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Virus replication in the upper respiratory tract after H1N1pdm virus challenge. Nasal washes were collected on days 1, 2, 4, 7, and 9 after H1N1pdm virus challenge, and virus titers were determined by endpoint titration in MDCK cells. Arithmetic mean titers are given, and error bars indicate standard errors. *, P < 0.05; **, P < 0.001.
