Estimating human-to-human transmissibility of hepatitis A virus in an outbreak at an elementary school in China, 2011.

Hepatitis A is caused by hepatitis A virus and occurs worldwide. Estimating the transmissibility, which is usually characterized by the basic reproductive number R0, the mean number of secondary infectious cases generated by a single primary infectious case introduced into a totally susceptible population, provides crucial information for the effort required to stop infection spreading. Hepatitis A virus is usually transmitted indirectly through contaminated food and environment. An outbreak from March to June 2011 was reported to have occurred at an elementary school of 698 pupils in China and it was found that the outbreak was due to direct transmission between school children. Based on the symptom onset date and the social contact network of the children, in this study we estimate the serial interval (i.e. the gap in symptom onset between an infectee and its infector) and use different statistical methods to estimate R0. Combining with the positivity of IgG antibodies tests, we develop a compartmental transmission dynamics model which includes both asymptomatic and symptomatic infections to estimate the overall R0. Our analysis suggests a serial interval of mean = 23.9 days and standard deviation = 20.9 days. The different statistical methods suggest estimates for R0 in the outbreak varying from 2.1 to 2.8, and the estimates from the transmission dynamics model are consistent with this range. Our estimates are in agreement with that from one study in England but are higher than that from one study in the United States. Our transmission dynamics model suggests that the proportion of symptomatic infections is about 9%, implying that there were about 344 asymptomatic infections along with the 32 observed symptomatic cases. Furthermore, it is shown that the inclusion of asymptomatic infection in the epidemic process increases the estimate of R0 but does not do so greatly provided that the proportion of symptomatic infections is constant over the outbreak and there is no difference in transmissibility between symptomatic and asymptomatic infections.