This question has actually been pretty thoroughly researched, so it should be possible to come up with a conclusive answer. A systematic review and meta-analysis of the collected data was published in the British Journal of Medicine in 2017. The review was funded by the National Institutes of Health (NIH). No companies standing to benefit from the sale of vitamin D supplements were involved in funding the study and none of the authors had financial ties to any such companies. That makes me quite prone to trust the data.
The review included data from 25 randomized double-blind placebo controlled trials of vitamin D as a means to prevent respiratory infections. Individual patient data was collected from 10,933 individuals, out of a total of 11,321 who took part in the trials. These two facts make it more likely that the data can be trusted – only randomized double-blind placebo controlled trials (the highest quality type of study) were included in the review, and individual patient data were collected from 97% of all participants across the 25 studies, allowing the investigators to do their own independent analysis of the complete data set.
The studies were conducted between 2009 and 2016. The duration ranged from eight weeks to 1.5 years and the number of study participants ranged from 40 to 3,000. The ages of the participants varied from newborn to 95 years old. Roughly 2,000 participants were healthy adults and 4,000 were healthy children. A further 2,000 were adults with some type of health problem, and the remaining 2,500 or so were children with some health problem.
Some of the studies gave vitamin D as a bolus, while others gave it daily. The daily doses ranged from 7.5 µg (300 IU) to 100 µg (4,000 IU). The bolus dosing intervals varied from weekly to monthly to quarterly, and there was a big variation in dose size in the bolus groups too. This does make the review messy, and makes it harder to get good statistics, because each group getting a certain dose size and dose interval actually ends up quite small, even though the total number of participants is over 10,000.
Ok, let’s get to the results. In the groups getting a vitamin D supplement, 40% had a respiratory infection, while in the groups getting a placebo, 42% had a respiratory infection. This difference was small, but statistically significant, but still small enough that, if that was all there was too it, most people probably wouldn’t think it was worth taking vitamin D to avoid respiratory infections.
However, luckily, there is actually a lot more to it than that. There was a lot of variation in results between the individual studies, so the authors did a sub-group analysis to see if the variation between the studies could be explained. What they found was that among people with a vitamin D deficiency at the start of the studies, the percentage getting a respiratory infection during the study periods dropped from 55% to 41% with a vitamin D supplement. That is a big effect (14% absolute reduction) and it was statistically significant. Any medication that achieved an effect size that big would be a blockbuster and make billions of dollars for the company that invented it. However, among individuals with normal vitamin D levels to begin with, no benefit was found to taking a supplement. So the entire 2% overall benefit seems at this point to be due entirely to a big benefit among those who are deficient.
But wait, there’s more. Another finding was that daily and weekly dosing was protective against infection, but that more infrequent bolus dosing (monthly or quarterly) was not protective. What this means is that there is a clear advantage to taking smaller doses of vitamin D frequently rather than large doses occasionally. When the ineffectiveness of bolus-dosing was accounted for, it was found that the decreased risk of infection was significantly bigger than it had initially appeared. Among people with a vitamin D deficiency who took vitamin D daily or weekly, the proportion that got an infection dropped from 60% to 32% . That is an absolute risk reduction of 28 percent! Among people without deficiency, the absolute risk reduction was 6% .
When looking at treatments, it’s always important to look not just at primary effect, but also side effects and safety. The review found that vitamin D supplementation at the doses used in the studies was safe, with no difference in adverse events between the vitamin D groups and the placebo groups.
Conclusions: Vitamin D isn’t going to magically make you immune to respiratory infections, but it will likely decrease the frequency with which you get them by a bit if you are not deficient, and by a lot if you are deficient. People who are likely to be deficient are those who don’t get a lot of sun (the elderly frequently fall in to this category), those with darker skin living in northern latitudes, those covering large amounts of skin whenever they are outside (a lot of muslim females fall in to this category). If you belong to any of these categories, you should definitely be taking a vitamin D supplement. If you don’t, it’s unlikely to hurt, and it might help.
The fact that infrequent bolus dosing doesn’t work suggests that it’s not enough to just get lots of sun during the summer and then no sun during the winter. So even if you have light skin but live in northern latitudes, you may well benefit from taking a daily or weekly vitamin D supplement during the winter months.
Ok, so you’ve decided to take a vitamin D supplement based on this review. What dose should you be taking? Unfortunately, the review does not mention anything about dosing, or the relationship between dose size and effect. But the main benefit seems to come from going from being deficient to having a normal level in the blood stream. The Recommended Dietary Allowance (RDA) of vitamin D for an adult under 70 is 600 IU (15 µg), and for an adult over 70 it’s 800 IU (20 µg). If you’re taking a supplement that gives you that amount, it’s probably enough. If you want to be on the safe side and be absolutely certain that you’re getting enough to decrease your risk of infection, you could take a bit more than that. According to the FDA, doses up to 4,000 IU (100 µg) are completely safe for adults. The daily dose I would recommend would thus be somewhere in the interval 1,000-4,000 IU per day. Personally I take a supplement containing 3,000 IU per day.
You might also be interested in my article about whether vitamin D supplements can be used to prevent depression or my article about whether you should take fever lowering drugs when you have an infection.