Household size is critical to varicella-zoster virus transmission in the tropics despite lower viral infectivity.

The epidemiology and severity of infections can vary dramatically in different geographical regions. Varicella zoster virus (VZV) is a particularly tractable model for investigating such global differences, since infections can be unambiguously identified. VZV is spread by aerosol to cause chickenpox, which, in temperate countries, is a relatively benign childhood infection; yet in tropical countries it tends to occur at later age, a trend associated with markedly increased severity including complications, hospitalization, and overall burden of care. To investigate global differences in the epidemiology of chickenpox we studied a population in Guinea Bissau, which in contrast to other tropical countries has an unexpectedly early age of infection with VZV, comparable to temperate latitudes. In this study we used detailed records from over 3000 houses during an outbreak of chickenpox, combined with viral genetic information on routes of infection, to obtain precise estimates of disease transmission within and between houses. This community contains many large households in which different families live under a single roof, in living quarters divided by partitions. Our data show that household infectivity in tropical Guinea Bissau is reduced four-fold compared with temperate climates (14.8% versus 61-85%), with an intermediate rate between members of the same family who are in more intimate contact (23.5%). All else being equal, these lower infection rates would be expected to lead to a later age of infection as is commonly seen in other tropical countries. The young age of infection, which had drawn our attention to the Guinea Bissau population, can however be explained by the exceptionally large household sizes (mean 14.5 people). We have combined genetic and demographic data to show that the epidemiology of chickenpox in tropical Guinea Bissau is dependent on the interaction of the social and physical environments. The distinctive clinical presentation of VZV and its ubiquitous distribution make it an attractive model for estimating the variables that contribute to global differences in the transmission of airborne viruses.