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On the basis of the ability of the attenuated cold-adapted strain of influenza A virus to suppress disease production in ferrets simultaneously infected with epidemic influenza virus (P. Whitaker-Dowling, H.F. Maassab, and J.S. Youngner, J. Infect. Dis. 164:1200-1202, 1991), an evaluation of the ability of the cold-adapted virus to modify clinical disease in humans was made. Adult volunteers with prechallenge serum hemagglutination-inhibition titers to the influenza A/Kawasaki/86 (H1N1) virus of < or = 1:8 received either 10(7) 50% tissue culture infective doses of the wild-type A/Kawasaki virus or a mixture of 10(7) 50% tissue culture infective doses of each of the wild-type virus and a cold-adapted A/Kawasaki reassortant virus by intranasal drops in a randomized, double-blind fashion. Symptoms and wild-type virus shedding were assessed daily for 6 days following challenge. Results were compared with those derived from another group of volunteers who received only cold-adapted virus. Volunteers who received the mixed inoculum of cold-adapted and wild-type viruses had lower symptom scores than those who received wild-type virus alone, suggesting that coinfection with the cold-adapted virus may modify wild-type virus infection, but the differences were not statistically significant in this small study. The data demonstrate that administration of cold-adapted influenza A virus to humans at the time of wild-type virus infection is a safe procedure.