HPIV Seasons

In the United States, human parainfluenza viruses (HPIVs) commonly cause respiratory tract illnesses.

There are four types of HPIVs and two subtypes that circulate at different times of the year.

• HPIV-1 infections often cause croup in children. There are usually more cases in the fall of odd-numbered years.
• HPIV-2 infections can also cause croup. HPIV-2 infections occur more commonly in the fall each year. It is less frequently detected than HPIV-1 and HPIV-3.
• HPIV-3 infections usually occur in spring and early summer months each year. However, HPIV-3 infections can occur throughout the year, particularly when HPIV-1 and HPIV-2 are not in season.
• HPIV-4 (subtypes 4a and 4b) seasonal patterns are not as well characterized.

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