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Flu; Why is it Seasonal?

Posted by Phil Heler, MD on February 14, 2019

As we speak there has been at least three main mechanisms of action fir the transmission of flu in winter

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Flu; Why is it Seasonal?

Why do flu epidemics happen in winter? Why is flu seasonal? Last week I wrote a comprehensive piece on flu and why it is a good idea to have the flu vaccination. The fundamental reason of course is that flu is highly contagious. Adults can spread the virus one day prior to the appearance of symptoms and up to seven days after symptoms begin. Since the onset of Swine Flu in 2009 caused by a subtype of influenza Type A (H1N1pdm09) in Mexico, most epidemics in the UK since this time have been linked to this specific flu strain. Flu vaccines have also taken this account, so we all have some level of immunity which is good news. However, since writing this, I have had a few emails asking me why is flu seasonal. This is a very good point.


This is not a very easy question to answer and there really are a potpourri of possibilities. This is something that science has been wrestling with for some time. What we do know is that in temperate regions flu epidemics recur at the same time each year. Meanwhile in the northern hemisphere the flu season is typically November to March, while in the southern hemisphere epidemics last from May until September. Although seasonality is one of the most familiar features of flu, it is also one of the least understood. As we speak there has been at least three main mechanisms of action that have been suggested and researched.

Low Vitamin D status compromises our immune system

The first issue relates to Vitamin D (or the ‘Sunshine Vitamin). We have only enjoyed about 24 hours of sunshine in Buxton this January! Vitamin D and its metabolites influence our immune system and response to infections. A body of latest research from 2018, carried by the National Institute for Health Research (NIHR), found that the use of vitamin D led to a 45 per cent reduction in viral upper respiratory infections in COPD sufferers (COPD stands for Chronic Obstructive Pulmonary Disease). Lead researcher Professor Adrian Martineau said: “Our study shows that giving supplements to vitamin D-deficient COPD patients nearly halves their rate of potentially fatal attacks”. 1.2m people have COPD in the UK, which is the cause of five per cent of the UK’s total deaths (around 30,000 per year), and costs the NHS £800m per year. Vitamin D is a cheap and very easy remedy but may also help many of us stave off flu!


Flu; Relative Humidity favours transmission

It has also been suggested that a key mechanism is relative humidity (RH) and ambient temperature. Much of the work on this question was done using some very unfortunate little guinea pigs, in the name of science, in 2007. This body of research exposed these poor animals to a range of RH (20-80%) and temperatures (5-30 degrees). Infected guinea pigs were housed with what were called naïve guinea pigs, in order to assess what the optimum conditions were for flu transmission. It was found conclusively that flu transmission rates were high at lower RH (20-35%) and completely blocked at 85%. Transmission also appeared to increase at 5 °C and with greater frequency than at 20 °C, while at 30 °C, no transmission was detected. Overall the data implies that at low RH, typical with indoor heating, and cold temperatures, courtesy of winter, the flu virus can spread very well!!

Our behaviour Changes in Winter

A third it is the effect of cold weather on our behaviour. It is true that we all spend more times indoors in enclosed environments. Schools play a key role in the seasonality of epidemic diseases. It has been shown that in winter breaks, such as half term, there is a 25% reduction in flu transmission (Cauchemez et al 2009). This is also supported by a study in India published in the Journal of Epidemics in 2013. India was badly affected by Swine flu in 2009. The subsequent analysis suggested that the school holidays had a significant influence on epidemiology. It was estimated that school holidays reduced the transmission rates by 14-27% in different regions of India, relative to levels seen outside holiday periods.