Why is flu a winter problem?

At this time of year there is typically an epidemic of media hysteria about influenza. Recently, the perceived panic has focused on flu vaccine shortages.

But why is flu more prevalent in the winter, when the virus is clearly with us all year round? One suggested explanation is that in winter we tend to spend more time indoors in close contact with each other, allowing the virus to spread more easily between us. But many of us spend the whole year in close contact with others in our workplaces and on our crowded public transport.  

A more likely factor is winter weather. Cold air is generally less humid than warm air and, as a result, the mucus in our noses tends to dry out and be less effective as a defence against airborne infections.

Unfortunately, the flu virus thrives best in cold, dry air. It is protected from low temperatures and low humidity by a buttery coating that is rapidly dissolved when it comes into contact with the warm environment of the respiratory tract.

A further aspect of flu infection — and one that has only recently begun to be appreciated fully — is the role of cholecalciferol (vitamin D₃). Growing evidence suggests that this vitamin can be a major factor in preventing influenza infections.

Our main source of vitamin D is its natural production when the skin is exposed to ultraviolet-B light. But as winter approaches, the British sunshine becomes too weak for the vitamin’s manufacture and in any case we tend to wrap ourselves up well and lose the benefit of any sunlight that is still available. The vitamin reserves we built up during the summer begin to flag, giving the flu virus a greater opportunity to attack.

So, whether or not you have had had a flu jab this winter, it might be a good idea to boost your vitamin D levels by eating foods such as oily fish and eggs, or by taking an appropriate vitamin supplement. Alternatively, fly south for a couple of weeks of health-giving winter sunshine.