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Centers for Disease Control & Prevention - USA,Ctr Dis Control & Prevent;
Prince Wales Hosp,Chinese Univ Hong Kong;
Virol Reference Dept,UK Hlth Secur AgcyDept Epidemiol,Univ Michigan;
VIRAL CHALLENGE MODEL; TEST-NEGATIVE DESIGN; SEASONAL INFLUENZA; UNITED-STATES; VACCINE EFFECTIVENESS; CIRCULATING VIRUS; COMMUNITY BURDEN; H1N1 INFLUENZA; SOUTH-AFRICA; INFECTION;