Catching One Cold After the Other? 10,000 IU Vitamin D per Week + Gargling May Help. Plus: Gargling Thrice A Day or Gargling With Green Tea Are Even More Effective!

Gargling and vitamin D (10,000 IU/week) keep you infection free.

Not too long ago, everyone would have thought you'd picked the wrong letter from the vitamin alphabet if you told him to use "vitamin D" supplements to ward off upper respiratory infections. For decades this has been the still unconfirmed prerogative of vitamin C, which appears to work only in men & women with chronically low intakes of vitamin C...  These days, however, vitamin D can - supposedly - do everything and Mr. and Mrs. Joe & Jane Average won't be surprised to hear that Emma C Goodall and colleagues from the McMaster University and other research institutions call vitamin D, or to be more precise the provision of vitamin D3 supplements a promising intervention for the prevention of URTI. 
To test their hypothesis that the combination of gargling in D-supplementation would ward off the common and not so common cold(s), the scientists randomized 600 students into 4 treatment arms:

• vitamin D3 and gargling, 
• placebo and  gargling,  

The students completed weekly electronic surveys and submitted self-collected mid-turbinate nasal flockedswabs during September and October in 2010 or 2011. Symptomatic students also completed
an electronic symptom diary, which was intended to test for possible effects of vitamin D on the severity and length of the upper respiratory tract infections (URTIs). The primary and secondary outcomes were the occurrence of symptomatic clinical URTI and laboratory confirmed URTI respectively.

Figure 1: Effect of gargling and vitamin D supplementation on URTI occurance (laboratory confirmed), viral load, symptom duration and symptom severity (Goodall. 2014)

As you'd expect, not all students completed the study - of 600 participants, only 471 (78.5%) completed all survey. 43 (7.2%) even registered without completed a single of the questionnaires. Of those students who did responde, 150 (25.0%) reported clinical upper respiratory tract infections (laboratory testing identified 70 infections; 46.7 per 100 URTIs).
If we look at the effects of vitamin D in isolation, we will see that seventy of those 150 participants had been randomized to the the vitamin D3 groups. The other 80 subjects, were in one of the "no-suppplementation" groups. Based on this data the scientists calculated a "significantly lower risk for laboratory confirmed URTI (RR: 0.54, CI95 :0.34-0.84, p = 0.007)" and "a significantly lower mean viral load  measured  as  log10 viral  copies  /  mL" for the vitamin D3 supplemented study participants.

Interestingly, these results stand in contrast to previous studies, where a different dosing scheme, i.e. 100,000IU/month of vitmamin D3 supplements, did not yield a significant reduction in URTI incidence among the 322 healthy adults who participated in a 2012 study by Murdoch et al. that was conducted between February 2010 and November 2011 in Christchurch, New Zealand and analyzed both the D-intake and the changes in 25-OHD levels (Murdoch. 2012).
Reference:

• Goodall, Emma C., et al. "Vitamin D3 and gargling for the prevention of upper respiratory tract infections: a randomized controlled trial." BMC Infectious Diseases 14.1 (2014): 273.
• Jolliffe, David A., Christopher J. Griffiths, and Adrian R. Martineau. "Vitamin D in the prevention of acute respiratory infection: systematic review of clinical studies." The Journal of steroid biochemistry and molecular biology 136 (2013): 321-329.
• Laaksi, Ilkka, et al. "Vitamin D supplementation for the prevention of acute respiratory tract infection: a randomized, double-blinded trial among young Finnish men." Journal of Infectious Diseases 202.5 (2010): 809-814. 
• Mao, Song, and Songming Huang. "Vitamin D supplementation and risk of respiratory tract infections: A meta-analysis of randomized controlled trials." Scandinavian journal of infectious diseases 45.9 (2013): 696-702.
• Murdoch, David R., et al. "Effect of Vitamin D3 Supplementation on Upper Respiratory Tract Infections in Healthy AdultsThe VIDARIS Randomized Controlled TrialVitamin D3 and Upper Respiratory Tract Infections." Jama 308.13 (2012): 1333-1339.
• Satomura, Kazunari, et al. "Prevention of upper respiratory tract infections by gargling: a randomized trial." American journal of preventive medicine 29.4 (2005): 302-307.