Introduction to Coronaviruses and COVID-19
Coronaviruses are group of viruses which can affect both mammals and birds. Seven of them are known to infect humans and can cause a range of symptoms, resulting in illnesses from a mild common cold to the more severe SARS (Severe Acute Respiratory Syndrome) or MERS (Middle East Respiratory Syndrome). They are spread by droplet or aerosol infection - from an infected person sneezing, coughing or talking since the droplets can either be inhaled or consumed when they land on a surface which is then touched and then transferred to the eyes, nose or mouth.
Coronaviruses are zoonotic, this means that they normally exist in animals but can infect humans in what are called spillover events. Transmission from animals to human may be airborne, through food, close contact, through there faces or salvia or via something else called a vector. They get their name from their appearance, which is crown-like (the Latin for crown is corona). It has also been said that electron micrograph images of the virus look like solar coronas, which are the auras that surround the sun and other stars.
In 1960 researchers isolated two viruses with crown like structures that were causing common colds in humans. Scientists then realised that viruses earlier identified in sick chickens and pigs also had the same structure and it was in 1968 that the term coronavirus was given to the entire group.
The novel coronavirus was discovered in Wuhan in December 2019 and has been named SARS COV-2 with the resulting disease called COVID-19 (Coronavirus disease of 2019). Initially it was an epidemic in China, but as a virus new to humans, it spread quickly around the world, being officially declared a pandemic by the World Health Organisation in March 2020. By the end of that month there had been around 30,000 deaths and half a million people infected. A third of the world's countries have been forced to introduce preventative measures to slow the spread, some even going into complete lockdown. It has now spread to nearly every country in the world with more than 20 million known to be infected and over 700,000 deaths have been recorded.
How did it start?
It is unclear how and where the virus was initially caught by the so-called 'patient zero', but it is suspected to have been passed from a bat to an intermediate (which some scientists believe was a scaly anteater called a pangolin) and then to humans at a "wet market" in Wuhan, which sold both live and dead animals. These markets are often difficult to keep clean as animals can be slaughtered on site and are typically very busy, which helps the spread of disease. Also human activity such as deforestation, land clearance for human settlements, industrial agriculture etc often brings humans and animals closer together creating opportunity for zoonotic spillover.
Coronaviruses have a genomic proofreading mechanism which keeps the virus from mutating in such a way that could weaken it. They also have a special trick that makes them even more dangerous: they often swap chunks of their RNA with other coronaviruses which is called recombination. This recombination can lead to new versions of the virus that can infect new cell types and jump into other species. This recombination often happens in bats which carry 61 viruses known to infect humans.
SARS COV-2 shares 96% of its genetic material with a virus found in a bat cave in Yunnan, China, however it has a way of entering cells that is particularly efficient making it different from the Yunnan bat virus which does not seem to infect people. The pangolin has been found to host a coronavirus with a part of the virus used to getting into the cells which is almost indentical to the human version, however, the rest of the coronavirus was only 90% genetically similar, so some researchers think that the pangolin was not the intermediary. As the virus both mutates and recombines it makes identifying the source particularly difficult.
What does it do?
SARS COV-2 is a virus that causes many different symptoms which can vary greatly from person to person. However, the most common are a persistent cough, a fever and a loss or change to your sense of smell or taste, other symptoms can include aching muscles, lack of energy, breathlessness, sore throat, headaches and chest pain. The infection is mild in about 80% of cases and more may be carrying the disease with no symptoms at all (asymptomatic). However in 20% of cases it leads to more serious severe cases leading to pneumonia sepsis and even death.
Although 2 of the other coronaviruses that can kill humans, SARS COV (SARS) and MERS COV (MERS) killed over 1,600 people worldwide, SARS COV-2 is far deadlier mainly due to the fact that once it infects a person it can lie undetected for a while letting that individual unknowingly infect others.
How has it affected different countries?
The number of COVID-19 cases/1 million people in each country around the world has varied with factors such as the demographics and density of the population, the geographical spread in the early days of the virus and the ability to contain and the ability to perform sufficient and timely testing and the speed and effectiveness of the government response. The death rate does not necessary correlate with the number of cases and as each country measures its COVID-19 deaths differently a more reliable measure is to look at is the excess deaths over previous years. The chart below shows the excess deaths over the initial 11 week peak in each country.