Mar 13 2020
Myth: infectious diseases are seasonal because of weather changes and depend significantly on local climate parameters such as temperature, humidity, and rain.
Fact: Seasonal change in the incidence of infectious diseases is a common phenomenon in both temperate and tropical climates. However, the mechanisms responsible for seasonal disease incidence, and the epidemiological consequences of seasonality, are poorly understood with rare exception. For instance, in the Northern Hemisphere, the flu season can begin as early as October and can last as late as April or May. In the temperate regions of the Southern Hemisphere, influenza activity typically occurs during April – September. In the tropics, influenza activity occurs throughout the year.
Factors influencing apparent seasonality are:
Congregation of children during school terms;
Congregation of adults for work and entertainment in contained spaces;
Time spent outdoors is typically longer in the summer, although winter sports such as skiing also allow more time outdoors;
Seasonal use of heating and air-conditioning. Both heaters and air conditioners are essentially vacuum cleaners without a bag collecting dust. They suck air in, warm or chill it, and expel air back into the contained space. They also accumulate water inside of units. The filters which are expected to collect dust are usually unable to catch smaller particles, such as viruses.
Central heating and air-conditioning in private homes, communal, office and industrial buildings with no windows or with non-opening windows;
The prime example is an outbreak of severe pneumonia among attendees at an American Legion convention in 1976. The initial outbreak involved 182 persons and was linked to contamination of the air conditioning system at the hotel where the convention was held;
The relative importance of seasonal factors depends on the local context. For example, winter peaks in the incidence of measles in temperate regions are likely to be caused not only by school terms, but also by lower indoor humidity, which favors survival of the virus in the air. In the tropics, measles incidence peaks during the dry season and the association with school terms is not apparent. Instead, increased survival of the virus in the dryer air may be the key determinant of these dry season peaks;