FACTBOX: How does weather affect the spread of flu?

(Reuters) - Weather experts are studying swine flu to see if climate could influence its spread and severity.

Determining the impact of cold, heat, dryness and humidity on the H1N1 strain -- which has killed up to 149 people in Mexico and had milder effects elsewhere -- could illuminate the countries and regions most vulnerable to infection.

Climate information may also provide clues into whether the virus is likely to return year after year on a seasonal basis.

Following are details about seasonality of regular flu, and the weather and other factors affecting its patterns, according to data from the World Health Organization, World Meteorological Organization, and leading scientists:

COLD AND DRYNESS

Flu incidence peaks in the winter season in temperate parts of the world, generally described as areas outside the tropics.

Cold and dry conditions help the virus survive outside the body, meaning that droplet particles take longer to evaporate and remain airborne for more extended periods in winter.

HEAT AND HUMIDITY

Influenza outbreaks do occur in the tropics, though less frequently than in areas with less heat and humidity, and without the seasonality seen in areas with wintry weather.

Poor laboratory and health data, especially from Africa and Latin America, have made it difficult for international experts to track viral transmission in tropical regions.

SCHOOL CYCLES

School cycles have also been shown to affect the seasonality of flu. Winter school breaks tend to reduce flu transmission to children by as much as 25 percent, mainly because they provide fewer opportunities for the virus to spread among large groups.

OTHER FACTORS

The precise strain of virus circulating, which can change from flu season to flu season, can also affect how virulent it is and how many people may fall ill in a given outbreak. The levels of immunity in the population, vaccination programs, hygiene practices and other practices also play a role.

(Reporting by Laura MacInnis)