Bronchial hyperresponsiveness in normal subjects during attenuated influenza virus infection

Fourteen healthy male subjects with hemagglutination-inhibition antibody titers of 1:8 or less to homologous influenza A virus were studied. Six subjects received live, attenuated influenza virus by nasal drops and by aerosol. Although infection occurred in these six subjects, with the development of 4-fold or greater increases in hemagglutination-inhibition antibody titers, they remained asymptomatic. Eight subjects received placebo via the same route, and did not develop symptoms and showed no increase in antibody titer. Prior to administration of virus or placebo, histamine diphosphate aerosol increased airway resistance only slightly, and there was no difference between the virus and placebo groups. Two days after inoculation, bronchomotor responses in the placebo group were unchanged (p greater than 0.05), but in the virus-infected group, bronchomotor responses were significantly greater than in the preinfected state (p less than 0.01). Isoproterenol hydrochloride reversed and prevented the increase in airway resistance after histamine, suggesting that the bronchoconstriction was caused by smooth muscle contraction. Our findings indicate that transient, asymptomatic respiratory virus infection augments airway smooth muscle responses.