A coronavirus is a common type of virus that usually causes mild illnesses, but can lead to more serious infections
Electron micrograph of Coronavirus, the cause of Severe Acute Respiratory Syndrome (SARS)
Coronaviruses are a group of viruses that usually cause mild illnesses, such as the common cold. However, certain types of coronavirus can infect the lower airway, causing serious illnesses like pneumonia or bronchitis. Most people get infected with coronaviruses at some point in their lives and the majority of these infections are harmless. The new coronavirus that causes the covid-19 illness is a notable exception.
Coronaviruses have extraordinarily large single-stranded RNA genomes – approximately 26,000 to 32,000 bases or RNA “letters” in length. Coronavirus particles are surrounded by a fatty outer layer called an envelope and usually appear spherical, as seen under an electron microscope, with a crown or “corona” of club-shaped spikes on their surface.
What is covid-19?
The virus that causes covid-19 is known as SARS-CoV-2. It appears to have first emerged in Wuhan, China, in late 2019. The outbreak has since spread across China to other countries around the world. The WHO declared the outbreak a pandemic in March 2020.
Commonly reported covid-19 symptoms include a fever, a new continuous cough and a loss or change in sense of smell or taste. In the most severe cases, people with the virus can develop difficulty breathing, and may ultimately experience organ failure and death. Some people experience long-lasting symptoms following their initial infection, which are often referred to as “long covid”.
Coronaviruses replicate their RNA genomes using enzymes called RNA-dependent RNA polymerases, which are prone to errors, but genomic analysis suggests that covid-19 is mutating slowly, reducing the chance of it changing rapidly to become more deadly. In December 2020 two highly transmissible coronavirus variants were identified in the UK and in South Africa. Neither new variant appears any deadlier.
Following the initial identification of the novel coronavirus as the cause of covid-19 in early 2020, vaccine development progressed at unprecedented speed. Within a year, a number of covid-19 vaccines had received approval or emergency use authorisation in several countries, and vaccination programmes were underway.
In the UK, three covid-19 vaccines had been given temporary authorisation for emergency use in adults as of 8 January 2021, including two mRNA-based vaccines – one developed by Pfizer and its German partner BioNTech and another developed by US company Moderna – as well as a viral vector-based vaccine developed by the University of Oxford in partnership with AstraZeneca. But there are concerns that people in poorer nations could be left without access to any vaccines until as late as 2024, due to wealthier nations purchasing the majority of doses.
It still isn’t clear if any of the existing covid-19 vaccines can prevent people from spreading the coronavirus or how long any immunity they provide might last.
In June 2020, dexamethasone, a widely available steroid that dampens the immune response, became the first medicine shown to reduce deaths in covid-19 patients. The RECOVERY trial of more than 2000 people found that it reduced deaths in people on mechanical ventilators by a third – and by a fifth in those who received oxygen but not ventilation. Many other potential treatments are being explored, particularly the possibility of giving people a dose of antibodies against covid-19.
Other severe coronaviruses
At least two other types of human coronavirus – Middle East respiratory syndrome coronavirus (MERS-CoV) and severe acute respiratory syndrome coronavirus (SARS-CoV) – are known to cause severe symptoms.
SARS-CoV first emerged in 2002 in Guangdong, China as an unusual pneumonia, which developed into life-threatening respiratory failure in certain cases. The virus rapidly spread across 29 countries, infecting more than 8000 people and killing about 800.
The MERS-CoV epidemic appeared in Saudi Arabia in 2012, with people experiencing similar symptoms to SARS-CoV but dying at a much higher rate of 34 per cent. Unlike SARS-CoV, which spread quickly and widely, MERS-CoV has been mainly limited to the Middle East.
How coronaviruses spread from animals to people
Coronaviruses are zoonotic, meaning that they can be transmitted to people from animals. Both SARS-CoV and MERS-CoV originally came from bats, though other animals – including camels in the case of MERS – can act as intermediaries that spread coronaviruses to humans.
Many of the early cases of covid-19 were traced back to a large seafood and animal market in Wuhan. The virus is thought to have come from bats, possibly via an intermediary animal. In response, Chinese officials enacted a ban on eating and trading wildlife in February, a measure that could become a permanent law.
There have also been concerns about covid-19 spreading from people to animals, which could then transmit mutant forms of the coronavirus back into humans. Onward transmission of SARS-CoV-2 into non-human animals has already been documented with pet cats and dogs, captive tigers and lions, as well as mink.
- This article was last updated on 8 January 2021