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.