Three separate studies have suggested that the mass quarantine measures taken to limit the spread of the deadly coronavirus in China may have potentially made it more "insidious" and harder to detect.

Governments around the world have taken increasingly drastic measures to slow the outbreak of the virus, which has been declared by the World Health Organisation (WHO) as a pandemic and infected more than 125,000 people globally.

"The WHO has been assessing this outbreak around the clock and we are deeply concerned both by the alarming levels of spread and severity, and by the alarming levels of inaction," WHO director-general Tedros Adhanom Ghebreysus told reporters earlier this week.

"We have therefore made the assessment that COVID-19 can be characterised as a pandemic."

Sweeping travel bans have since been put in place, schools closed, public events cancelled, and Italy has become the latest country to put its population into lockdown.

But a team of clinical researchers in Wuhan have said that locking down millions of people may have caused mutations in the genetic make-up of the coronavirus, that resulted in milder symptoms of the pneumonia-like illness or no initial symptoms at all in the early stage of infection.

Authorities locked down Wuhan, the city where the new virus strain first emerged in December, on January 23, confining its 11 million people to their homes, halting transport and closing public areas.

The drastic measures were later extended to other cities in the Hubei province, affecting nearly 60 million people, and have since been praised by the WHO for containing the virus' spread.

"There's no question that China's bold approach to the rapid spread of this new respiratory pathogen has changed the course of what was a rapidly escalating and continues to be a deadly epidemic," said Dr Bruce Aylward, who led the WHO delegation.

However, respiratory specialist Zhang Zhan and his colleagues at the Renmin Hospital of Wuhan University have noticed an unusual trend among patients with COVID-19 that may have been caused by the quarantine measures.

"We found that the initial clinical characteristics of patients admitted after January 23 began to differ from those of patients admitted before, which brought new diagnostic challenges," the doctors said in a paper submitted on March 2 to SSRN, a website run by The Lancet for research that has not yet been published.

"Some common systemic symptoms of COVID-19 such as fever, fatigue, phlegm and muscle pain were more prominent in patients admitted before January 23, but more insidious in later patients."

Patients who tested positive for the virus after January 23 did not experience these symptoms as much as those who tested positive before Wuhan went into lock down.

There was a 50 per cent decrease in fever, 70 per cent decline in fatigue and an 80 per cent drop in muscle pain, according to the paper. Some of the 80 patients in the study could have been asymptomatic, it said.

While initial symptoms may have been milder after the lockdown was imposed, the study found that there was no evidence that transmission or the pathogen had weakened.

The doctors said the observations could be "characteristics of virus mutation".

A separate study by researchers from Peking University and the Chinese Academy of Sciences, published on March 3, reached a similar conclusion on the effect of quarantines.

The team, led by bioinformatics researcher at Peking University, Lu Jian, analysed genomic sequence data of the virus from 103 samples collected from patients in China and other countries.

They found the earliest cases in Wuhan might have evolved into two competing strains - an earlier one and a newer, "more aggressive" one.

The newer strain accounted for 96 per cent of the samples collected in early January, suggesting it was more infectious, according to the study published in the National Science Review.

But that more aggressive strain was less prevalent in samples from China and other countries taken after the initial outbreak in Wuhan - falling to about 60 per cent of the total.

The researchers attributed that change in the virus to the mass quarantines.

"These human intervention efforts might have caused severe selective pressure against the (aggressive) type," they wrote in the paper.

A third study, led by Jiang Yongzhong from the Hubei Provincial Centre of Disease Control and Prevention, also found that the genetic make-up of the virus had changed after quarantine measures were imposed.

The study also said that mutations "had occurred across the world … providing the possibility of widespread adaptation".