Incubation period as part of the case definition of severe respiratory illness caused by a novel coronavirus

Abstract

Non-specific symptoms of acute respiratory viral infections make it difficult for many countries without ongoing transmission of a novel coronavirus to rule out other possibilities including influenza before isolating imported febrile individuals with a possible exposure history. The incubation period helps differential diagnosis, and up to two days is suggestive of influenza. It is worth including the incubation period in the case definition of novel coronavirus infection.

Figure 1. Probability of coronavirus infection given the incubation period of a case

The observed length of the incubation period of a case can partially help differential diagnosis. A. The probability of coronavirus infection given the incubation period, when comparing between coronavirus infection and influenza virus infection as possible diagnoses. We use 50% for each of the two viruses (i.e. coronavirus vs influenza virus) for a conservative argument to avoid an underestimation of the risk of novel coronavirus. Since known coronaviruses are classified into severe acute respiratory syndrome (SARS)-associated virus and non-SARS, and because influenza viruses are crudely classified as type A and B viruses, there are four possible combinations for comparison. HCoV stands for human coronaviruses other than severe acute respiratory syndrome (SARS), while Flu A and Flu B stand for influenza A and B viruses, respectively. B. The probability of coronavirus infection given the incubation period, using empirically observed viral aetiology data as a prior information among influenza-like illness cases in Madagascar [11] with a total of n=177 samples for those aged below 5 years. The observed number of isolates, i.e., Flu A (n=40), Flu B (n=5), human coronaviruses (n=12) and others (n=120), were used to calculate q_i in equation (1). n=12 for ordinary human coronaviruses is used as if it gave the frequency of a novel coronavirus, only for now, for the exposition of our theoretical idea. The incubation periods of viruses other than influenza viruses and human coronaviruses were assumed to be uniformly distributed from Day 1 to Day 10, for a conservative argument to avoid an underestimation of the probability of novel coronavirus.