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How does the coronavirus spread from one person to another?

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The new coronavirus is transmitted from one person to another as it is ‘shed’ by an infected person into the environment. The virus must survive that environment before being picked up by another person, who either inhales the virus in respiratory droplets or makes contact with virus-contaminated surfaces.

Infected people shed virus

Inside an infected person, the new coronavirus (SARS-CoV-2) multiplies in the airways and lung cells. This is shown by its presence in the saliva and spit of infected people. It gets there because, when infected cells die, viruses are released into the thin layer of fluid lining the airway and lung. The virus is also sometimes found in the faeces but rarely in blood.[1][2] The way that the virus is released into the environment, either through coughing or sneezing, is called shedding.

Studies have found that on average, the new coronavirus continues to be detected in people for 7–20 days after they develop initial symptoms, and sometimes for more than one month. The tests may be detecting the genetic material of dead viruses, so it does not mean that someone continues to be infectious for this length of time. This theory is supported by the fact that scientists have only been able to grow the virus in the lab from samples taken less than ten days after symptoms started. This strongly suggests that infectiousness is significantly reduced after this point.[3] This is why people who develop COVID-19 are required to stay at home for 7 days from the start of symptoms.

It would make sense that infected individuals with symptoms shed more of the new coronavirus into the environment than individuals without symptoms because of coughing, sneezing, or a runny nose. However, scientists think infected people are contagious before they develop symptoms too. This is based on two types of study, both indirect. The first is that the new coronavirus has been detected in mucus samples taken from recovering patients who no longer show symptoms,[3] although this needs to be confirmed in similar samples taken from people before they show any symptoms. The second is indirect evidence: the average time between one person getting symptoms, and someone they infect getting symptoms, is approximately four days: shorter than expected.[4] This suggests a number of infected people will have caught the virus from someone who had not yet developed symptoms. This makes it more difficult to control the spread of the virus because it is being passed on by people who may not know they are infected.

Children seem to resist the new coronavirus better than adults, showing milder symptoms. Children with milder symptoms probably shed less virus, and so may be less infectious.[5] But this remains to be confirmed.

Virus survival in the environment

Preliminary studies have shown that the new coronavirus survives for three hours in the air in tiny water droplets artificially generated by a machine. But we do not know if humans breathing or coughing can generate droplets full of virus in the same way as the machine, or exactly how long they last in different conditions, for instance indoors or outdoors. The virus can survive longer on hard surfaces: 24 hours on cardboard and two to three days on plastic and steel.[6] It is not yet known which route of transmission – by inhaling virus in droplets or through contact with virus-contaminated surfaces – is most important for the spread of infection.

Infecting a new host

Scientists already know that the new coronavirus survives in droplets in the air and on surfaces. So it is likely that the two main routes of transmission are by breathing in the virus; or by the virus on surfaces being transferred to the eyes, nose and mouth by people’s hands. It is not yet known which of these two routes is most important; advice on social isolation, hand washing and avoiding face touching aims to interrupt both. It is not known whether the faecal–oral route is important in the spread of the virus, though this too would also be prevented with hand washing. It is still unclear whether the virus can pass between a pregnant woman and the fetus; there is so far no evidence that it can.

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References

  1. Zhou F, Yu T, Du R, et al. Clinical course and risk factors for mortality of adult inpatients with COVID-19 in Wuhan, China: a retrospective cohort study. The Lancet. 2020 Mar. DOI: 10.1016/s0140-6736(20)30566-3.

  2. Wang W, Xu Y, Gao R, et al. Detection of SARS-CoV-2 in Different Types of Clinical Specimens. JAMA. 2020 Mar DOI: 10.1001/jama.2020.3786.

  3. Woelfel R, Corman VM, Guggemos W, et al. Clinical presentation and virological assessment of hospitalized cases of coronavirus disease 2019 in a travel-associated transmission cluster. medRxiv. 2020 Mar. DOI: 10.1101/2020.03.05.20030502.

  4. Nishiura H, Linton NM, Akhmetzhanov AR. Serial interval of novel coronavirus (COVID-19) infections. International Journal of Infectious Diseases. 2020 Mar. DOI: 10.1016/j.ijid.2020.02.060.

  5. Cai J, Xu J, Lin D, et al. A Case Series of children with 2019 novel coronavirus infection: clinical and epidemiological features. Clinical Infectious Diseases. 2020 Feb. DOI: 10.1093/cid/ciaa198.

  6. van Doremalen N, Bushmaker T, Morris DH, et al. Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1. The New England Journal of Medicine. 2020 Mar. DOI: 10.1056/nejmc2004973.

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