Why does the flu have a season?

Why Flu Has A Season

Most cases of influenza in the United States fall between the months of November and March, with the peak of infection rates in January and February. But that doesn't mean it is tied only with cold weather. Other places in the world have their flu season in warmer months.

It had been long held that this was most probably due to school children returning to schools and people being in closer proximity indoors in winter where they could pass all their germs around more easily. One of the most commonly cited studies used as a basis for this hypothesis was the "Seattle Virus Watch", done by John Fox, Carrie Hall, and friends. Another hypothesized explanation had been that our Vitamin D production is lower in winter due to less exposure of our skin to sunlight, and since Vitamin D improves the immune system's ability to fight off infections, our defenses were made weaker in winter with Vitamin D deficiency. Another commonly held belief was that in drier air our mucous tissues dry out and can crack making the viruses more easily introduced to the body. Some combination of all these factors may be at play.

However, the most recent studies have all seemed to point more to the different absolute humidity levels in winter compared to those in summer in the Northern Hemisphere. In February 2010 results were published of a study that used mathematical modeling of 31 years of data about the absolute humidity levels. That study, which was supported by the US National Institutes of Health (NIH) and in association with Oregon State University, reproduced the same seasonal variations in incidence of flu as has been observed for centuries or longer (although over that time frame the reason for the variations had not been known).

Most scientists now agree that there is a correlation between the absolute humidity levels and the spread of the flu, which several more recent studies have suggested. The studies indicate that variation in the absolute humidity is a major cause of seasonal cycles of influenza.

This also helps explain why in some locations in the world the flu season is during the warmer times and not in fall and winter. It is more the effect of absolute humidity and not the temperature itself that makes that difference. In areas with the flu season in warmer months, the absolute humidity is lower then, making the dry conditions that influenza viruses prefer despite warm temperatures. In the US, the absolute humidity levels are lowest during the peak of the flu season in January and February, the colder season.

Flu viruses like it dry: they spread more easily when it is drier and also survive longer outside a host in those conditions. There is some speculation that significantly increasing the humidity in your home in winter could help prevent flu infections.

Absolute humidity is a measure of the total amount of water in the air (without factoring in the temperature). Relative humidity, which is the measurement that we usually get in weather reports, is based on a combination of temperature and moisture. In some areas of the US in summer there can be four times more water vapor in the air than on a winter day. It is believed that this impacts the seasonal variations since flu does not like more water vapor. The saturation level of water vapor in the air varies with the temperature. Warm air can "hold" the same amount of water vapor at 30% relative humidity as cold air has at 60% relative humidity.

Therefore, while it may not be totally proven as the sole cause of flu seasons, it is very likely that the variation in absolute humidity is a major impacting factor for why we have seasonal flu infections.