Why is Flu Infection More Common in Winter?

In northern latitudes, flu season peaks November through March, typically the coldest months of the year. In the southern hemisphere, winter, and flu season, is May through September. So there are two different flu season, one in each hemisphere. But the reason for the flu's seasonality has long remained a mystery.

Infection Risk Factors That Aren't Seasonal

There are many factors that can increase a person's risk of becoming sick from any infectious respiratory illness. An immune system run down from lack of sleep and other body-abusing habits won't work well to protect a person from disease. Bad hygiene and infrequent hand washing increase the transmission of infectious disease. But these risk factors don't necessarily fluctuate seasonally.

Theories on Why Flu Is Seasonal

Researchers have speculated on, and investigated, many different causes of influenza's seasonality. Does flu peak in winter because people spend more time near each other while staying indoors during inclement weather? Reduced levels of sunlight in winter, which result in the body producing less vitamin D, has also been suspected as contributing to seasonal outbreaks of influenza. Remember mom's favorite explanation-that being exposed to cold weather could make you sick? (That belief is actually how the common "cold" got its name).

But there are holes in all of these theories. If close quarters was the main contributor to the spread of influenza, why wouldn't flu season begin as soon as school's back in session, in September? Kids are notorious sharers of germs. Vitamin D deficiency does appear to be a factor in increasing a persons susceptibility to catching influenza (Cannell, 2006), but still doesn't provide a complete explanation.

How Cold, Dry Weather Helps the Flu Spread

Cold weather does increase the number of influenza infections, but not in the way that people have traditionally believed. You can't catch a cold or the flu just from being outside on cold weather. Flu is caused by infection with the influenza virus, and it turns out that the influenza virus is better able to spread at lower temperatures and humidity levels.

Flu viruses do not spread in the same way that cold viruses do. Cold viruses are mainly spread by direct contact, such as when a person touches a surface where cold viruses have been left by the hands of others, or when an infected person shakes hands.

In contrast, flu viruses are transmitted through the air, in the aerosol droplets of coughs and sneezes. Influenza viruses are more stable (last longer) in cold air. And low humidity levels help the infected droplets remain in the air for a longer period of time, making it more likely that someone else will breathe them in. When the air is more humid, these droplets pick up water, get bigger, and fall to the ground, where they can't cause infection (Lowen, 2007).

Reducing Transmission of the Flu

The best defense against catching the flu is to get vaccinated for seasonal and novel H1N1 influenza, and to wash hands and use hand sanitizer at regular intervals. To prevent the spread of colds and flu, always cover coughs and sneezes with fabric, such as a sleeve or tissue, rather than with hands.

Sources

Cannell J. et a. (2006) "Epidemic influenza and vitamin D". Epidemiol Infect, Volume 134, Issue 6.

Lowen, A.; Mubareka1, S.; Steel,J.; Palese, P. (2007). "Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature". PLoS Pathogens, October Issue.