What Makes Flu Seasonal?

Just as flu season gathers force here in the northern hemisphere, it’s petering out in the southern half of the globe. No matter where you are, you’re more susceptible to the flu in the winter months. Even if, let’s say, some research physicians expose you to live flu virus in the middle of summer, you’re still less likely to get sick than if the same doctors hit you with the same virus in the dead of winter. Why? One big risk factor for flu infection is a lack of vitamin D. We naturally produce vitamin D when we’re exposed to sunlight, and as the days shorten in the winter, we produce less and less of it. That led these researchers to think that flu epidemics could be a sign of widespread vitamin D deficiency, due in part to decreased solar radiation. So it seems darkness makes the flu go ’round. (HT: Jason Tetro) [%comments]

Dr. Manak

Isn't it due to the fact that protien coat of the flu virus deteriorates under high temperature and humidity? Hence flu transmission is lower in summer?


Is is possible that we are more vulnerable to the flu when it's cold outside, which lowers our immune system?


I have often disbelieved the traditional answer for colds and flu, that we catch more in the winter because we are in confined spaces more.

This new answer is as good as any, but would not appear to be the final word. Why, for example, is Flu seasonal even in countries close to the equator? There was still plenty of sunlight in most of Mexico last winter when H1N1 ran rampant.


The flu virus is resistant to cold, dry environments while it's weakened by warmth and humidity.

Another Mike

can you take Vitamin D over the counter to prevent catchign the flu?


Well the lack of sun light argument does not explain the epidemic in Mexico...


We are indoors more, and children are in school.


It's always sunny and warm in Mexico so maybe N1H1 started out as stronger than average flu... And Mexico doesn't have any schools - that's why illiterate Mexicans have to come here to find work.


I'm seeing what appears to be evangelizing of one theory over the others -- is it not likely that its a combination of all of these factors? I'm no doctor, but...

Ed Haines

Mexico does indeed have schools. In fact, some of them are quite good. Their illiteracy rate is higher than in the US but seems to be improving while ours is either unchanged or somewhat worse than in the past (depending on how you define illiteracy). They come here for work because the work is available.


Vitamin d production does not require sun light it requires hard UVB. The high energy ultra violet is absorbed by the atmosphere, so only exposure close to noon makes vitamin d. Any smog, cloud, pollution, clothes, make up absorbs the hard UVB, as does glass and buildings so if you work indoors a sunny climate does not help. There are many people who live near the equator who are vitamin d deficient.

It is normal at the equator for the flu season to follow the rainy season.

Charles S

There needs to be a large scale study using therapeutic amounts of D3 (at least 5000 i.u. or .125 milligrams- that's 1/8 of a milligram folks!) to measure the effect on seasonal and swine flu. It should be organized immediately to verify the rumored success re: preventing flu. In the interim the anecdotal evidence is overwhelming and I type that comment based on personal experience. Part of my job requires close interface with Latino workers of all ages in a small office. I can report directly that of the dozens of colds/flus contracted by dozens of workers in this environment I have had none.

I have studied vitamins and supplements for thirteen years. I have watched the "flavor of the month" come and go in regards to 'alphabetic' vitamin/supplement claims. None are even in the same universe as vitamin D3. In fact the combined therapeutic effect of all of them don't come close to that of D3. The citizens of this country need to wake up and recognize the necessity of D3 supplementation year round. The FDA, the CDC, the NIH, and the AMA need to admit defeat and acknowledge the universal requirement of at least 50 ng/ml of activated D3 in blood serum. The drug companies won't make a nickel, and in fact will be forced to cancel production of at least half of their molecules. Face it folks- D3 is the essence of the Sun and the ultimate gift to mankind. We can't function without it- lots of it- and the sooner this is recognized the sooner we cut medical healthcare costs. LAST BUT NOT LEAST...THOSE WITH THE HIGHEST LEVELS OF ACTIVATED D3 IN THEIR BLOOD HAD 7 YEARS LESS CHROMOSOMAL WEAR THAN THOSE WHO HAVE THE LOWEST LEVELS.!!!




This Vitamin D theory is almost certainly correct. In the next 5 to 10 years everyone will fully understand the importance this amazing Vitamin....which is really not a Vitamin at all...but rather a hormone.

Low levels of Vitamin D are a main contributing factor to everything from Cancer, Diabetes, High Blood Pressure, Schizophrenia and more. There is a very plausible link to Autism as well that is being researched.

The only reason this is not getting the attention is deserves is because there is no payday for Pharmaceutical companies. The cure for Vitamin D deficiency is free sunshine (in moderation) or supplements which cost pennies a day.

Charles S

'Why, for example, is Flu seasonal even in countries close to the equator? There was still plenty of sunlight in most of Mexico last winter when H1N1 ran rampant.'

The reason flu was 'rampant' is due to the fact that so many Mexicans are severely vitamin D3 deficient. Incredible- but true. The darker the skin, the less efficiently D3 is synthesis zed. D3 deficiency is widespread and documented beyond dispute in Central America. In Costa Rica an asthma study found conclusive evidence for hospital visits and D3 deficient children. More than 1/2 the children were considered deficient- using the old, discredited level of activated serum D3. The point in all of this is that the level of D3 necessary to be most therapeutic is at least 50 ng/ml. Nothing less or you'll get sick like you always have...cancers, heart attacks too!

Investigate the evidence and medical research at: Vitamin D3 Council. Be certain you are sitting down, as you are likely to keel over when you read the facts. Nothing, absolutely nothing in this world is more important than raising activated D3 levels to at least 50 ng/ml. It is more important than quitting smoking, eating right, losing weight, exercising- combined. The proof of that is the fact D3 deficiency incurs a 54% higher risk of death- all cause mortality. Smoking is only 23% and the rest combine not even close!

Ever wonder why people with great cholesterol, HDL, LDL numbers have heart attacks? You guessed it- low D3. More important than all those other factors. Ever wonder why plaque builds in the arteries? Is it the American lifestyle, environmental factors, etc.? Well it's due to low D3, which is the ultimate anti-inflammatory, ultimate antioxidant (at least 350 X more potent than vitamin E).

In the next few years there will be irrefutable evidence that a vitamin D3 deficiency is the ultimate cause of autism. The facts are there and the writings on the wall. D3 deficiency will be shown to be the root cause of schizophrenia, diabetes, most cancers (see DINOMIT theory of cancer), obesity, crooked teeth, gum disease, most heart diseases, low I.Q., dementia, MS (already proven), and a host of other plagues. While this sounds fantastic and absurd it is not. And the reason it is all true? D3 is THE molecule derived from direct sunlight. It is the tangible essence of the source of all life on Earth. We need oxygen, water, D3, food and shelter (in just about that order) to thrive. Living in glorified caves is distinctly unhealthy and the last 75 years provide proof of that.

Read up on D3 and supplement everyday and live the Golden Years in better health. In 5 years there will be standard D3 testing for everyone and reaching the 50 ng/ml serum level goal will reduce heath costs more than any other single factor. Look it up and see how D3 supplementation will cut costs by more than 5 trillion per dec


Ben McFarland

If that's so, wouldn't Seattle be more affected by seasonal flu, just like it's more affected by seasonal affective disorder from the long gray winters?

Ted Hutchinson

The charity Grassrootshealth D Action offer cost price 25(OH)D tests by post.. So you can check your Vitamin D status is above 55ng/ml 135nmol/l for $40. It's a simple fingerstick test with just 2 drops of blood on the sample sheet.

About 1000iu/daily/vitamin D3 is generally required. Dr Davis of the Heartscanblog finds his Wisconsin patients generally require around 5000~6000iu/daily to achieve levels around 60ng/ml 150nmol/l. There is however a great variation in response so testing is advisable so you can be certain what you are doing is effective.

Vieth has shown that 10,000iu/daily is a safe upper limit that allows a huge margin of safety so that even people with daily access to sunshine would not get near the level at which adverse events have been recorded.

Do make sure you use D3 Cholecalciferol not the form most doctors prescribe that is not only a lot more expensive but also less effective and shorter acting. To correct deficiency 50,000iu/D3 once a week is usual for 8 weeks.

There is a role for Vitamin D used in high doses as Stoss Therapy and detailed at The Vitamin D Council Website.

Those people living in URBAN areas or under a flight path do need to check their 25(OH)D if they are relying on sun exposure. Diet provides only a trivial amount, around 10% of daily needs. Atmospheric pollution prevents UVB reaching the ground in sufficient amounts. There are a lot of examples of urban people with regular extensive sun exposure but remaining D3 deficient. Full body sun exposure (or as near to as practicable) is generally required as least tanned areas of skin are most effective for D3 synthesis. Bear in mind that vitamin d synthesis is a self limiting process and once the D3 is made further sun exposure turns it into products the body doesn't use. So a short sun session, 20minutes full body is a lot more effective than 40minutes with only 50% of the body exposed or 80minutes of 25% skin surface.


Charles S

'If that's so, wouldn't Seattle be more affected by seasonal flu, just like it's more affected by seasonal affective disorder from the long gray winters?'

Compared to Miami Beach absolutely...unlesssssss....the inhabitants of the latter are paranoid re: direct sunlight and hide like moles to escape the sun. It comes down to personal habits- sun exposure without sun block SPF 8 (or more), a wee bit re: diet, and most of all supplementation. On average there is much less D3 making time in Seattle vs. Miami Beach but the lifestyle choices are the discriminating factor. 90% of all D3 comes from sun exposure...or does not. Therefore it's a matter of the serum level in the blood- if no sun then does one supplement? Age and general health are very important- obesity- also dark skin or light skinned.

There are other factors regulating flu transmission but what works very effectively is to keep healthy D3 levels and gain near immunity from flus and colds. There is a tremendous amount of anecdotal evidence on this from doctor's offices and clinics who have seen it work themselves. Absolutely no doubt about it. The challenge now is to re-educate Americans that it is D3, not vitamin C that beats colds and the flu.

Factoid: 97% of the most popular, available sunscreens block beneficial UVB rays (they make the D3 precursor) and allow the damaging UVA to penetrate to the lower dermis, wreaking carcinogenic havoc (and wrinkles). Wearing these sun blocks give the false impression of safety (skin will not turn red) while in fact it is just the horrible opposite. This surreal scenario is the rule, not the exception, and one wonders how this could happen in America?



I find this research flawed, perhaps these findings where in an unrealistic setting. I am in the military (Navy) and we can get underway for days or months on end. Now, for some of the sailors' onboard, sunlight is a non observed action. We have individuals that work nights, and sleep days. Unless, a person is an avid smoker, or finds time to PT (physically train) top side; they can go long periods without exposure. However, these individuals never fall ill to any of the particular viruses that are around during the season. So why is that?


My child came home from school saying that they learned to cough and sneeze into their elbow with Germy Wormie, and I was totally taken aback. I always covered with my hands. But I went to the website and now I get it, hands touch, elbows don't!! Kids can touch 300 surfaces in 1/2 hour and they hate to wash their hands. This is a simple thing that can make a huge difference.


Mexico may enjoy the tropical sunshine, but the pollution in Mexico City means you won't get a suntan - and you won't make any Viramin D, as both need UV.

Also: has anyone invented an effective repellant for vitamin pill salesmen? There's a nasty swarm of them buzzing this blog.