Brief Report

Abstract

Background Few previous studies have investigated the association between the severity of an infectious disease and the length of incubation period. Methods We estimated the association between the length of the incubation period and the severity of infection with the severe acute respiratory syndrome (SARS) coronavirus, using data from the epidemic in 2003 in Hong Kong. Results We estimated the incubation period of SARS based on a subset of patients with available data on exposure periods and a separate subset of patients in a putative common source outbreak, and we found significant associations between shorter incubation period and greater severity in both groups after adjusting for potential confounders. Conclusions Our findings suggest that patients with a shorter incubation period proceeded to have more severe disease. Further studies are needed to investigate potential biological mechanisms for this association.

Association between Severity of MERS-CoV Infection and Incubation Period

“…We estimated the incubation period of MERS-CoV cases during the recent MERS outbreak in South Korea and found that patients who died had a shorter incubation period than patients who survived. In a previous study, we found that the length of incubation period in patients infected with SARS coronavirus was also correlated with severity of the disease, with a shorter incubation period for patients who died (9). The pathogenesis of MERS-CoV and SARS coronavirus infection is similar (11), with a rapid progression to respiratory failure and intubation occurring ≈1 week after onset of symptoms and up to 5 days earlier in MERS patients than in SARS patients (4,12).…”

“…For most cases, information on exposure was recorded as intervals >2 days during which infection was believed to have occurred, rather than exact dates of presumed infection. For the subset of patients without available exposure data, we assumed that their incubation time was 0-21 days because 21 days was the longest incubation period reported (9,10). Data for patients is provided in online Technical Appendix 1 (http://wwwnc.cdc.gov/EID/article/22/3/ 15-1437-Techapp1.xlsx).…”

“…To estimate incubation period distribution, we fitted a gamma distribution that enabled interval censoring (6) by using Markov Chain Monte Carlo methods in a Bayesian framework (online Technical Appendix 2, http://wwwnc. cdc.gov/EID/article/22/3/15-1437-Techapp2.pdf) (9). In this analysis and analyses described below, we specified flat priors for each parameter and drew 10,000 samples from the posterior distributions after a burn-in of 5,000 iterations.…”

“…However, this analysis could not account for potential confounders. Therefore, we specified a multivariable logistic regression model in which death was the binary response variable and predictors included age, sex, and the incubation time for each patient (9). We performed this analysis by using an exact likelihood approach and incubation times resampled from the 10,000 posterior samples in each iteration (online Technical Appendix).…”

“…In 2015, South Korea had the largest outbreak of MERS-CoV infections outside the Arabian Peninsula (6). In a previous study, we reported that patients who died of severe acute respiratory syndrome (SARS) coronavirus infection had a shorter incubation period compared with infected patients who survived (9). The objective of this study was to examine the association between severity of MERS-CoV illness and length of incubation period.…”