Discussion
A systematic review was undertaken to describe the distribution of incubation period of SARS-CoV-2. The results suggested that the mean incubation period of SARS-CoV-2 is 6.24 days, ranging from 2.33 to 17.60 days. These values are within the range quoted by international organizations reporting 1 to 14 days [4], except for Zhang and al reporting a longer incubation period.
The incubation period might be discussed as an epidemiological parameter but also as a link with the early pathogenic process.
Without any vaccination or treatment available against SARS-CoV-2, incubation time is paramount for isolating infected cases and the quarantine of case contacts [8]. In the light of the results, a quarantine period of at least 14 days would be necessary to limit the transmission of the virus from the exposed cases. The retrospective investigation of cases is also associated with a relevant incubation definition. In the context of nosocomial cases or confined environments (i.e. classrooms in schools or cruise ships), knowing incubation will help for identification of exposed individuals.
The duration of incubation could also be relevant for establishing the prognosis of COVID-19 as illustrated in other studies (i.e. acute human immunodeficiency viruses for which a shorter incubation was associated with a faster disease progression [9]).
Integration of the virus to the host cell corresponds to the moment when a subject becomes is infected and the onset of the incubation period. Promptly after its emergence, the SARS-CoV-2 strain was identified to be responsible for human transmission during the incubation period [10]. In addition, previous studies have suggested that a short incubation period could have an impact on the severity of infection caused by coronavirus species [11]. Therefore, more knowledge about the link between the duration of incubation period and the viral load is essential to tailor clinical decision making and assess COVID-19 severity.
Some limits should be discussed. All studies were made from Asian case series and cohorts which might have induced a selection bias. Although these data can be extrapolated to other populations, that can be done carefully if incubation period is associated with underlying diseases or prevalence of some individual characteristics (i.e. obesity). Additionally, it has already been shown that one virus can have various incubation periods in different populations [12], which could be explained by host factors, the potential of mutation of the viral strain and the unknown animal reservoir. This meta-analysis was based on available data online. Some studies are still at the pre-print stage and had not yet been peer-reviewed. Definition of cases might differ across studies. Furthermore, study populations are made up of cases that have been investigated as part of point source outbreaks where incubation period was not the main goal of investigation.