A new US study on the Influenza viruses, published in the journal Nature Chemical Biology, has shed light on why winter is the flu season. The research, conducted by the National Institute of Child Health and Human Development (NICHD), one of the National Institutes of Health, has found out that viruses which cause flu coat themselves in fatty material that hardens and protects them in colder temperatures.

This gives them an edge in the cold temperatures of the winter season. The special insulator coating melts in the lungs and the virus is freed to infect the host.

"Like an M&M in your mouth, the protective covering melts when it enters the respiratory tract," said Joshua Zimmerberg, lead author of the study, quoted by Reuters. "It's only in this liquid phase that the virus is capable of entering a cell to infect it," he said.

Influenza viruses have an outside envelope protein, called hemagglutinin, hence the "H" in their names. In a warm environment such as a cell, the hemagglutinin is liquid enabling the virus to bud out. However, the envelope hardens at cooler temperatures, through a process called ordering.

This means that if the Influenza viruses are in a warm environment but outside an infectable host, such as is the case in summer, they die rather quickly and can hardly infect another person.

A panel of federal vaccine advisers recommended last week that all children aged from six months up to 18 should receive annual flu shots. The current recommendation is that children 6 months to 5 years of age are vaccinated. The recommendation coming from the U.S. Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices would cover an additional 30 million children.