Although chickenpox is a generally mild, self-limited illness of children, it can cause fatal disease in adults. Accumulating reports from tropical countries showed a high prevalence of seronegativity among the adults, implying that varicella diseases could become a heavy burden in tropical countries. However, in the situation of humanitarian emergencies in tropical areas, chickenpox has largely been ignored as a serious communicable disease, due to lack of data regarding varicella mortality and hospital admissions in such a context. This is the first report describing an outbreak of chickenpox in a refugee camp of tropical region. In 2008, we experienced a varicella outbreak in ethnic Lao Hmong refugee camp in Phetchabun Province, northern Thailand. The attack rate was 4.0% (309/7,815) and this caused 3 hospitalizations including one who developed severe varicella pneumonia with respiratory failure. All hospitalizations were exclusively seen in adults, and the proportion of patients 15 years old was 13.6% (42/309). Because less exposure to varicella-zoster virus due to low population density has previously been suggested to be one of the reasons behind higher prevalence of susceptible adults in tropics, the influx of displaced people from rural areas to a densely populated asylum might result in many severe adult cases once a varicella outbreak occurs. Control interventions such as vaccination should be considered even in refugee camp, if the confluence of the risk factors present in this situation. © 2010 Shimakawa et al; licensee BioMed Central Ltd.

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