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expert reaction to news reports that the China coronavirus may spread before symptoms show

There have been reports that the Wuhan coronavirus may be transmittable whilst asymptomatic. 


Monday 27th January

Professor Mark Woolhouse, Professor of Infectious Disease Epidemiology, University of Edinburgh

“In my view it is premature to conclude, on the basis of the evidence currently available, that the new virus can be transmitted before symptoms appear. The anecdotal report from China that a single instance of asymptomatic transmission had occurred is based on the history of a single cluster of cases, it is open to alternative interpretations and there is no indication that the publicised information has been confirmed (as it could be for example, by analysis of the viral genome sequences from the patients involved). This is flimsy evidence on which to base such an important conclusion. If the National Health Commission of China have stronger evidence then it is important that they share it promptly.

“In any case, this is a new virus and we are still learning about it, including how and when transmission can occur. Further, robust research on this point is urgently needed. That is because the question is crucially important. In the absence of any treatment or vaccine our main hope of controlling the epidemic is the rapid identification of cases and the immediate prevention of onward transmission through patient isolation and infection control. The efficacy of those interventions would be compromised if significant levels of transmission occurred before symptoms appeared and the patient reported to a health care facility.”


Professor Bill Keevil, Professor of Environmental Healthcare, University of Southampton, said:

“Since I cited the Lancaster University study suggesting an Ro of 3.6-3.8, there have been several other published estimates, one from China suggesting 3.8 and the MRC Unit in London’s very recent estimate of 2.6 (but uncertainty range 1.5-3.5). These are worryingly high numbers and support the rapidly increasingly numbers of infections. It is essential to maintain genome sequencing to understand how and how fast the virus is mutating and whether this points to development of more lethal forms. As I stressed, good containment and hygiene measures are essential, especially hand hygiene which is a principal transmission route – probably also for symptomless transmission. Wearing face masks may give a false sense of security if aerosol coughs contain submicron particles. Our paper on the related human coronavirus 229E showed it survived over 4 days on common materials such as plastics, ceramics, glass and stainless steel, making regular surface cleaning and hand hygiene paramount. Several authors have advocated everyone wearing gloves, as we do in microbiology labs, carefully removing them and then washing hands.”


Professor David Heymann, Professor of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, said:

“Virus transmission occurs when there is a high enough level of virus in the blood and body secretions of an infected person to another. Some infections can be passed from one human to another a few days before symptoms occur because there are high levels of virus and the virus then causes disease symptoms – this depends on how the virus is transmitted and that is not fully understood at present – it is known that this virus can pass from one to another through close physical contact, and more and more evidence suggests that it can be passed by droplets that spread face to face by a cough or sneeze directly on the face from one to another as was SARS.  As this is a newly identified virus in humans more evidence is required to fully answer the question and that is why it is important for all contacts of persons who develop disease to be identified and observed for fever so that they can be tested if they develop a fever to determine if the fever is caused by the virus.”


Prof Jonathan Ball, Professor of Molecular Virology, University of Nottingham, said:

“Defining the scale of asymptomatic transmission remains key: if this is a rare event then its impact should be minimal in terms of the overall outbreak. But, if this transmission mode is contributing significantly then control becomes increasingly difficult. It’s looking like this coronavirus is behaving very differently to SARS and MERS, and this is a big concern. I would be surprised if WHO do not declare this as a Public Health Emergency of International Concern.”


Sunday 26th January

Professor Sheila Bird, Honorary Professorship at Edinburgh University’s College of Medicine and Veterinary Medicine and formerly Programme Leader at MRC Biostatistics Unit, Cambridge, said:

“If transmission does occur pre-symptoms or is suspected to do so, at some point soon-ish it may be instructive & important for the Chinese health authorities to try to measure within-household transmission, taking account  of household size and ages of members of the household. Little has been said as yet about the estimated fatality-rate for patients whose clinical course is completed (ie recovered alive, or deceased) but I’m sure that public health official know how important it is to do so – even if only for hospitalized cases in the first instance.”


Professor Wendy Barclay, Department of Infectious Disease, Imperial College London, said;

Many of the respiratory viruses that spread amongst humans do transmit even in the absence of symptoms, including influenza and other cold viruses.  They are carried into the air during normal breathing and talking by the infected person. It would not be too surprising if the new coronavirus also does this.  If this does prove to be the case then controlling the spread does become more of a challenge, and measures like airport screening are unlikely to stem the virus effectively.”


Michael Head , Senior Research Fellow in Global Health,Faculty of Medicine, University of Southampton, said;

“If ‘symptomless spreading’ is confirmed, it would not be too surprising. Other respiratory infections such as measles and influenza can both be spread, without the infected person showing symptoms. A key factor would be the extent of the person-to-person transmission. If transmission between people is not too extensive, then the impact of symptomless spreading would not be too great. If there is significant levels of person to person transmission, this would make containment of the outbreak harder.”


Nathalie MacDermott, NIHR Academic Clinical Lecturer, King’s College London, said;

“While the suggestion that the 2019-nCoV virus may be contagious during the incubation period, as reported by a doctor in Zhejiang province in relation to a cluster of cases linked to an individual who had yet to show any symptoms, is concerning it is not surprising. It is known that several other coronaviruses and respiratory viruses from other virus families can be spread during the incubation period, the period during which a person is infected but has not yet developed symptoms. This was taken account of in some of the modelling exercises undertaken by colleagues recently, and was likely given the degree of spread of the outbreak within China. There is often the question of whether individuals who may be infected with the virus but never show any symptoms (asymptomatic or subclinical cases) may also be contagious to others. The report from Zhejiang suggests this may be the case, but further confirmation is required. This report highlights the importance of identifying individuals who may have travelled from affected regions or had contact with a known case of 2019-nCoV infection to establish if they may have symptoms and to request they limit their attendance at public gatherings in order to try and contain the spread of infection. While these findings may cause concern, the possibility of transmission of virus during the incubation period and from asymptomatic individuals has been considered by public health authorities and the WHO, and has been included as a factor within response plans to tackle an outbreak of an airborne virus.”


Professor Paul Hunter, The Norwich Medical School, University of East Anglia said:

“Current headlines are stating that the Wuhan coronovirus is being spread by asymptomatic people. Whilst there appears to be good evidence from at least one report this is surprising. If person to person spread from people without symptoms became common then this would be extremely worrying. It would also be very surprising.

The consensus from the SARS outbreak was that only patients with symptoms spread the infection. Patients with influenza can transmit the infection before becoming ill but only for a day at most before symptoms develop.  

The primary way that Coronaviruses are spread is by aerosols generated by coughs and sneezes.  By definition once a patient is coughing and sneezing they have already developed symptoms. 

A description of the early cases suggest that on the current outbreak affected patients are less likely report upper respiratory symptoms such as sore throats and runny noses which may even reduce the risk of person to person spread early in the illness. 

It remains the case that the best way to control this outbreak at present is to diagnose cases as early as possible, nurse them in hospitals practicing effective infection control and follow close contacts. 

I would also say that there is insufficient information about how many days after contact the secondary cases became sick to be absolutely sure.”


Notes to Editors:

There have been quotes from China experts suggesting it is happening (See Dr. Kwok-Yung Yuen quotes at this link, for example)

See translation:…t_75650029.htm

Expert: New coronaviruses are infectious when they do not have any typical symptoms during the incubation period
Published: 2020-01-26 14:35:08 | Source: Xinhua | Author: Huang Xiao Wangbing Yang

Xinhua News Agency, Hangzhou, January 26th, reporters learned from Sheng Jifang, director of the Department of Infectious Diseases of Zhejiang University First Hospital, deputy director of the State Key Laboratory of Infectious Diseases Diagnosis and Treatment, and member of the pneumonia expert group for the diagnosis and treatment of new coronavirus infection in Zhejiang Province. Case studies of pneumonia associated with a new coronavirus infection have been contagious when there were no typical symptoms during the incubation period.

‘We encountered a patient who came to Hangzhou from Wuhan to attend a conference. When he arrived in Hangzhou, he did not have any symptoms. He did not have the typical symptoms of cough and fever. But it didn’t take long before several colleagues he had contacted were infected Symptoms appeared one after another. But at this time, he still did not have the disease himself. After returning to Wuhan at the end of the meeting, he did not get the disease after another two days. ‘Sheng Jifang introduced.

Sheng Jifang said that after different people are infected with the new coronavirus, the course of disease development and clinical symptoms may be very different. It is possible that this person developed symptoms immediately after infection, while another person had no obvious symptoms during the incubation period after infection. Therefore, it is difficult for ordinary citizens to identify.

According to the ‘Pneumonitis Prevention and Control Plan for New Coronavirus Infection (Second Edition)’ released by the National Health and Health Committee on the evening of the 22nd, the longest incubation period of the disease is about 14 days based on the current knowledge of pneumonitis infected by the new Coronavirus.

‘This case tells everyone that even during the incubation period, when there are no typical symptoms, it is already contagious.’ Sheng Jifang said. ‘If you come back from Wuhan or have contact with the epidemic source, you should do your own quarantine. Do not contact others during the quarantine, and do not spread the virus to family, colleagues and friends. This is very important.’

Sheng Jifang hopes that those who have come out of Wuhan but have not yet reached 14 days should be isolated by themselves, or those who have symptoms should go to the hospital for screening in time for effective treatment.


All our previous output on this subject can be seen at this weblink:


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