Proportion of serum samples with contemporary HAI and CF titers ≥1∶10 in 4 control groups.

Group 1: T0 and T1: paired acute and convalescent serum samples from patients with A/H1N1pdm infection confirmed by RT-PCR.Group 2: serum samples submitted to our laboratory for HIV antibody screening and resulted HIV-negative in September 2008, from males matching the age group represented in the study population.Group 3: serum samples submitted to our laboratory for HIV antibody screening and resulted HIV-negative in September 2009, from males matching the age group represented in the study population.Group 4: serum samples submitted to our laboratory for HIV antibody screening and resulted HIV-negative, 2 months after the pandemic influenza peak in Italy (February 2010).