Case-to-Case Intervals of Rhinovirus and Influenza Virus Infections in Households

The incubation period is usually quoted in textbooks as one to two days for influenza virus and one to three days for rhinoviruses. Ideally, the incubation period is estimated from a single short exposure until onset of symptoms. Household studies of transmission as a reflection of incubation period are marred by the fact that the exact time when the transmission actually occurred is unknown. It could occur before onset of symptoms or, more likely, after a couple of days of symptoms when the source patient has increased secretions. The authors analyzed the case-to-case interval of laboratory-confirmed overt influenza virus and rhinovirus infections in household members to determine the usual time of occurrence of symptomatic illness in contacts after an index case became ill.