Letter to the Editor 2009 H1N1 Influenza Virus Seroepidemiology

I read the recent report of 2009 H1N1 influenza virus seroepidemiology in Shiraz with a great interest (1). Moghadami et al. concluded that “High seroprevalence of antibody against 2009 H1N1 in the sera of our subjects describes either a high level of pre-existing immunity against H1N1 in Iranian population or a high rate of asymptomatic infection in our area compared to other countries (1).” The question is whether this finding is an evidence of a pre-existing immunity or an indication of a high rate of asymptomatic infection. Indeed, some previous reports confirming the pre-existing immunity among the elderly are available (2). If there is a previous cluster of asymptomatic infection, there should be homogeneity in seroprevalence in all age groups. The elderly who have the highest prevalence of antibody should not develop more asymptomatic infections compared to the younger group since the elderly usually have poorer immune function and physiological status.