Image Credits: Atreyasai @ Scicatalyst
A novel type of coronavirus (2019-nCoV), previously unidentified in human beings, appeared in Wuhan, China, towards the end of December in 2019. Post identification of the outbreak, the infection quickly spread, affecting tens of thousands of people and killing a few hundred within a month. The International Committee on Taxonomy of Viruses (ICTV) then renamed the 2019-nCoV to SARS-CoV-2. On 11th February 2020, ICTV also announced that the acute respiratory disease associated with the virus would be referred to as the Coronavirus Disease 19 (COVID-19). Infections with SARS-CoV-2 are now widespread. As of 16th October 2020, the number of confirmed cases has risen to 39 million in 215 countries across the world. Molecular analysis of the virus revealed that COVID-19 is, in fact, a zoonotic disease, i.e., an infection transmitted from animals to humans. Well-known diseases like avian flu, ebola, plague, swine flu, and much more originated in animals and later isolated from human beings in a recombinant (modified) form. In general, a virus, bacteria, fungus, or parasites are the cause of zoonotic disease.
Studies have since revealed that SARS-CoV-2 has a high nucleotide similarity with the virus found in Rhinolophus affinis bats, bat CoV RaTG13. Yet, the sequence divergence is way too significant, suggesting that bats might not be the immediate reservoir host or the source of potential reinfection of humans. Recent studies have also revealed that an endangered mammalian species, Manis javanica, commonly known as the pangolin, could also harbour an ancestral virus related to SARS-CoV-2. However, there is insufficient proof to establish a natural origin of the virus from pangolins. Humans come in contact with bats and pangolins through coal mining or butchering and consumption of game meat. The search for the animal origin of SARS-CoV-2 goes on, rife with speculation that many mammals could be susceptible to SARS-CoV-2.
During the initial days of the outbreak in China, it was suggested that the patients infected with coronavirus might have been in contact with infected animals as a source of food or visited the market where they were sold. But, further investigations revealed that some individuals contracted the virus without doing so. This pointed towards a human to human transfer which was subsequently reported by many countries.
Coughing, sneezing and talking produce droplets of various sizes ranging from a few microns to a fraction of a millimetre. Until about a couple of months ago, it was believed that the primary mode of the human-human transmission of the virus was due to the exposure to cough or respiratory droplets, when close to an infected person. When a healthy person comes in close contact, i.e., within one metre from a person showing symptoms (e.g., coughing or sneezing), the droplets can enter their body via any mucosal surface like the nose or mouth and ultimately affect the lungs, WHO defines an airborne disease as diseases that are spread by aerosols. COVID-19 seems to be spread by droplets, and COVID-19 wasn’t considered an airborne illness.
Now, depending on the size of the droplets, we classify them as ‘droplets’ if they are larger than 5 microns, or ‘aerosols’ if they are smaller. By applying fluid mechanics to the respiratory droplets, it has been established that droplets (or aerosols) that are less than 10 microns in size may travel longer distances through airstreams, and those less than 50 microns may contaminate ventilation systems by remaining suspended in the air for long enough. At this point, the consideration of an airborne transmission route was imminent.
The surface-based infection couldn’t explain a lot of outbreaks indoors (like the Diamond Princess Cruise ship), in crowded and poorly ventilated settings at various parts of the world. On 6th July 2020, a group of 239 scientists from 32 different countries published an open letter addressed to the World Health Organisation (WHO) and other competent authorities, urging them to consider the potential of the airborne transmission route of the coronavirus, to explain the spread of the virus indoors through tiny aerosol.
A significant consequence of considering the airborne transmission would be the changes to the WHO's current social distancing guidance of 1m of physical distance, which is bound to increase to a higher value. Many governments also rely on these guidelines to formulate their policies for controlling the pandemic, and those policies also require a significant revision. Although WHO is not entirely convinced with the scientists’ argument, we can definitely expect new guidelines involving measures to counter the airborne spread.
Transmission may also occur through fomites, materials that are likely to carry the infection (e.g., metallic surfaces, furniture, etc.), in the immediate environment around the infected person. Scientists from National Institute of Health (NIH) and Centers for Disease Control and Prevention (CDC) in the USA, and those from the University of California, Los Angeles (UCLA) and Princeton University have conducted extensive studies on the viability of the coronavirus and observed that it remains stable from several hours to a couple of days in aerosols, and on different surfaces like cardboard, copper, plastic and stainless steel. The virus was detected in aerosols (< 5 μm) for up to three hours, an hour more on metallic surfaces, up to a day on cardboard, and on plastic and stainless steel, it goes up to a couple of days. The virus was also detected in sewage and faeces and has raised a hypothesis of faecal-oral transmission. All possible environmental routes from faeces to mouth were considered. Five categories of water-related and excreta-related diseases have been proposed for the faecal-oral hypothesis. As confirmed by WHO, there is a lack of evidence relevant to the faecal-oral transmission of SARS-CoV-2.
Something that all of us can do to avoid getting infected would be to clean our hands frequently; not touch our eyes, nose or mouth; and be careful to cover our mouth and nose with a tissue while coughing or sneezing, and discard it after use. Ideally, one has to maintain a minimum distance of one metre from one another to avoid contact with the droplets from an infected person. But is this physical distancing always possible? Definitely not. The only other thing that we could do is to wear a fabric mask to prevent the spread to others. The indirect transmission through fomites can be prevented by cleaning and disinfecting the objects of frequent contact using common household disinfectants. Given the necessity to wash our hands frequently, there has been an increased demand for hand sanitisers, and this has boosted the production by many folds.
There have been several doubts and misconceptions regarding the transmission and controlling the spread of the novel coronavirus. While some of the doubts about the role of vectors like mosquitos and flies seem genuine, most of the facts about the “death” or “killing” of the virus have been baseless without any proper scientific explanation. “WhatsApp University'' has played an essential role in propagating these misinformation. A few of the most popular ones are related to high or low temperatures. Firstly it was believed that bathing in hot water kills the virus. As much as it sounds convincing, it is totally wrong. Average human body temperature fluctuates between 36.5°C to 37°C. Can the water temperature get any higher? If it does, we will burn our skin before the virus dies. Secondly, it was also believed that residing in a place with either hot and humid weather or really cold and freezing weather would control the transmission. We reject this on a similar line of argument that humans are homeotherms or organisms that maintain their body temperature. India, a country that clearly suits the above criteria, has more than 7.3 million confirmed cases. This validates the above argument. Additionally, some people started arguing towards increased consumption of alcohol to prevent infection, as the prescribed type of alcohol-based sanitiser contains more than 60% ethyl alcohol. This is also absurd as increased consumption of alcohol only increases the risk of health issues.
It has also been well established that the SARS-CoV-2 virus uses ACE2 or angiotensin-converting enzyme as a receptor to enter human cells. Having a really high affinity for ACE2, a few SARS-CoV-2 virions are sufficient to infect humans. A disparity was observed in the age-group that got infected by the virus. Even though cases have been reported from all the age groups across the world, infected children seem to have milder symptoms and appear to be less susceptible to infection than adults. The lower risk among children is hypothesised to be due to differential expression of ACE2 gene in the nasal epithelial cells of children. These cell receptors were found to be lowest among children below the age of 10 and increased with age. This hypothesis is also supported by the analyses of confirmed and suspected cases of COVID-19 among children under 18 years by the Royal College of Paediatrics and Child Health. Studies by an international panel of experts in WHO has also shown no child to adult transmission in China. Yet, there is not enough research to conclude that infected children are less likely to pass on the infection than adults.
With cases of COVID-19 rising rapidly globally and overwhelming health workers and hospitals, the race to find treatments has dramatically accelerated.
This article is an extended and updated version of the one written as a part of the Scicatalyst COVID-19 Special Edition Vol. VIII | Part 1 - June 2020. Illustrator: Atreyasai. Editors: Ananya Dash, Rachita Dash, and Mandira Choppella.