The flu season is long overdue in the big cities

During the winter, the humidity decreases "and thus, the virus remains viable longer in the air, which effectively widens the cloud of risk," he explained.

Researchers have found, however, that in large cities, where people are stormed, dry weather in winter is less important.

"If an infected person is sitting right next to you, the specific moisture content does not matter," said Dalziel. "It helps the virus to find a host even when the weather is not optimal."

Dalziel and his colleagues combined six years of influenza case data with census data showing where people live and work in 603 different cities across the United States.

Investigators have found that in large metropolitan areas, influenza cases are more prevalent, including at the beginning and at the end of the season when the weather is not conducive to transmission. New York and Miami are examples of cities where the flu season is longer, said the study's authors.

On the other hand, small towns tend to have short, well-grouped influenza seasons around the usual peak in winter. Atlanta and Nashville are examples, where the flu hits for a more concentrated period.

The researchers noted that the risk of contracting influenza remains the same in all cities.

Viboud said, "This does not show that some cities are safer than others against the flu, it shows the relative difference in the timing of the cases."

The data shows that public health officials must take into account the structure of their metropolitan area when planning each influenza season, the researchers suggested.

Large cities with longer influenza seasons will have to prepare for cases to start appearing earlier, for example.

"On the other hand," said Viboud, "in small towns, a more intense epidemic could overburden the health care system, making it particularly difficult to cope with epidemics."

The study shows how the rise of megacities could make a new influenza pandemic more likely, said Dr. Amesh Adalja, senior researcher at the Johns Hopkins Center for Health Security in Baltimore. He was not involved in the new study.