Explained | When can people transmit the novel coronavirus?

The main driver of nCoV transmission is people who exhibit overt symptoms. Such people will spread the virus more readily through coughing and sneezing, WHO says.

Updated - November 28, 2021 11:35 am IST

Published - February 06, 2020 09:02 pm IST - CHENNAI

A disinfection worker at Budapest's Liszt Ferenc Airport on February 5, 2019.

A disinfection worker at Budapest's Liszt Ferenc Airport on February 5, 2019.

The spread of the novel coronavirus in mainland China continues unabated. On February 5, mainland China recorded 3,694 new confirmed cases of novel coronavirus (2019-nCoV) and 73 deaths, taking the total number of confirmed cases to 28,018 and deaths to 563 . This is the second consecutive day when the number of daily new cases rose sharply. On February 4, mainland China recorded 3,887 new confirmed cases and 65 deaths.

The Hubei province alone, the epicentre of the outbreak, reported 2,987 new confirmed cases and 70 new deaths on Wednesday. With this, the Hubei province so far has 19,665 confirmed cases and 549 deaths.

Which countries/regions outside mainland China have reported nCoV cases?

As on February 5, 28 countries/regions have reported 243 cases of the virus.


With 35 cases, Japan has reported the most number of cases. It is closely followed by Singapore with 28; Thailand 25; Hong Kong 21 cases including one death; South Korea 19; Australia 14; Germany and Malaysia 12 each; Taiwan, and the U.S. 11 each; Macau and Vietnam 10 each; France 6; U.A.E. 5; Canada 4; India 3; Philippines 3 cases including one death, Italy, U.K., and Russia 2 cases each; Nepal, Cambodia, Finland, Sri Lanka, Belgium, Spain, Sweden and Cambodia 1 case each.

How many human-to-human transmissions have been seen outside mainland China so far?

As on February 4, the World Health Organization said 27 cases of human-to-human transmission of the virus had been reported from nine countries outside of China.

The first case of human-to-human transmission outside China was reported from Vietnam. It was followed by Japan, Thailand, South Korea, Germany, Singapore and the U.S.

So far, 15 healthcare workers in mainland China have been infected through such transmission. The main route of transmission currently in China is believed to be through human-to-human transmission as the Huanan Seafood Wholesale Market in Wuhan city, which is considered as the source of the virus, was shut down on January 1.

Can the virus be transmitted during the incubation period?

China’s National Health Commission Minister had first warned that the virus might be spreading even during the incubation period when symptoms do not show up in a person infected with it. Then on January 30 , the New England Journal of Medicine (NEJM) published a Correspondence that reported a Chinese woman who had purportedly not shown any signs or symptoms of infection having transmitted the virus to a German during the incubation period.

The NEJM paper was, however, found to be misleading with the journal Science reporting that The Robert Koch Institute (RKI), the German government’s public health agency, and the Health and Food Safety Authority of the State of Bavaria having established, based on a conversation with the Chinese woman, that she had indeed had a few of the symptoms while in Germany. According to the Science , she “felt tired, suffered from muscle pain, and took paracetamol, a fever-lowering medication”.

What is WHO’s position on virus transmission during the incubation period?

WHO’s Situation Report posted on February 1 said: “WHO is aware of possible transmission of the novel coronavirus from infected people before they developed symptoms. Detailed exposure histories are being taken to better understand the pre-clinical phase of infection and how transmission may have occurred in these few instances.”

It then stresses that “asymptomatic infection may be rare, and transmission from an asymptomatic person is very rare with other coronaviruses, as we have seen with Middle East Respiratory Syndrome (MERS) coronavirus. Thus, transmission from asymptomatic cases is likely not a major driver of transmission.”

The main driver of nCoV transmission is people who exhibit overt symptoms. Such people will spread the virus more readily through coughing and sneezing, WHO says.

The Public Health Agency of Sweden says on its website that scientific evidence on the coronavirus causing severe acute respiratory syndrome (SARS) shows that it “does not infect at all during the incubation period . There is therefore much to suggest that similar would also apply to the new coronavirus”.

Can people exhibiting mild symptoms transmit the virus?

Two laboratories have been independently monitoring virus shedding by patients exhibiting few or minor symptoms and being treated in Munich. These patients have “symptoms of common cold rather than viral pneumonia”, according to a press release.

The doctors at the two laboratories could isolate the infectious virus from the pharynx (part of the throat). Both laboratories “found signs of viral replication in the pharynx” besides the lungs.


Based on these observations they say that persons who have “mild or early symptoms of common cold (sore throat, signs of sinusitis, feeling unwell without fever) may be able to transmit the novel coronavirus to other persons”.

“Because of the immediate relevance for infection control, the participating institutions have decided to release this information prior to completion of studies and formal scientific publication,” the researchers said. The observations have not been published in any journal post peer-reviewing. The findings have not been independently verified by other labs, either.

What do people with mild symptoms spreading the virus mean for infection control?

The Chinese woman who infected the German colleague is an example of a person exhibiting mild, non-specific symptoms spreading the virus to others.

If further studies find that some people infected with the virus have only mild symptoms, it would mean that many people might have gone or will go undetected. So the actual number of cases might be higher. More studies are needed to confirm if people with mild, non-specific symptoms might be spreading the virus and infecting others.

Like in the case of the Zika virus, can the nCoV be transmitted from a mother to the child (vertical transmission)?

In China, a baby born to a mother who tested positive of the virus was found to have been infected 30 hours after birth.

“This reminds us to pay attention to mother-to-child being a possible route of coronavirus transmission,” chief physician of Wuhan Children Hospital’s neonatal medicine department, Zeng Lingkong, told Reuters.

But it is also possible that the baby was infected after birth probably due to close contact with the mother. No details are yet available to confirm or refute the claim by the Chinese doctor.

Vertical transmission (mother-to-child) is confirmed if the virus is transmitted from the mother to the foetus, during delivery or from breastfeeding. According to WHO, there have been no reported cases of vertical transmission in the case of Severe Acute Respiratory Syndrome (SARS) or Middle East Respiratory Syndrome ( MERS ).

According to the New York Times , the hospital disclosed details of a second case involving an infant who was born healthy but got infected 16 days later. The mother and the baby’s nanny were diagnosed with the virus after the baby was born.

Though the median age of patients is between 49 and 56 years, these two cases suggest that even infants can get infected with the novel virus.

Does a negative result mean the person is not infected with the novel coronavirus?

Not always. There have been at least a few instances when people have initially tested negative for the novel coronavirus before being testing positive. It is not clear at what stage of infection the initial tests and final tests were done.

Of the 1,155 people tested, as on February 6, in India for the virus, only three have been found positive. The Ministry of Health and Family Welfare said in a press release on February 6 that all 645 evacuees from Wuhan had tested negative. In addition, 510 samples had been tested by ICMR network laboratories and only three had turned out as positive. All the three people who tested positive are being kept in isolation wards in Kerala.

Li Wenliang, an ophthalmologist in Wuhan, believed to be a whistle-blower on the coronavirus outbreak, felt unwell and started coughing after treating a patient who later developed a fever. Dr. Li’s test results came back negative a few times before his infection was confirmed.

Similar cases have been reported from elsewhere. In Canada , a Chinese woman initially tested negative before further tests confirmed infection. In another case in Canada, a student from Wuhan, who was asymptomatic, initially tested negative. A second test confirmed the case as positive. In Japan , a person who returned from Wuhan had symptoms such as fever and coughing but initially tested negative. An additional test discovered the infection, according to a February 1 news report.

The failure to detect at the first instance could be due to early stages of the infection. “Our testing procedures are evolving and getting more and more precise,” Ontario’s Chief Medical Officer of Health David Williams said in a statement .


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