Covid-19 incubation period can be up to 24 days instead of 14, says new research from China

Covid-19 incubation period can be up to 24 days instead of 14, says new research from China

The incubation period for Covid-19 could be as long as 24 days, instead of the previously thought 14 days, according to new research by researchers in China.

The study, based on data gathered from 1,099 patients diagnosed with the new coronavirus at 552 different hospitals in China showed that the incubation period for the virus ranged from zero to 24 days, with less than half of patients (43.8%) having a fever when they first see a doctor. The numbers increased to 87 percent after hospitalisation. About 67.7 percent of patients also had a cough.

This could mean that a more extended period of isolation/quarantine is required than the 14 days many health authorities around the world are currently using, including Singapore.

About three dozen researchers in hospitals and medical schools in China led by Dr Zhong Nanshan, the epidemiologist who discovered the severe acute respiratory syndrome (SARS) coronavirus back in 2003, noted in the study that early identification methods could be defective given what we now know, meaning that many infected people could go undiscovered.

The frequency of Covid-19 cases with no fever is higher than those in SARS and middle east respiratory syndrome (MERS), the study found. So surveillance measures and case definition guidelines that focus heavily on fever detection could lead to these infected patients being missed initially.

The study was published on Sunday (9 February) on a medical research archive called medRxiv. The site noted that the paper is a ‘prepublication’ and has not been peer-reviewed, so it should not be used to guide clinical practice.

CT scans fail to identify many who are infected

Another revelation of the study is that relying on CT scans to identify those who might be infected with Covid-19 is not entirely reliable since only half of those who underwent CT scans showed ground-glass opacity, and only 46 percent showed bilateral patchy shadowing.

On 27 January, China’s National Health Commission revised its diagnostic criteria to no longer require CT scans showing pneumonia in identifying suspected cases. However, there was a concern that the nucleic acid tests (NATS) alone were producing too many false-negative results, leading some doctors to suggest that CT scans be used as a basis for diagnosing the infection.

Most transmissions are human-to-human

The study also showed that only 1.18 percent of patients had close or direct contact with wildlife, which is where experts believe the virus might have emerged. However, about 31.3 percent had been to Wuhan City, the epicentre of the virus, and 71.8% had close contact with those from Wuhan.

This tracks with reports of people being infected at gatherings and events – such as the business meeting held in Grand Hyatt Singapore which led to several infections. This presents a risk that the infection could spread further via a small group of people transmitting the virus to others.

The study concluded that Covid-19 is transmitted via the conventional route of respiratory droplets and direct contract. However, it also found that 6.5 percent of 62 stool specimens tested positive for the virus, while testing of four more patients show the presence of the virus in their gastrointestinal tract, saliva, and urine.

The researchers, therefore, urged for protection measures to take into account possible transmission via gastrointestinal secretions.

Lower fatality rate and a threat to medical workers

The study also showed that about 2.09 percent of patients are healthcare workers, shining a light on how the outbreak could be a threat to medical workers on the frontlines.

Though China’s health authorities have not said how many medical workers have been infected, a different paper released by South Central Hospital of Wuhan University said that 40 doctors and nurses were confirmed to be infected as of 28 January.

Finally, researchers found that the fatality rate of Covid-19 is lower compared to that of SARS or MERS which have been reported to be 15 percent and 35 percent respectively.

According to the World Health Organisations (WHO) latest situation report on 11 February, there are now 43,103 confirmed cases of Covid-19 around the world, with 42,708 being in China, which reported 2,484 new cases in just one day. As the country that is hardest hit by the virus, China has also reported 1,017 death due to the virus.

Only one person has died due to the virus outside of China, though he was from Wuhan in Hubei Province, and is believed to have been infected before leaving the country.

In Singapore, the Ministry of Health (MOH) announced two new cases yesterday (11 February), bringing the total up to 47, with 25 of those being locally transmitted cases. Of the confirmed cases, nine have fully recovered from the infection and have been discharged from hospital, said MOH.

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