Even before SARS-CoV-2 swept around the world, scientists had been warning of the global threat posed by viruses like it. Although COVID-19 vaccines should help to end the current coronavirus pandemic, it is unlikely to be the last.
In December 2019, Wei Guixian was working as a shrimp-seller at the Hua’nan Seafood Market in Wuhan, China, when she developed cold-like symptoms. After visiting her doctor's surgery for treatment, the 57-year-old returned to work, but within a week she had to be admitted to Wuhan Union Hospital, along with several other market workers. They were some of the first human victims of the SARS-CoV-2 coronavirus that has now infected more than 150 million people globally, and counting. Although Wei eventually recovered, at least 3 million others have died from COVID-19.
SARS-CoV-2 is the seventh member of the coronavirus family to infect humans, and the third to trigger serious, life-threatening disease in some of those who are infected with it. Although the other two – Middle East Respiratory Virus (MERS), first identified in 2012, and SARS-CoV, which caused the 2002-2004 SARS outbreak – both killed more people than they infected, differences in transmission dynamics and disease pathologies meant that there were far fewer infections and deaths overall.
Where is it circulating? The COVID-19 pandemic is ongoing, with SARS-CoV-2 infections worldwide. The MERS coronavirus is also still a threat, with 2,589 cases reported between April 2012 and April 2021, including 940 deaths. Most of these cases have been on the Arabian Peninsula. There have been no recorded cases of SARS since 2004.
Hundreds of other coronaviruses are circulating in wild animals, particularly bats.
Pandemic threat: Although the development of COVID-19 vaccines may help to end the current COVID-19 pandemic, ongoing high rates of infection in many countries could lead to the emergence of further variants capable of evading vaccine-induced immunity. This could mean people continue to develop COVID-19.
MERS also has pandemic potential: At the moment, the MERS coronavirus is usually contracted through direct or indirect contact with infected camels or camel products, but there have been cases of human-to-human transmission. The worry is that more transmissible variants may emerge, which rapidly spread. This scenario becomes more likely as climate change increases the frequency of droughts, and prompts farmers in countries, such as Kenya, to switch cattle for camels, potentially bringing a greater number of infected animals into close contact with humans.
Then there are the hundreds of other coronaviruses circulating in animal species such as bats, pigs and pangolins, which have the potential to jump to humans, mutate and become more transmissible. The sale of live wild animals in traditional food markets makes such events more likely.
How is it spread? Human coronaviruses spread through respiratory droplets produced when people cough, speak, sneeze or breathe. Those standing within two metres of an infected person are at greatest risk of inhaling these viral particles, but there is mounting evidence that the virus may also linger in smaller, airborne droplets, making spending prolonged periods indoors with an infected person risky, even if you’re some distance away.
Viral particles can also contaminate people’s hands and the surfaces they touch, triggering an infection if someone else touches them and subsequently touches their mouth, nose or eyes. Coronaviruses can additionally be excreted in faeces – although this isn’t considered a major route of transmission in the case of COVID-19.
Case fatality rate: This is difficult to calculate for COVID-19 because many people experience no, or only mild symptoms, meaning they are never tested. Testing capabilities also vary between countries, giving rise to very different estimates of the case fatality rate depending on where people live, from 0.1% to more than 25%. However, these are averages; in reality, mortality varies by age (with older people, and those with underlying conditions including diabetes and obesity, at greatest risk) and it seems to show some difference between COVID-19 variants.
MERS has an estimated case fatality of 35%, while for SARS, estimates range from 3-15%.
Incubation period: Most people infected with SARS-CoV-2 develop symptoms within 1 and 11 days, with an average incubation period of 5-6 days. For SARS, the incubation period is usually 2-7 days but may be as long as 10 days, while for MERS it ranges from 2-14 days.
Symptoms: For COVID-19, SARS and MERS, the most commonly reported symptoms are fever, cough and shortness of breath – although many people with COVID-19 also report losing their sense of smell and/or taste. Gastrointestinal symptoms such as diarrhoea are also sometimes associated with COVID-19 and MERS.
Diagnosis: Polymerase chain reaction (PCR) testing is most frequently used to diagnose all three diseases, because of its ability to detect low levels of virus. Antigen tests, such as lateral flow tests, are also used to diagnose COVID-19, although they are less accurate.
So far, the World Health Organization (WHO) has authorised the emergency use of three COVID-19 vaccines: those manufactured by Pfizer/BioNTech, AstraZeneca and Johnson & Johnson. Other countries have authorised additional vaccines, and the WHO is expected to authorise more such products in the coming months. All of the vaccines that are currently being used involve training the body against the “spike” protein on the surface of the virus, which it uses to infect human cells.
Various treatments have also been authorised in some countries to curb the progression of COVID-19 in people who have been hospitalised with the disease, including anti-inflammatory drugs such as dexamethasone, the antiviral drug remdesivir and blood thinners such as heparin, to reduce the risk of blood clots. Monoclonal antibody drugs have been authorised for the treatment of infected people at high risk of serious disease in some countries, including the USA. These attack the coronavirus's spike protein and make it harder for the virus to infect new cells.
No vaccine or specific treatment is currently available for MERS, although vaccines and treatments are in development.
Scientists are also trying to develop “pan-coronavirus” and universal vaccines, which would protect people against future variants of SARS-CoV-2, the viruses that cause SARS and MERS, and possible future human coronaviruses as well.
COVID-19, SARS and MERS all started with a spill-over of a coronavirus from animals to people. Although the COVID-19 pandemic is still raging, reducing contact with wild and farmed animals that are capable of becoming infected with SARS-CoV-2 will reduce the chances of the virus gaining further mutations that enhance its transmissibility or ability to cause severe disease. So will reducing the number of human infections through vaccination and physical distancing. Reducing contact with wild animals should also reduce the chances of other coronaviruses spilling over into humans, and adapting to infect us more easily.
Developing robust surveillance and contact tracing systems is also essential, both to keep track of, and contain new variants, as they emerge, ensuring that COVID-19 vaccines remain effective, and to rapidly identify and quash any future human coronavirus threat.