Volume 7, Number 3—June 2001
Seasonal Variation in Host Susceptibility and Cycles of Certain Infectious Diseases
|Pathogens peak at characteristic times in all seasons of the year||Winter: influenza, pneumococcus, rotavirus Spring: RSV, measles Summer: polio, other enteroviruses Fall: parainfluenza virus type 1|
|Timing and duration of peaks for each pathogen are similar from year to year||Measles: regular pattern since 1703 (1) Influenza: annual peak varies by only 5 to 10 weeks in the United States (6)|
|Onset of epidemics often occurs simultaneously in areas that are geographically dispersed and have different weather conditions and diverse populations||Influenza: simultaneous outbreaks across North America, 16 European countries, and 6 Chinese provinces (7) Pneumococcus: simultaneous outbreaks in seven surveillance areas (8)|
|Latitude is a critical determinant of timing and magnitude of peaks||An increasing magnitude of seasonal peaks as distance from the equator increases has been documented for polio (9) and rotavirus (10) and reported for influenza (11).|
|Pathogens can be detected in the off-season despite lower incidence of disease and virtual absence of epidemics||Meningococcus: no decrease in carriage in the off-season, despite absence of epidemic disease (12) RSV: sporadic summer viral isolation but no epidemic spread (13) Influenza: sporadic summer isolation, occasional clusters of disease without epidemic spread (14)|
RSV = respiratory syncytial virus. RSV peaks in the winter or spring in the United States, depending on location. For simplicity, it is listed here as a spring pathogen.