Epidemiology of COVID-19

1. Introduction

Coronoviruses have been reported as causes of mild and moderate respiratory infections forover 50 years [1]. Even though this group of viruses have been isolated from many different animals, bats are accepted major natural reservoir of coronaviruses [2,3].

Four human coronavirus, 229E, HKU1, NL63 and OC43, are known as causes of common cold in humans [1]. However, recently-detected coronaviruses, SARS CoV (2002), MERS-CoV (2012) completely altered all known approaches about this virus group because these viruses caused severe acute respiratory infections and nosocomial outbreaks. In the end of 2019, a novel coronavirus, now known as SARS-CoV-2 (2019), suddenly emerged in Wuhan, China. The World Health Organization declared that the epidemic is a public health emergency of international concern on January 31, 2020. As of April 16, 2020, the emerging coronavirus infection, COVID-19, has been spreading worldwide, causing over 2 million cases and over 137 thousand of death.

In as much as this disease recently taken a significant place in our daily practice, as a new group of respiratory diseases, due to their higher rates of transmissibility, hospitalization, intensive care unit admission, severity of disease, mortality and so on, we should pay more attention than past to prevention and treatment of coronavirus infection.

2. Origin of the SARS CoV-2 virus

The first cases of coronavirus disease COVID-19 were directly related to an animal market in Wuhan, China. Early investigations suggested that the origin of SARSCoV-2 may be bats. Zhou et al. demonstrate that SARS CoV-2 possesses 96% nucleotide identity with a bat coronavirus, for instance BetaCoV/RaTG13/2013 [4].

3. Epidemic curve of infection

An epidemic curve of infection is a statistical chart used in epidemiology to visualize the onset of the coronavirus outbreak. In an epidemic curve, there are three zones: increasing, plateau, and declining phases.

Increasing phase: This period is affected by many different parameters such as country demographics, age distribution, preparedness of health system to an outbreak, implementation of some preventive measures, country reaction time to a pandemic, reaction of society to new implementing rules. Different countries can exhibit quite different curve patterns and these facts could complicate to make any assumption about pandemic pattern for a country (Figure 1). However, it seems that this period takes generally 3 or 4 weeks for COVID-19 (Table 1).

Plateau phase: At this phase disease incidence is stable. According to daily country reports it takes 2 or 3 weeks for COVID-19 (Table1).

Decreasing phase: Today we have only China’s data about this phase (Figure 2), showing that 2 or 3 weeks later disease activity could be detected very low levels.

4. Incubation period

The incubation period of COVID-19 was defined 5.2 days (95% CI, 4.1–12.5 days), 5.1 days (95% CI, 4.5 to 5.8 days) and 4 days in three separate study [5,6]. However, in a familial cluster of 5 patients, this period was reported between 1 and 19 days [7]. These data show that the incubation period of COVID-19 was similar to MERS and SARS and it’s a bit longer than influenza [8].

5. Severity of infection

COVID-19 has been reported form all aged patients. Severity of infection could be varied from asymptomatic infection to critical disease. Clinical severity of COVID-19 was defined in 5 groups as asymptomatic, mild, moderate, severe, and critical. Diagnostic criteria of these groups were as follows [9, 10];

1. Asymptomatic infection: Without any clinical sign of symptoms with positive SARSCoV-2 PCR test.

2. Mild: Symptoms of acute upper respiratory tract infection, including fever, fatigue, myalgia, cough, sore throat, runny nose, and sneezing without pneumonia.

3. Moderate: With pneumonia, frequent fever and cough; some may have wheezing, but no obvious hypoxemia such as shortness of breath.

4. Severe: Rapid progression around 1 week, dyspnea, with central cyanosis, oxygen saturation less than 92%, with other manifestations of hypoxemia.

5. Critical: Patients with acute respiratory distress syndrome (ARDS) or respiratory failure, shock, multiple organ dysfunction.

This clinical classification is also important because it gives some clues about prognosis and mortality of COVID-19. Most COVID-19 cases (81%) were classified as mild or moderate disease in adults, and in children most cases were mild. [9,11,12]. In critical cases mortality could be as higher as 50% in adults [9,11].

6. Route of transmission

In spite of the fact that some animals are thought as origins of the virus, a major transmission mode for SARS CoV-2 is human-to-human transmission like SARS CoV and MERS CoV. Accepted transmission routes of COVID-19 are droplet and contact transmission [18]. Nevertheless, explaining or excluding of other possible transmission routes is necessary for controlling disease in the community.

7. Transmission period

People infected with SARS CoV-2 can be contagious prior to onset of symptoms. In a study, 13% of patients were contagious before onset of symptoms [30]. In a familial cluster, 3 of 8 laboratory-confirmed patients were asymptomatic [31]. Presymptomatic transmission would be a challenge to contact tracing [32]. These kind of reports show that asymptomatic carriers emerge as an important target group to be taken into consideration in the control of the disease.

Viral shedding might be longer than previously thought. In a retrospective multicenter cohort study of 191 adult patients, the median duration of viral shedding was found to be 20 days (IQR 17.0–24.0) and the longest observed duration of viral shedding in survivors was 37 days [33].A study of postdischarge surveillance of patients after clinical recovery, two patients were found positive [34].

8. Susceptible ages

Although COVID-19 was reported from every aged patient, patients with advanced age seem more susceptible to infection. An early report shows that the most affected age group was middle ages (Table 2). Among children and young adults, infection rates were very low and it changed between 0.8%–4.0%. In addition, asymptomatic infection rate is quite high in this ages.

9. Transmissibility and R0

Early reports from China show that a reproductive number for COVID-19 was 2.2 to 2.7 days [5]. This means that the number of infected person will be doubled every 6–7 day. A reproductive number of the outbreak on the Diamond Princess Cruise Ship also supports this preliminary data [35]. However, Sanche et al. found that a serial interval was 5.7 days (95% CI, 3.8–8.9 days) [36]. In another study which prefer to use real-time reproduction number (Rt) instead of basic reproduction number (R0), Yuan et al. showed that their Rt values for Italy, Germany, France and Spain were 3.1, 4.43, 6.56, and 3.95, respectively [37]. Liu et al. found that average R0 was 3.8 (1.4–6.49) in their review of 14 studies [38]. This study also showed that even in the same geographic region, different Ro values can be calculated by using different methods and assumptions. It seems that we need more data to determine more accurate R0 value.

10. Case fatality rate

Overall case fatality rate was 6.3 according to WHO situation report as of April 13, 2020 [39]. Nevertheless, a remarkable difference in mortality rates was also noted between countries as given Table 3. It was noted that this rate is extremely higher in country with older-aged populations. In Italy, the median age of those died from COVID-19 was 78 years, whilst the median age of patients was 62 [40]. In Turkey, the current case fatality rate is 2.1% which is a significant issue to bear in mind. Mortality associated with COVID-19 is a multifactorial process and underlying diseases and health care burden as well as ages could be related to higher mortality [41].

10.2. Risks for disease and death

Disease progression can be rapid and the median survival time can be as short as 5 days in advanced aged patients [28].

In a metaanalysis which evaluated 46,248 patients from eight studies, most prevalent comorbidities were hypertension, diabetes mellitus, cardiovascular diseases and respiratory diseases. Another finding of this study was that these comorbidities were more likely detected in severe patients [47]. Another metaanalysis showed that hypertension, cardiovascular diseases, diabetes mellitus, smoking, chronic obstructive pulmonary disease, malignancy, and chronic kidney disease were most frequently detected underlying diseases among hospitalized patients [48].

In a summary report form China CDC, preexisting comorbid condition can increase fatality rate by 10.5% for cardiovascular disease, 7.3% for diabetes, 6.3% for chronic respiratory disease, 6.0% for hypertension, and 5.6% for cancer [9].

According to the Italian weekly report on death from COVID-19, most frequently detected comorbidities were hypertension, diabetes mellitus, ischemic heart disease, and chronic renal failure as 72%, 31.5%, 27.4%, and 23.5%, respectively. This report showed that patients without any comorbidities accounted for only 2.8% of those died from COVID-19 [40].

In a pooled analysis, risk of severe disease and mortality was higher in hypertensive patients nearly 2.5 fold increases especially among advanced aged individuals [49].

12. Conclusion

Every day we learn new data about the current pandemic of COVID-19. Since the outbreak has not been over yet, the updates should be followed closely to monitor the disease and the risk factors as well as therapy modalities.

Acknowledgments/disclaimers

Cemal BULUT, MD, Prof. worked with the Turkish Red Crescent, Ministry of Health and Disaster and Emergency Management Presidency in different disaster such as Pakistan’s Flood Disaster, Somalia, Darfur Sudan, Banda Aceh, Endonesia.

Yasuyuki KATO, MD, MPH, Prof. is the former Chief Physician of Division of Preparedness and Emerging Infections Disease Control and Prevention Center National Center for Global Health and Medicine of Japan.