Backcalculation of the disease-age specific frequency of secondary   transmission of primary pneumonic plague
  Aim: To assess the frequency of secondary transmissions of primary pneumonic plague relative to the onset of fever. Methods: A simple backcalculation method was employed to estimate the frequency of secondary transmissions relative to disease-age. A likelihood-based procedure was taken using observed distributions of the serial interval (n = 177) and incubation period (n = 126). Furthermore, an extended model was developed to account for the survival probability of cases. Results: The simple backcalculation suggested that 31.0% (95% confidence intervals (CI): 11.6, 50.4) and 28.0 % (95% CI: 10.2, 45.8) of the total number of secondary transmissions had occurred at second and third days of the disease, respectively, and more than four-fifths of the secondary transmission occurred before the end of third day of disease. The survivorship-adjusted frequency of secondary transmissions was obtained, demonstrating that the infectiousness in later stages of illness was not insignificant and indicates that the obtained frequencies were likely biased on underlying factors including isolation measures. Conclusion: The simple exercise suggests a need to implement countermeasures during pre-clinical stage or immediately after onset. Further information is needed to elucidate the finer details of the disease-age specific infectiousness. 
Hiroshi Nishiura