Influenza is a winter disease. For decades, scientists have wondered why the virus is epidemic in the winter months but relatively absent the rest of the year. The first step to answering the question is to look at patterns. While we take a break from influenza during the North American summer, flu attacks the Southern Hemisphere, and in the tropics, flu rates are relatively stable throughout the year. Could it be the weather?
What makes summer different from winter? People move indoors, the air is colder, and there is a lot less sunlight. Sunlight has many effects on people. Light and dark cycles cause changes in levels of brain chemicals such as melatonin. Skin exposed to sunlight produces vitamin D, and many North Americans are vitamin D deficient in the winter. Which of these might make flu “catchier”? Some? All?
Influenza is spread mainly through respiratory droplets. When a flu victim sneezes and coughs they send out thousands of droplets of flu-laden fluid. When these droplets are the right size, they linger in the air for the next person to inhale. As the cold forces us inside, people are exposed to more droplets; or so goes one hypothesis. It’s hard to come up with a really good human influenza study. Unlike cold viruses, which have been tested in humans, influenza is a potentially deadly virus. It would in most cases be unethical to expose research subjects to it. Ferrets are often used as stand-ins for humans, but as you might imagine, they aren’t the most cooperative subjects.
Generally, we’re stuck looking for patters, for example looking at influenza rates and sunlight patters and making inferences, or giving vitamin D or placebo to a population and then seeing who gets the flu. Results of vitamin D studies have been mixed, and it’s unlikely to be the only or even primary answer to flu transmission. One of the most intriguing intriguing ideas is that it isn’t so much we who change during the winter but the virus itself. Studies in guinea pigs have shown that flu is easier to transmit in cold, dry conditions. This month in the open access journal PLoS One, a group is reporting a clever experiment, one that involves coughing mannequins.
The scientists placed coughing, breathing mannequins in a room where the temperature and humidity can be measured. Air samples were collected and tested for flu. What they found was dramatic. The infectivity of aerosolized flu expelled by the dummies dropped off rapidly as the relative humidity rose to about 43%.
This has huge implications for controlling flu epidemics—potentially. If these findings can be verified in real people, control of indoor humidity may help stem epidemics. Right now, the relative humidity in my house is 27%. It’s not terribly comfortable, but I’m not worried about catching the flu at home. I don’t know what the relative humidity is at my office, the hospital, or on the bus. Might modestly humidifying these areas lessen the impact of flu epidemics? I look forward to finding out.