Seasonal trends of human parainfluenza viral infections: United States, 1990-2004.

Background: Human parainfluenza viruses (HPIVs) are important causes of upper and lower respiratory tract illness among children and adults.

Methods: We describe seasonal trends for individual HPIV serotypes and respiratory syncytial virus in the United States using data on the percentage of specimens with positive test results reported to the National Respiratory and Enteric Viruses Surveillance System during the period 1990-2004. Onset and conclusion dates for peaks in activity were determined with the Early Aberration Reporting System's cumulative sum method C2 by detecting periods when the number of positive HPIV test results was significantly greater than that observed for preceding weeks for each serotype.

Results: During the study period, increases in the percentage of positive HPIV-3 and HPIV-2 test results occurred annually during April-June and October-November, respectively. Increases in the percentage of positive HPIV-1 test results occurred biennially during September-December during odd-numbered years. During years when HPIV-1 was not circulating, more HPIV-3 activity was reported, either as a longer spring season or as a second smaller period of increased activity noted in the fall. Seasonal peaks in respiratory syncytial virus activity occurred annually during November-April.

Conclusions: We provide a national perspective for HPIV activity during the 15-year study period and demonstrate distinct seasonal peaks in activity for HPIV-1, HPIV-2, and HPIV-3. In addition, our data suggest that there is an interaction between HPIV-3 and HPIV-1 activity, which may have implications in future prevention strategies.