The epidemiology of COVID-19

Spread mainly through respiratory droplets or close contact, SARS-CoV-2 induced diseases has been growing dramatically in accordance with the published data from the World Health Organization (WHO). Although the number of confirmed cases in China has decreased a lot from late February 2020, and there is no report of COVID-19 deaths on 6th April, the confirmed cases of CoVs worldwide are still expanding with a vengeance. The spectrum of illness presentation or severity profile also affects triage and diagnostic decision-making, along with the therapeutic options and prognostic expectations. Till now, the exact source of the current outbreak of COVID-19 remains unclear, but the dynamic model is similar to the classic zoonotic emergence to human-to-human transmission. The mortality rate of SARS-CoV-2 (∼3.8%) is lower compared to that of MERS-CoV (37.1%) or SARS-CoV (10%), but the number of infections is more than ten times higher. With respect to the lack of evidence that companion animals might be a source of infection, patients with COVID-19 are the prime source of infection, and those with severe conditions are more infectious than those with mild conditions. Intriguingly, asymptomatically infected persons or patients in incubation has also been demonstrated to shed the infectious virus, serving as a potential infection source to drive the transmission of the COVID-19. In addition, researches focused on the follow-up of recovered patients revealed that the tested samples of rehabilitees continuously showed a positive RT-PCR result, implicating that asymptomatic infection during incubation or recovery from COVID-19 may pose a daunting challenge to disease control and prevention.

The incubation period refers to the time between exposure to the virus and initial symptoms. A research report on the early propagation dynamics of COVID-19 unveiled that the average incubation period of COVID-19 was around 5 d, and its 95% distribution was 12.5 d. Another study analyzing the travel history and symptoms in 88 confirmed cases revealed a similar average incubation period of around 6 days. In addition, there was an unusual case with an incubation period of up to 19 d. Although such a long incubation period may be a low probability event (the condition of 14 d was suggested by experts for quarantine), the longer incubation time indicates the adjustment of screening and control policies.

The early outbreak data of COVID-19 largely follow exponential growth. Disparate models based upon the clinical progression of the disease had been proposed to assess the basic reproductive ratio R0. A retrospective analysis of the first 425 identified cases demonstrated that in the early stages of COVID-19, the R0 was assessed to be 2.2. Nevertheless, deterministic compartmental models based upon the likelihood and a model analysis revealed that the control reproduction number Rc might be as high as 6.5 due to the estimation of four generations of viral transmission and serried social contacts. In this regard, it is noteworthy that R0 estimates may vary in the light of numerous biologics, social behavior, and environmental factors. In general, the basic R0 assessed by the majority of researches ranges between 2 and 4. According to the WHO data updated on May 28, 2020, more than 215 countries have reported 5593, 631 confirmed cases, including 353, 334 deaths. The grand total case fatality rate of global cases outside China is 1.31%. The US had also recorded the largest number of coronavirus deaths in a day, with more than 1810 deaths reported on April 7th, according to data from Johns Hopkins University. Given the condition that the population of all races and ages is generally susceptible, there is an urgent need to further implementing the timely diagnosis, along with efficient isolation of patients, to cut down the R0 of SARS-CoV-2 and control the epidemic outbreak.

Endocrinology and Metabolism: Open Access is a peer reviewed journal which focuses on the publication of current research and developments on the endocrine glands and its secretions with their coordination with metabolism and reproduction.

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