100 days for the vitamin D levels to plateau (thus, no benefit is expected until then)
Respiratory tract infection eliminated in 36 percent of people
Types of Respiratory Infections

Vitamin D did particularly well with
Moraxella catharralis
Staphylococcus aureus
Pseudomona aeruginosa
Fungal infection
Adverse effects: fewer with vitamin D than with placebo

Suspect that Vitamin D caused the reduction in:
Gastrointestinal
Cardiovascular
Infections
The RTI aspects

📄 Download the English 2012 PDF from VitaminDWiki
📄 Download the English 2015 PDF from VitaminDWiki
See also VitaminDWiki
VitaminDWiki RTI studies26 as of Sept 2021
Respiratory Tract Infection risk reduced 2X by Vitamin D loading doses – meta-analysis Jan 2021
Respiratory tract infections in childhood – vitamin D is needed, no consensus of how much – Oct 2015
Vitamin D and Respiratory Tract Infections – meta-analysis with charts June 2013
Vitamin D response time is 3-6 months, not much benefit in first 4 months – RCT July 2017
Hypothesis: Respiratory problems will decrease with increased vitamin D – Jan 2012
Respiratory Virus risk reduced 35 percent by Vitamin D (14,000 IU weekly) – RCT Oct 2018
Respiratory diseases helped by vitamin D if initially have low level – RCT review Jan 2015
Common cold incidence reduced by two thirds (500 IU for IBD with low vitamin D) – RCT Jan 2019
Vitamin D does not reduce Infectious Disease when not enough is given – March 2015
Inflammatory diseases: review of vitamin D, with many tables – May 2014 which has the following summary table

See also web
Prevention of New Respiratory Episodes in Children with Recurrent Respiratory Infections: An Expert Consensus Statement - Nov 2020
1,000 IU of vitamin D was not enough for recurrent RTI
-
- Has a chart showing a very strong relationship with season of low Vitamin D at 30 degrees North latitude in China

- Note RTI in Middle East peaks in the summer, not winter, because they stay away from the hot hot sun.