Low secondary transmission of 2009 pandemic influenza A (H1N1) in households following an outbreak at a summer camp: relationship to timing of exposure

Following an outbreak of 2009 pandemic influenza A (H1N1) at a residential summer camp for boys aged 10-16 years, we assessed secondary household transmission of the novel virus after their return home. Of 212 study participants who attended camp, 49 had confirmed or probable influenza for a primary attack rate of 23%. Of 87 exposed household contacts who did not attend camp, only three instances of probable transmission were observed, for a household secondary attack rate of 3.5%. All secondary cases occurred in households where the ill camp attendee returned home 1 day after onset of illness, with an attack rate of 14.3% in household contacts in this category. Returning home after peak infectivity to others and advanced warning prior to reintegration of sick individuals into the household probably contributed to the overall low secondary attack rate observed.