2019 nCOV cryoEM structure

Side and top views of the structure of a protein – S protein – of 2019-nCoV.          Retrieved from the paper by Wrapp and others published in the journal Science.

With new COVID-19 cases being reported in India, there has been a deluge of information – news stories, articles and awareness campaigns – around it. Containment, social distancing, transmission, flattening the curve, epidemic, and pandemic are some terms that you will now find in most articles. In this short post, I try to explain some of these terms.

COVID-19, SARS-CoV-2, 2019-nCOV: COVID-19 (Coronavirus Disease) is an infectious disease caused by the most recently discovered coronavirus. Coronaviruses are a family of viruses that have RNA as their genetic material. Other viruses of this family include MERS (Middle East Respiratory Syndrome coronavirus), SARS (Severe Acute Respiratory Syndrome coronavirus) and viruses that cause common cold (E.g. Rhinoviruses). The new coronavirus was initially called 2019-nCOV (2019 novel coronavirus) when it was first identified in China. The virus is now named SARS-CoV-2 (Severe Acute Respiratory Syndrome coronavirus 2). WHO announced the name of the new coronavirus disease as COVID-19 on February 11, 2020.

Endemic: A disease, or an infectious agent that causes it, is called an endemic when it is usually present in or is restricted to a community in a certain geographical area. For example, Kyasanur Forest Disease (KFD), commonly known as monkey fever is a viral disease endemic to South India. This is a disease seen in specific areas in South India. KFD is an endemic prevalent in Shimoga district of Karnataka.

Epidemic: When the number of disease cases is in excess than the normally expected numbers, then the disease is called an epidemic. The numbers of normally expected cases are defined specifically for each case. When the number or density of susceptible cases exceed a threshold (called an epidemic threshold), the event is defined as an epidemic.

Outbreak: When the disease cases that occur are more than what is normally expected, it is called an outbreak. For example, there were two outbreaks of Nipah in Kerala (2018 and 2019). Although epidemic and outbreak have overlapping definitions, an outbreak can be seen as the process of occurrence of an epidemic.

Pandemic: When a disease epidemic spreads across a continent or worldwide, it is called a pandemic. WHO characterized COVID-19 as a pandemic on March 11, 2020.

Infected person: A person who has the infectious agent, which causes the disease, in his body.

Incubation period: This is the time in which the infectious agent (say, a virus) is present in the host (an infected person), but the person has no symptoms. The incubation period for COVID-19 is 14 days or less. According to a study, the average incubation time for COVID-19 is 5.5 days. Less than 2.5% of the cases develop symptoms in 2.2 days and 97.5% of the cases develop symptoms in 11.5 days. Only 1 in 10,000 cases developed symptoms after 14 days.

Convalescent period: This is the time when an infected person recovers from the illness.

Transmission: The way in which a disease spreads from one person to another is called transmission. If the disease is transmitted from an infected person to a healthy person, it is called contact transmission. Contact transmission can be direct – when there is physical contact between the infected person and the healthy person, or indirect – where the infectious agent spreads by indirect means without direct physical contact. Indirect transmission can include droplet transmission or transmission by fomites.

Droplet Transmission: When an infectious agent (say, a virus) is transmitted through respiratory droplets that are propelled into the air by sneezing or coughing, it is called droplet transmission. When an infected person coughs or sneezes, droplets of moisture of different sizes contaminated with infectious agents are propelled out in the air. When these droplets contact the eyes, nose or mouth of a healthy person, he/she can get infected. Droplets are usually heavier and settle down to the ground quickly because of gravity. These droplets can be propelled to about 1 metre depending on the size of the particle and the force with which it is expelled.

Aerosol Transmission: When an infectious agent (say, a virus) is transmitted through aerosols (suspension of fine particles in the air) propelled into the air by sneezing or coughing, it is called aerosol transmission. When an infected person coughs or sneezes, aerosols of moisture of different sizes contaminated with infectious agents are propelled out in the air. Aerosols are lighter and can remain in the air for sometime before it falls. They can be propelled up to 3 metres depending on the size of the particle and the force with which it is expelled. SARS-CoV-2 is reported to be stable in aerosols for 3 hours.

Transmission by Fomites: Fomites are non-living objects that can be contaminated with infectious agents and can transmit the disease. These include water, plastics, metals – doorknobs, keyboards, phones, handrails etc. If an infected person sneezes or coughs and the droplets fall on fomites, the infectious agents can remain active on fomites for hours or days. When a healthy person touches an infected surface and then touches his/her nose, eyes or mouth, the infectious agent can get inside the body. A recent study has shown that SARS-CoV-2 can be viable for 3 hours in aerosols, and up to 3 days on stainless steel, copper and cardboard surfaces.

Local Transmission: When the source of infection is present with a locality, there is local transmission of the disease. That means if a person in Bangalore gets infected by someone who is in Bangalore, then there is a local transmission. This is referred to as stage II of the epidemic.

Community Transmission: When one cannot relate confirmed cases by chains of transmission (i.e., when one cannot trace the source of infection of all the different people in the chain) for a large number of cases, then it is called community transmission. In other words, if a person who has not come in contact with anyone known to be infected and has not travelled to any country when the virus is spreading tests positive, then there is community transmission. This is referred to as stage III of the epidemic.

Social distancing: It is the practice of maintaining more than the usual physical distance from other people. The purpose is to stop or slow down the spread of a disease that can be transmitted by physical contact, droplets, or fomites. The more distance you maintain from people, the lesser the risk of getting the disease.  

Flattening the curve:  This is a term used to indicate preventing a sharp peak of infections. During an epidemic, the disease spreads quickly as infected people come in contact with healthy people, who in turn come in contact with more people. Let us say there is one infected person (the person may or may not have developed symptoms). He/She hangs out with 3-4 friends. There is a possibility that these 3-4 people are now infected. These people now meet with more people, who in turn meet other people. Suddenly there is a sharp increase in the number of cases when all of these people develop symptoms within a few days. This overwhelms the healthcare system. If people practise social distancing, the chances of infection are lesser and, therefore, the sharp peak can be flattened as lesser cases are present in a given time. This can reduce the burden on the healthcare system.

Quarantine: The process of isolating people who have been exposed to an infection that can spread. The aim of quarantining is to contain – prevent the further spread of – the disease. Quarantine for 14 days is recommended for suspected COVID-19 cases.

Herd immunity: This is indirect protection for susceptible people in a community. In an outbreak, if a large proportion has become immune to the infection, they indirectly protect people who are not immune to the infection by disrupting the spread. Consider the situation with COVID-19. Let us say healthy people get infected. They become sick, isolate themselves (especially from people who are at higher risk of death by infection because of other health conditions) and eventually recover. When the number of recovered people increases in a population, they indirectly protect those who have not been infected as the chain can now be broken quickly.

Covidiot (informal use): This term is used to refer to a person who ignores the warnings regarding public health or safety and hoards goods denying them from their neighbours during the COVID-19 pandemic.





6 thoughts on “COVID-19: key terms you should know

  1. Thankyu so much sir for doing such a great effort regarding this…..these days ppl really need to check this out… your contribution means a lot…it will very helpful for everyone…


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