Home Immunity Vitamin D can cut your risk of flu by half

Vitamin D can cut your risk of flu by half

  • Have you ever wondered about the seasonal variation in colds and flu’s?
  • Why is influenza seasonal in every country?
  • Given the amount of international travel, why doesn’t the flu hit both hemispheres at the same time?
  • Why do epidemics occur coincidentally at the same latitudes?
  • Where is the virus between epidemics?

Have a look at the following graphs. Because I’m in New Zealand I’m using data from our country – you will find however the exact opposite pattern if you live in the northern hemisphere. The highest ‘flu rates are in August. They are very low in the summer months November –  March.flu nz year round


flu hospital discharges nz


Compare this to the levels of vitamin D measured in blood samples in labs in Auckland (22,000 Auckland adults, 25 (OH) D by age)

vit d blood levels nzThis graph shows the difference in Vitamin D levels in ethnic groups, those with darker skin who avoid the sun are likely to be highly deficient.

vitamin d ethicity nz

You may have noticed that the decrease in blood vitamin D is the exact opposite of the flu rates. This pattern of low vitamin D coinciding with high rates of flu occurs worldwide.

This suggests we should make sure we have blood vitamin D levels of at least 60nmol/ L during the winter to reduce risk of colds and flu. Most researchers now consider 50nmol/L (20 ng/ml) to be the absolute minimum, and 75 – 100 nmol/L to be more optimal (30 – 40 ng/ml)

So would it be smart to supplement with Vitamin D3?

Studies suggest yes.

Supplementation in Japanese school children

This was a randomized controlled study, 167 children in each group, the vitamin D group received 1200IU D3 per day, the other group a placebo.

Children received the supplement from December 2008 – March 2009 (the Japanese winter)


Influenza A occurred in 18 / 167 vit D group, and 31 / 167 placebo group

Asthma attacks were also reduced: 2 children in Vit D3 group had asthma attacks vs 12 in placebo group.

(Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren)

Mongolian School children study

247 children, received either vitamin D fortified milk (300iu) or plain milk

Those who received Vit D had half the risk of upper respiratory infection. Vitamin D levels increased to a mean of 47 nmol/l from a baseline of 20nmol/l

(Randomized trial of vitamin D supplementation and risk of acute respiratory infection in Mongolia.)

Adults with impaired immunity and frequent respiratory tract infections, Sweden

140 patients who suffered several respiratory tract infections every year, many of whom required antibiotics. Patients were randomized to receive 4000IU vitamin D3 or placebo daily for 12 months. Each person documented every respiratory infection and the number of times they had to take antibiotics. Nose and throat swabs were taken with every infection.

Results: 25 (OH) D levels rose from 50nmol/l to 133 in treatment group. The vitamin D3 group had 76 positive microbial swabs (bacteria or fungi) vs 159 in the placebo group. There was significantly less fungi (Candida spp. and Aspergillus spp.) and bacteria:  S Aureus.

At 12 months no pathogens could be detected in nasal swabs of vitamin D3 group, whereas the placebo group continued to show both normal and abnormal flora.

The amount of antibiotics taken by treatment group was 60% less than placebo group.

Another interesting documented result was less adverse events in the Vitamin D3 group. Significantly more patients in the placebo group reported cardiovascular problems, such as heart failure and thrombosis, as well as higher rates of non respiratory infections.

(Vitamin D3 supplementation in patients with frequent respiratory tract infections: a randomised and double-blind intervention study – Full)

How much vitamin D should you take?

In the summer, build up your vitamin D3 levels by exposing skin around the middle of the day, without sunblock. Just a few minutes exposure is enough for those with light colour skins, up to 30 minutes for those with darker skins. In the winter supplement with 1000 to 4000IU per day. To check your Vitamin D levels, you can go directly to Labtests (New Zealand) and pay for it – around $45.00. People respond differently to supplementation so this will allow you to see how much you need to take to build your levels to between 30 – 40 ng/mmol or 75 – 100 nmol/L.

See this earlier post for more on Vitamin D, and this with regards to sun exposure and Vitamin D



  1. Hi Julianne I read your post with great interest. Our laboratory was involved in a Vit D/non-influenza respiratory virus study here in Christchurch. The results were disappointing but I believe, reading around the subject, that it was almost designed to fail to establish Vit D as a reducer of respiratory viral infection. Namely, monthly doses were given and I understand Vit D has a fairly short half-life. I have been quoted 13 days by the section head of our steroid lab. So the study subjects would have been vulnerable for some of that month. Getting a cold this month reminded me to pick up another script for Vit D as I’d run out about a month ago!

    The study purposely excluded influenza A and B. The rationale being that these are seasonal viruses. I wonder if an opportunity to compare rates of all respiratory viral infections in ‘flu immunised vs non-immunised had been missed as the study was done on Canterbury District Health Board staff who have a very high rate of immunisation against influenza A and B (7,000 people so far this season).

    • I had a look at that study. Perhaps a daily dose is better than a large monthly dose, or non seasonal viruses don’t respond to Vitamin D supplementation as well as seasonal as you said. Perhpas populations that are more prone to viruses like the ones in these studies show an effect.
      Still no harm can come from supplementation and the most vulnerable will likely benefit.

  2. Great post…. I started taking vitamin D supplements about a year ago and I have never had a cold or flu since.
    I work a lot with the public – different people every single day, sometimes even those sick with colds. Plus I travel on public transit for 1.5 hours every day…. and those buses are hotbeds of virus transmission.

    But still I did not get sick…
    In addition to the Vit D, I do take a probiotic as well, which probably also helped to a certain degree…

  3. Given the problem with low vit D in NZ (recognised in the South Island at least), it’s surprising that it is only available in meaningful does by prescription. I was surprised to find that I could not buy vit D in NZ in the same doses as are available in the UK. Since you have to pay $35 + to see a doc, and then pay for the prescription, that’s makes it pretty expensive for many people. Similar problems with iodine by the way – I get mine when I visit Australia.

    • Yes, it’s a pain in the butt having to have it prescribed. On the plus side, I think it’s only $1 a month and if you’re seeing your GP every 3 months as I am, it’s no extra cost for having them prescribe it. I do think the daily dose is a much better way of going about it. I look at the packets of cholecalciferol vermin poison nailed to the trees in the back country and wonder!

    • I bought a bottle of 10,000 iu caps when I was in UK a year ago. I’ve been taking 2 – 3 a week, but ended up with too high vit D. I’ve found 2000iu a day is enough for me.

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