INTRODUCTION
The current coronavirus disease 2019 (COVID-19) pandemic is one of the worst catastrophes to engulf humanity in recent times. The World Health Organization (WHO) had declared this outbreak as a pandemic on March 11, 2020 and so far it has claimed more than 182,467 lives in 214 countries (April, 2020).
THE AGENT
The causative agent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a newly emerged coronavirus, so that the human population is immune naïve and totally susceptible to it. Coronaviruses are spherical or pleomorphic enveloped RNA viruses. They have distinctive club shaped spikes on their surface giving the appearance of “solar corona”. It is the new virus, which is the seventh member of the Coronaviridae family (Fig. 1). At 125 nm, it is slightly larger than other influenza viruses. It has four structural proteins which help in its adherence entry, replication assembly and exit. It binds to its target cells through angiotensin-converting enzyme 2 (ACE2), which is expressed by epithelial cells of the lung, intestine, kidney, and blood vessels.
THE RESERVOIR OF INFECTION
The emergence of the virus was linked to a local Huanan (South China) Seafood Market in Wuhan, Hubei Province, China in December 2019. Another hypothesis is that coronaviruses that are phylogenetically close to SARS-CoV-2 have been detected in pangolins (scaly anteaters), especially in Malayan pangolins (also known as Sunda pangolins) that are obtained in antismuggling operations in Guangdong Province and the Guangxi Zhuang Autonomous Region in China.1 This species, which is located throughout Southeast Asia, may be a potential intermediate host of SARS-CoV-2. Another theory was regarding bats being potential reservoirs.2 However, the important source of infection was found to be human. They may be cases, asymptomatic carriers or even fresh cadavers.
THE HOST
Some studies have reported an age distribution of adult patients between 25 and 89 years old. Most adult patients were between 35 and 55 years old, and children and infants were less affected.3 The median age of patients was found to be 59 years, with the majority (59%) being male.4 It was suggested that the population most at risk may be older people (>60 years of age) and those with comorbidities such as coronary artery disease, diabetes, hypertension renal pulmonary, and hepatic dysfunction.5 Pregnant women, immunocompromised patients, steroid therapy or malignancy also comprise high-risk groups.
TRANSMISSION DYNAMICS
The disease has a high transmissibility measured by a high basic reproduction number R0. Different studies of COVID-19 have estimated the basic reproduction (R0) range to be from 2.6 to 4.71 (Fig. 2).6,7
The virus can be spread through respiratory droplets between people who are within about 6 feet of each other, through fomites, fecal-oral route, vertically and possibly by aerosol. Contact transmission may occur when a subject touches a surface or object contaminated with the virus and subsequently touch their mouth, nose, or eyes.8
Fig. 2: The population may be divided into the following four classes: Susceptible, exposed (asymptomatic), infectious (symptomatic), and removed (i.e., isolated, recovered, or otherwise noninfectious). A fraction of exposed individuals have a subsequent travel history and are eventually detected in their destination country.Source: Kucharski AJ, Russell C, Diamond C, Liu Y, Edmunds J, Funk S, Early dynamics of transmission and control of COVID-19: a mathematical modelling study. Lancet. 2020. Available from: https://doi.org/10.1016/S1473-3099(20)30144-4.
Environmental Factors
Thus social gatherings, religious events, meetings, overcrowding as well as poor sanitation are conducive to explosive transmission.
Natural History
The average incubation duration of COVID-19 was estimated to be 4.8 ± 2.6, ranging from 2 to 14 days (Fig. 3).9 The symptomatic patients may have mild flu-like symptoms (81%) or progress to severe (14%) and critical disease with acute respiratory distress syndrome (ARDS), shock and multiorgan failure in (5%). The case fatality rate varies in different countries but has been reported as around 2.5–5%.104
Fig. 3: Natural history of disease.Source: Emergency Medicine Cases (2020). Ep 137 COVID-19 Part 1—Screening, Diagnosis and Management. [online] Available from: https://emergencymedicinecases.com/COVID-19-screening-diagnosis-management [Last accessed April, 2020].
DISEASE BURDEN
As of now (April 19, 2020) the total number of confirmed cases globally is 2,317,758 and number of deaths are 129,509 (Figs. 4 and 5). In India, there are 15,722 cases with 521 deaths while 2,463 people have recovered (Fig. 6).
STRATEGIES FOR PREVENTION AND CONTROL
Some of the most important measures are to avoid close contact with people suffering from respiratory infections (social distancing), frequent hand washing with sanitizers containing more than 70% alcohol. Cough etiquette (covering while coughing and sneezing) is to be practiced and masks are to be universally used. Health education is of prime importance in influencing people.
Personal protective equipment are to be used while caring for cases by healthcare workers and standard infection prevention and control practices are to be undertaken in hospitals.5
Fig. 4: Human confirmed cases of 2019-nCoV infection in the world (as on 24:00 UTC+8, February 20, 2020).Source: Sun J, He W, Wang L, Zhou J, Veit M, Su S, et al. COVID-19: Epidemiology, Evolution, and Cross-Disciplinary Perspectives. Trend Mol Med. 2020. Available from: https://doi.org/10.1016/j.molmed.2020.02.008.
Fig. 5: Number of COVID-19 cases worldwide (as of April 19, 2020).Source: ORF (2020). Tracking COVID-19 in South Asia and beyond [online]. Available from: https://www.orfonline.org/covid19-tracker/ [Last accessed April, 2020].
The chain of transmission can be interrupted by lockdown procedures in areas. In the absence of effective vaccine these measures should be meticulously followed.6
Fig. 6: Number of COVID-19 cases in India (as of April 19, 2020).Source: ORF (2020). Tracking COVID-19 in South Asia and beyond [online]. Available from: https://www.orfonline.org/covid19-tracker/ [Last accessed April, 2020].
Moreover, five pronged approach to tackling the disease is diagnosis, notification, isolation treatment and quarantine. A proportion of testing capacity must be reserved to support public health efforts to assess the severity of the disease. Although this approach may result in many negative test results, such set-aside capacity will permit understanding of the spread of the epidemic. Testing in unexplained clusters or severe cases of acute respiratory infections, regardless of a patient's travel history, may be a sensitive way to screen for chains of transmission that may have been missed.
Therefore, apart from testing of symptomatic cases and contacts of suspected and confirmed cases, the Indian Council of Medical Research (ICMR) has also recommended pool testing for asymptomatic individuals in a community and cluster testing by rapid kit for people participating in gatherings.7
EPIDEMIOLOGICAL INVESTIGATION
This is an important tool in future planning, implementation and evaluation that aids in prevention and control of a disease. Synthesis of data is possible from simultaneous surveillance studies, epidemiologic field investigations, and case series. Conducting descriptive and analytical studies in well-defined settings such as schools, workplaces, or neighborhoods (community surveys) can help in describing the household and community attack rate; rapid assessment of the severity of the epidemic by case counts, hospitalizations, and deaths in a well-defined population and extrapolating that rate to the larger population can help in subsequent preparedness and response.
CONCLUSION
Epidemiology aims to analyze disease occurrence and utilize the knowledge to formulate the means of dealing with a health crisis for the wellbeing of society.11 The severity and transmissibility of this COVID-19 pandemic is so explosive that the health system is being overwhelmed in many countries resulting in failure to provide emergency medical assistance to critical cases.12 Thus it is imperative to study the epidemiological determinants of disease and device strategies to fight this scourge.
REFERENCES
- Liu P, Chen W, Chen JP. Viral metagenomics revealed sendai virus and coronavirus infection of Malayan Pangolins (Manis javanica). Viruses. 2019;11:979. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6893680/.
- Li W, Shi Z, Yu M, Ren W, Smith C, Epstein JH, et al. Bats are natural reservoirs of SARS-like coronaviruses. Science. 2005;310:676–7. Available from: https://www.ncbi.nlm.nih.gov/pubmed/16195424.
- Chen N, Zhou M, Dong X, Qu J, Gong F, Han Y, et al. Epidemiological and clinical characteristics of 99 cases of 2019 novel coronavirus pneumonia in Wuhan, China: a descriptive study. Lancet. 2020;395:507–13. Available from: https://doi.org/10.1016/S0140-6736(20)30211-7.
- Li Q, Guan X, Wu P, Wang X, Zhou L, Tong Y, et al. Early transmission dynamics in Wuhan, China, of novel coronavirus-infected pneumonia. N Engl J Med. 2020;382(13):1199–207. Available from: https://www.ncbi.nlm.nih.gov/pubmed/31995857.
- Wang W, Tang J, Wei F. Updated understanding of the outbreak of 2019 novel coronavirus (2019-nCoV) in Wuhan, China. J Med Virol. 2020;92:441–7. Available from: https://onlinelibrary.wiley.com/doi/full/10.1002/jmv.25689.
- Wu JT, Leung K, Leung GM. Nowcasting and forecasting the potential domestic and international spread of the 2019-nCoV outbreak originating in Wuhan, China: a modelling study. Lancet. 2020;395(10225):689–97. Available from: https://www.ncbi.nlm.nih.gov/pubmed/32014114.
8 Shen M, Peng Z, Xiao Y, Zhang L. Modelling the epidemic trend of the 2019 novel coronavirus outbreak in China. bioRxiv. 2020. Available from: https://doi.org/10.1101/2020.01.23.916726.
- Liu T, Hu J, Kang M, Lin L, Zhong H, Xiao J, et al. Transmission dynamics of 2019 novel coronavirus (2019-nCoV). bioRxiv. 2020. Available from: https://doi.org/10.1101/2020.01.25.919787.
- Backer JA, Klinkenberg D, Wallinga J. The incubation period of 2019-nCoV infections among travellers from Wuhan, China. Euro Surveill. 2020. Available from: https://www.ncbi.nlm.nih.gov/pubmed/32046819.
- Lai S, Bogoch II, Watts A, Khan K, Li Z, Tatem A. Preliminary risk analysis of 2019 novel coronavirus spread within and beyond China. 2020. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7001240/.
- Lipsitch M, Swerdlow DL, Finelli L. Defining the epidemiology of COVID-19—studies needed. N Engl J Med. 2020;382:1194–6. Available from: https://www.ncbi.nlm.nih.gov/pubmed/32074416.
- Ming WK, Huang J, Zhang CJ. Breaking down of healthcare system: mathematical modelling for controlling the novel coronavirus (2019-nCoV) outbreak in Wuhan, China. bioRxiv. 2020. Available from: https://doi.org/10.1101/2020.01.27.922443.