Backcalculation of the disease-age specific frequency of secondary transmission of primary pneumonic plague

TL;DR

This study estimates the timing and frequency of secondary transmissions of primary pneumonic plague relative to disease onset, highlighting the importance of early intervention and providing a methodological framework for infectiousness assessment.

Contribution

It introduces a backcalculation method to estimate disease-age specific transmission frequency and extends it to account for case survival probabilities.

Findings

Approximately 28-31% of secondary transmissions occur within the first three days of disease.

Most secondary transmissions happen before the end of the third day.

Infectiousness in later stages is significant, suggesting early countermeasures are crucial.

Abstract

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.