Patients with mild coronavirus symptoms may be highly infectious, study warns

Coronavirus patients with mild symptoms are most infectious within a week of contracting the disease but are unlikely to pass the virus on after 10 days, German researchers have revealed.

In the study of just nine patients – one of the first one of the first to map when people actively transmit the illness to others – scientists found that patients with mild symptoms emit extremely high amounts of the virus at an early stage of their infection.

“Peak shedding” – when a person with Covid-19 is most infectious – typically occurs within five days of picking up the disease, and patients emit 1,000 times more virus than during peak shedding of a Sars (severe acute respiratory syndrome) infection. 

This very high rate helps to explain why the virus has spread so rapidly across the globe, with well over 100,000 people in more than 100 countries now infected, the researchers say. By comparison Sars, a closely related coronavirus, infected some 8,000 people and killed 800 in 2002-03. 

“This is in another dimension compared to Sars,” Dr Clemens Martin Wendtner, co-author of the report and a professor at the Ludwig-Maximilians University in Munich, told The Telegraph.

“The bottom line is that you are infectious even when you have no lung disease. You don’t have to be seriously ill to pass the virus onto other people.

“This virus is spreading even in very asymptomatic patients,” he added.   

The study, which was published as a pre-print this week but has not yet been reviewed by experts, analysed the viral load of nine patients from Munich and Berlin. 

Researchers took samples from the patients at different points during their infection – only if the virus replicated did the experts deem the disease to be transmissible and the patient a risk to others. Scientists were able to grow virus from all patients at some point. 

While patients might have traces of the coronavirus in their system for weeks, meaning tests would come back positive, after eight days of mild illness the team found that it was no longer possible to grow the virus from throat or sputum swabs – suggesting patients are not infectious once they start to recover. 

But two patients, who showed some signs of pneumonia, were infectious for slightly longer – 10 or 11 days. Researchers were also unable to grow the virus from stool, blood or urine samples. 

Critical details could impact public health advice

Dr Wendtner said that mass gatherings – including sports matches and conferences – should be cancelled as people with mild symptoms are highly infectious. 

He added that the results suggest doctors can release people from hospital isolation after 10 days, even if the coronavirus is still detected in tests. 

“In virology you often have shedding of virus for weeks and months, but it would be nonsense to keep these patients in hospital for an extended period of time,” he said.

“From a public health point of view we could discharge patients after 10 days – we think their antibodies kick in and neutralise the infection around day five or six.” 

Jonathan Ball, a professor of molecular virology at the University of Nottingham who was not involved in the research, said the study was important because it is one of the first to look at when patients are infectious.

“The numbers recruited are small but this is thorough research,” he told The Telegraph. “This is a study of what could potentially be the bulk of people infected and the bulk of people transmitting the disease – mild cases.

“Therefore the fact the study shows fairly high levels of virus replication and shedding in individuals likely to have symptoms of a cold, rather than a lower respiratory chest infection such as a cough or shortness of breath, is significant in terms of public health advice.” 

Prof Ball added: “The coronavirus has the perfect combination: a high viral load, it’s a respiratory infection, and it causes mild illness. It’s not going to be contained.”

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