Weekly supplementation with 10,000 IU of vitamin D3 in university students during September and October was associated with a non-significant, but potentially clinically important 20% risk reduction of clinical URTI. Importantly, vitamin D supplementation was associated with a statistically significant 46% risk reduction of laboratory confirmed URTI.
Results from previous trials of vitamin D3 supplementation for the prevention of URTI have been conflicting. Two placebo controlled RCTs in pediatric populations and one trial in adults with primary immune suppression have demonstrated that vitamin D3 significantly reduced the risk of clinical and lab-confirmed URTI and improved annual infection scores, respectively [17, 18, 20]. However, previous trials in healthy adult populations have not yielded statistically significant results. The effect estimates from our study are similar to those reported in pediatric trials. Urashima et al. reported that children, ages 6–15, receiving daily supplementation with 1200 IU vitamin D3 had a 42% lower risk of influenza A infection . Camargo et al., reported a 50% risk reduction in parent-reported URTI among children, ages 9–11, receiving daily supplementation with 300 IU of vitamin D3.
Consistent with previous studies in healthy adults, our primary analysis did not show that vitamin D3 significantly reduced the risk of clinical URTI; however the 20% relative risk reduction may be clinically relevant. Li-Ng et al. randomized 162 adults to 2000 IU vitamin D3 or placebo daily from December to March and reported no difference in self-reported URTI . Laaksi and colleagues randomized 164 men to 400 IU vitamin D3 or placebo daily from October through March but did not detect a significant difference in the number of days absent from duty due to URTI . However, the proportion of men who did not experience a URTI was significantly greater in the intervention group (51.3%) than in the control group (35.7%) . Murdoch et al. followed 322 adults randomized to monthly doses of 100,000 IU vitamin D3 or placebo . After 18 months, the study recorded 593 and 611 URTI episodes in the vitamin D3 and control groups respectively, however this was not a significant difference . Rees et al. performed a substudy of 759 participants in a trial of colorectal adenoma chemoprevention. Participants were randomized to receive 1000 IU vitamin D3, calcium carbonate, both or placebo daily and were followed for an average of 13 months which covered two winter seasons. The authors reported that supplementation did not significantly reduce the incidence of URTI . Bergman et al. followed 140 adults with increased susceptibility to respiratory infections at an immunodeficiency clinic. After one year of daily supplementation with either 4000 IU vitamin D3 or placebo, the authors reported a significantly lower infection score in the vitamin D3 group . Notably, our a priori secondary outcome which assessed laboratory confirmed URTI demonstrated a significant 46% relative risk reduction associated with vitamin D3 supplementation. This result is consistent with two recent meta-analyses by Charan et al. and Bergman et al. which reported that vitamin D3 supplementation significantly decreased the odds of respiratory infections (pooled OR: 0.58, CI95: 0.42-0.81, p = 0.001 and OR: 0.64, CI95: 0.49-0.84, p = 0.0014, respectively) [29, 30]. Although these results support the use of vitamin D3 supplementation, further meta-analysis, particularly an individual patient data meta-analysis, including the results from more recent trials and an exploration of heterogeneity between trials should be conducted.
Research has demonstrated that vitamin D plays a role in the innate immune response by stimulating the production of antimicrobial peptides, such as defensins and cathelicidins, and by enhancing the microbicidal action of monocytes and macrophages [31, 32]. For this reason we hypothesized that vitamin D supplementation might reduce the amount of virus in infected persons. Our data support this hypothesis: mean log viral load was significantly lower in the vitamin D3 group compared to the placebo group.
Our study differed in several potentially important ways from previous studies. It is well established that vitamin D levels fluctuate seasonally . Studies of Canadians have demonstrated a marked drop in vitamin D levels in the fall from their peak levels in the summer, and a high prevalence of low wintertime vitamin D levels [34, 35]. To capture peak rates of rhinovirus infection in students, we initiated our intervention in early September, rather than during late autumn and winter months. Thus, serum vitamin D levels did not have the same opportunity to decline as they naturally would during the autumn. Rather than having to overcome depletion in vitamin D levels, our study may have maintained and potentially enhanced vitamin D levels. However, in the absence of blood tests this cannot be proven. Additional differences include the frequency and quantity of vitamin D3 supplementation. Our study used a relatively high dose of 10,000 IU of vitamin D3 per week ingested as a single dose, an average of approximately 1400 IU/day. Although the optimal dosing regimen remains uncertain, it has been acknowledged that adherence with daily supplements is often suboptimal and larger, less frequent doses may be an effective alternative [36, 37]. A final methodological difference was our use of self-collected nasal swabs and laboratory confirmation of URTI. Our definition of clinical URTI may have been excessively broad and insufficiently specific which may have led to incorrectly classified events. It is possible that this definition captured episodes attributable to allergies or other causes, creating error in the statistical model and pushing the results towards a null effect.
Ultimately, our study was underpowered since the observed event rate was lower than predicted. This would contribute to larger variance in the estimates and uncertainty surrounding our results. This study was conducted over a relatively short period of time and while it captured peak rhinovirus activity, it was unable to capture URTI caused by other viruses. Consequently, it is uncertain whether vitamin D3 supplementation is beneficial for the prevention of non-rhinovirus URTI. An additional limitation was the lack of serum vitamin D testing without which we were unable to investigate whether individuals with lower baseline vitamin D levels benefitted incrementally compared to individuals with sufficient baseline vitamin D levels.
Gargling did not appear to reduce the risk of URTI in our study population, in contrast to previous reports . Although our collection period captured peak rhinovirus activity, gargling may be more effective for pathogens which predominantly colonize the oropharynx. Additionally, we were unable to observe whether, or how often, gargling was practiced by participants; gargling may need to be carried out more frequently than twice daily to be beneficial.