Time dependent risk of cytomegalovirus infection in Japan.

Cytomegalovirus (CMV), a major cause of congenital infections, has high morbidity and mortality rates associated with it. However, a decline in the proportion of anti-CMV antibody-positive individuals has been observed. The present study aimed to quantify the time-dependent transmission dynamics of CMV infection in Japan by analysing the seroepidemiological datasets for pregnant women collected from five cord blood banks from 1996 to 2009. By employing a mathematical model and using the maternal age distribution of child births from the census data, we computed the seroprevalence among the pregnant Japanese women as a function of time. A decreasing trend was observed for the force of infection, i.e. the rate at which susceptible individuals are infected, which decreased from 0.04 to 0.03 (/year) over the period from 1996 to 2009. While the total number of births has steadily declined in Japan over time, the estimated number of live births at risk of CMV infection has increased over time. Our data reveal that in 2009 in Japan, at least 0.3 million women may have been at risk of contracting a CMV infection during the perinatal period. Moreover, about 2,726 congenital CMV infections were expected to have occurred in 2009. The average age at infection has already reached the child bearing age, and it must be noted that the age at infection can be elevated even more, reaching close to 30 years old which is the ongoing mean age at child delivery. It must be remembered that, if vaccine can become one of the options for the control of CMV in the future, the vaccination can lead to further elevation of age at infection, which may coincide with further elevation of mothers' age of delivery in Japan.