How dangerous is the coronavirus and how does it spread?

 Scientists race to understand Covid-19 as number of confirmed cases continues to rise A Chinese worker dressed in a protective suit takes the temperature of a woman at a subway station in Beijing during the lunar new year and spring festival holiday © Kevin Frayer/Getty Share on Twitter (opens new window) Share on Facebook (opens new window) Share on LinkedIn (opens new window) Save Clive Cookson in London MARCH 18 2020Print this page520 Be the first to know about every new Coronavirus story Get instant email alerts Scientists are racing to understand the coronavirus disease, Covid-19, which is now a fast-growing global pandemic. The number of confirmed cases worldwide has exceeded 200,000 — and many epidemiologists believe the real total of infections may be close to a million because testing and reporting are so incomplete. Though Covid-19 has passed its peak in China, cases in some western countries are doubling or tripling every week. Public health experts fear the respiratory illness, which is believed to have started in a food market in Wuhan, will be at least as bad as Spanish flu in 1918-19. How dangerous is the new coronavirus? A transmission electron microscopy image of the first isolated case of the coronavirus © Courtesy of IVDC/China CDC via GISAID/Reuters Covid-19 is transmitted much more readily between humans than its closest relation, Sars, which caused outbreaks of serious disease in a few countries in 2003. The new coronavirus is, however, less dangerous to most people it infects than Sars. Computer modelling suggests that each new Covid-19 case infects 2.5 other people on average when no effort is made to keep people apart. The Chinese authorities have greatly reduced this “reproduction number” through drastic action to isolate cases and trace their contacts — and the rest of the world is rapidly introducing social distancing measures. The virus has caused severe respiratory disease in about 20 per cent of patients and killed more than 3 per cent of confirmed cases. Sars killed 10 per cent of infected individuals. Older people, whose immune defences have declined with age, as well as those with underlying health conditions, are much more vulnerable than the young. But fatality rates are hard to estimate in the early stages of an epidemic and depend on the medical care given to patients. For example, ventilators save lives by enabling people with pneumonia to breathe. Most experts believe the current fatality rate is exaggerated by serious under-diagnosis of mild cases; the best current estimate is that Covid-19 will kill around 1 per cent of those infected in a population with good healthcare. For comparison, seasonal flu has a mortality rate below 0.1 per cent but it infects so many people that it results in about 400,000 deaths a year worldwide. Spanish flu infected an estimated 500m people and killed 50m worldwide in 1918-19. Hypothetically, if Covid-19 affected half the world’s current population over the course of a year with a 1 per cent fatality rate, the death toll would be 35m — substantially increasing the number of deaths worldwide, which is around 60m for all causes in a typical year. How does the virus spread and what is the incubation period?  The most likely way to catch Covid-19 is to be physically close to someone shedding significant amounts of virus — which usually means a person with evident symptoms of disease, though there is growing evidence that people carrying the infection with few or no symptoms also play an important role in transmitting the disease. Respiratory infections are most commonly spread through the air by viral particles in droplets from a cough or sneeze, though health workers and family members are also vulnerable to infection through close physical contact with patients without good barrier protection. A study by the US National Institutes of Health showed that if the droplets fall on to a surface, the active virus is detectable for up to 24 hours on cardboard and two or three days on plastic and steel. However contaminated surfaces are not thought to be the most important transmission route. The incubation period between infection and symptoms appearing can range from two to 14 days. About five days is most common, according to the World Health Organisation. A study of hospital patients during the original Wuhan outbreak showed that they were potentially infectious for up to five weeks after first symptoms appeared. Do masks help protect against infection? There is considerable debate among public health experts about the preventive effectiveness of covering the face to prevent infection. © Kevin Frayer/Getty Although wearing face masks appears socially obligatory in some east Asian cities affected by coronavirus, the WHO and many governments say healthy people do not need to wear a mask unless they are taking care of a person with suspected Covid-19 infection. The advice may be partly an attempt to stop people who don’t need them trying to buy masks, which are in short supply in many places and urgently needed by health workers. However the most effective way to protect against Covid-19 is to minimise encounters with other people and if possible keep two metres away when you do meet. Clean your hands frequently, keep them away from your face and cover coughs and sneezes with the bend of your elbow or a tissue. What happens when you are infected? The virus multiplies within the lower respiratory tract, where symptoms develop. Early ones are a fever and cough. Most people will recover within a few days. But about 20 per cent go on to develop serious pneumonia as their lungs become inflamed; they may need a respirator to help them breathe. In some of the most severe cases, there can be a fatal “cytokine storm” in which the immune system goes into overdrive, overwhelming the body with cells and proteins that destroy other organs. Nurses in protective gear talk to people in the reception area of the First People's Hospital in Yueyang, Hunan Province, near the border with Hubei Province © Thomas Peter/Reuters How can doctors tell whether a patient has coronavirus disease?  Since Chinese scientists published the genetic sequence of the virus on January 10, laboratories anywhere in the world have been able to test patient samples for its presence. They use a procedure called polymerase chain reaction (PCR) to amplify and identify viral genes. But PCR is slow and requires specialist equipment and chemicals, which are in short supply in many countries, so researchers are rushing to develop faster, cheaper and more portable tests. At the same time scientists are carrying out detailed analysis of the full genetic code of the virus isolated from Covid-19 patients to track mutations as the epidemic proceeds. Although this surveillance is showing the inevitable emergence of what some are calling different viral “strains”, Covid-19 is genetically more stable than flu, with no significant changes detected so far that might make it significantly more virulent or transmissible. A third level of testing ìnvolves looking for antibodies in populations exposed to Covid-19, which will show how many people were infected but developed mild or no symptoms. This “serological” testing takes longer but results are expected soon. Can you be infected more than once? Reports continue to emerge of coronavirus “reinfecting” people who had apparently cleared the disease, but experts are casting doubt on how often reinfection takes place. A more likely scenario is that the immune system suppressed coronavirus to a low level — too low to detect in viral tests of swabs from the patients — and the virus then surged back, causing symptoms to reappear. How frequently this occurs — and whether it poses a serious risk to efforts to control the spread of Covid-19 infection — is unknown. Medical specialists still have much to learn about the way the new coronavirus interacts with the human immune system. If reinfection really does take place because people mount a very weak response to infection, that could be bad news for the development of a vaccine. Are drugs and vaccines being developed?  There has not been time to design new drugs to treat Covid-19, though some antiviral medicines may alleviate the symptoms and many existing drugs are being screened to find out whether they help. Chinese doctors are carrying out clinical trials with drugs normally prescribed for HIV — and another antiviral called remdesivir that was developed to treat Ebola. Their findings are expected very soon. Programmes to develop vaccines quickly to prevent Covid-19 infection are under way in dozens of academic and private labs around the world, some under the auspices of the Oslo-based Coalition for Epidemic Preparedness Innovations (Cepi) — a $750m partnership set up in 2017 by governments, industry and charities to prevent future pandemics. First to begin clinical testing is a candidate vaccine from Moderna, a US biotechnology company; the US National Institutes of Health started injecting it in healthy volunteers on March 16. Initial results, showing whether the vaccine is safe and elicits a good immune response, will be available this summer. But far longer and more extensive clinical trials will be required before regulators will approve a Covid-19 vaccine even in a global health emergency, so no vaccine is likely to be available for widespread use until early next year. How is the epidemic likely to develop?  Although scientists have learnt an astonishing amount about Covid-19 in the months since the disease was first detected in Wuhan, they do not know yet enough about the virus and how it affects the human immune system to forecast likely outcomes with any accuracy. A pandemic is already under way but there are various possible scenarios for its future development. One extreme is that the disease might affect as much as half the global population within a year, overwhelming health services and killing tens of millions of people. The other is the world takes strong enough action to suppress and then eradicate Covid-19. It is more likely that the virus will hang around for the indefinite future causing disease similar to flu but becoming less of a threat as time passes.
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