Abstract
BackgroundA measles outbreak involving 60 cases occurred in Yamagata, Japan in 2017. Using two different mathematical models for different datasets, we aimed to estimate measles transmissibility over time and explore any heterogeneous transmission patterns.MethodsThe first model relied on the temporal distribution for date of illness onset for cases, and a generation-dependent model was applied to the data. Another model focused on the transmission network. Using the illness-onset date along with the serial interval and geographical location of exposure, we reconstructed a transmission network with 19 unknown links. We then compared the number of secondary transmissions with and without clinical symptoms or laboratory findings.ResultsUsing a generation-dependent model (assuming three generations other than the index case), the reproduction number (R) over generations 0, 1, and 2 were 25.3, 1.3, and <0.1, respectively, explicitly yielding the transmissibility over each generation. The network data enabled us to demonstrate that both the mean and the variance for the number of secondary transmissions per primary case declined over time. Comparing primary cases with and without secondary transmission, high viral shedding was the only significant determinant (P < 0.01).ConclusionsThe R declined abruptly over subsequent generations. Use of network data revealed the distribution of the number of secondary transmissions per primary case and also allowed us to identify possible secondary transmission risk factors. High viral shedding from the throat mucosa was identified as a potential predictor of secondary transmission.
Highlights
Measles, a highly infectious disease, is caused by a virus belonging to the Morbillivirus genus in the Paramyxoviridae family.[1]
Owing to its substantial capacity for airborne transmission, the virus can be transmitted in an open space, yielding a large basic reproduction number; the average number of secondary cases generated by a single primary case in a fully susceptible population has been documented to range from 10 to 20.8–11 Immunization with the measles-containing vaccine (MCV) is an effective and promising preventive option, but the disease has yet to be eliminated from many parts of the world
As the local government of Yamagata performed contact tracing and raised public awareness via swift media announcements, we evaluated the control measures by estimating the generation-dependent reproduction number (R), which we defined as the average number of secondary cases produced by a single primary case in each respective generation, using our original generation-dependent model
Summary
A highly infectious disease, is caused by a virus belonging to the Morbillivirus genus in the Paramyxoviridae family.[1]. Vaccination against measles has been accepted as part of the routine immunization program since 1978. With this effort, the persistent chains of transmission for the local D5 strain were interrupted, with the last isolation of this genotype in May 2010.4,27,28 Japan was verified as measles-free in 2015. Using two different mathematical models for different datasets, we aimed to estimate measles transmissibility over time and explore any heterogeneous transmission patterns
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