No strong proof that people with no symptoms can infect others: Expert

Dr Shawn Vasoo, clinical director of the National Centre for Infectious Diseases, said that as far as is currently known, "transmission is predominantly still through symptomatic persons".
Dr Shawn Vasoo, clinical director of the National Centre for Infectious Diseases, said that as far as is currently known, "transmission is predominantly still through symptomatic persons".PHOTO: LIANHE ZAOBAO

There is no robust proof that the coronavirus can be spread by someone with no symptoms of the disease, said Dr Shawn Vasoo, clinical director of the National Centre for Infectious Diseases.

A letter in the New England Journal of Medicine on Jan 30 stating that this has happened has since been proven to be wrong, he said.

The letter by doctors from the University Hospital of Ludwig Maximilian University of Munich had said a German businessman caught the virus from a Chinese woman whom he had met when she was in Germany.

They said the woman became sick only on her flight home to Shanghai, and was later found to be infected with the coronavirus.

This had led to fears of asymptomatic transmission.

However, it later transpired that the doctors had not spoken to the woman at all, and had merely taken the German man's word that she had been well when they met. When she was interviewed in Shanghai, she said she had been sick while in Germany.

Other anecdotal reports include a letter in the Journal of the American Medical Association on Feb 21 by a team of doctors from China.

They described five members of a family in Anyang, China, who were infected by another family member who was presumed to be asymptomatic.

She had been in Wuhan, where the family and the doctors believe she was infected by the virus.

Without claiming it to be a case of asymptomatic transmission, they said: "The sequence of events suggests that the coronavirus may have been transmitted by the asymptomatic carrier."

Dr Vasoo said this report is limited in that it is not so clear about whether the patient in question had infected the others, or was infected by them.

 
 
 

It was presumed that she was the source of the infection simply because she had been to Wuhan, the outbreak's epicentre.

He added that in this case, "environmental contamination or exposure to other infected persons cannot be excluded".

Professor Paul Tambyah, an infectious diseases expert at the National University of Singapore Yong Loo Lin School of Medicine, said of this report: "That is the only case in the peer reviewed literature so far, and I am not totally convinced about it."

But he added: "I think it is certainly possible that people who are about to get ill may be able to transmit the infection."

People with influenza or chicken pox viruses can infect others "the day before the onset of symptoms", so it is possible that the coronavirus does too, he said.

Dr Vasoo said that as far as is currently known, "transmission is predominantly still through symptomatic persons".

 

Some who are said to be asymp-tomatic may actually have "mild symptoms that people do not make much of".

When asked about reports of asymptomatic carriers of the virus, a Ministry of Health (MOH) spokesman echoed the sentiments of Dr Vasoo about the limited information available.

The spokesman added that with nearly 80,000 patients worldwide, it is not surprising to find one or two outliers.

Instead of focusing on asymp-tomatic transmission, people should think about "what we can all do to protect ourselves", he said, reiterating the importance of personal hygiene.

 
A version of this article appeared in the print edition of The Straits Times on February 25, 2020, with the headline 'No strong proof that people with no symptoms can infect others: Expert'. Subscribe