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Beijing, Apr 17: China denied Friday it had covered up the extent of its coronavirus outbreak, as it responded to growing questions from Western powers led by the United States.

A foreign ministry spokesman acknowledged that the virus's rapid spread had contributed to undercounting that resulted in China raising its death toll earlier Friday, but he added "there has never been any concealment, and we'll never allow any concealment."

The allegations China is too close to the World Health Organization (WHO), were an attempt at "smearing" Beijing, Zhao said.

US President Donald Trump has questioned China's handling of the pandemic and whether it had been completely transparent since the virus emerged in the central city of Wuhan late last year.

On Thursday, British Foreign Secretary Dominic Raab and French President Emmanuel Macron also expressed doubts about China's virus response.

These doubts were spotlighted again on Friday when authorities in Wuhan, which has borne the brunt of Chinese deaths, abruptly raised its death toll by 50 percent -- or 1,290 deaths -- to a new total of 3,869.

That also pushed the nationwide death toll up sharply to 4,632, based on official national data released earlier in the day.

Wuhan authorities cited several reasons for the missed cases, including that the city's medical staff were overwhelmed in the early days as infections climbed, leading to "late reporting, omissions or mis-reporting".

Zhao said such miscounting was to be expected in the initial stages of a major disease outbreak.

US President Donald Trump -- under fire himself for initially denying the seriousness of the pandemic -- has accused the WHO of doing the same and being too trusting of China's assurances over the outbreak.

On Tuesday he announced a suspension of US funding to the world body.

Asked about the US allegations, Zhao defended the WHO and China.

"I think they are all smearing China and cooking up stories about China," he said, without specifying which countries he was referring to.

China has largely brought the contagion under control domestically via tough measures including the unprecedented lockdown of Wuhan and tens of millions of people in surrounding areas, but not before it spread worldwide.

London, May 20: The current physical distancing guidelines of 6 feet may be insufficient to prevent COVID-19 transmission, according to a study which says a mild cough in low wind speeds can propel saliva droplets by as much as 18 feet.

Researchers, including those from the University of Nicosia in Cyprus, said a good baseline for studying the airborne transmission of viruses, like the one behind the COVID-19 pandemic, is a deeper understanding of how particles travel through the air when people cough.

In the study, published in the journal Physics of Fluids, they said even with a slight breeze of about four kilometres per hour (kph), saliva travels 18 feet in 5 seconds.

"The droplet cloud will affect both adults and children of different heights," said study co-author Dimitris Drikakis from the University of Nicosia.

According to the scientists, shorter adults and children could be at higher risk if they are located within the trajectory of the saliva droplets.

They said saliva is a complex fluid, which travels suspended in a bulk of surrounding air released by a cough, adding that many factors affect how saliva droplets travel in the air.

These factors, the study noted, include the size and number of droplets, how they interact with one another and the surrounding air as they disperse and evaporate, how heat and mass are transferred, and the humidity and temperature of the surrounding air.

In the study, the scientists created a computer simulation to examine the state of every saliva droplet moving through the air in front of a coughing person.

The model considered the effects of humidity, dispersion force, interactions of molecules of saliva and air, and how the droplets change from liquid to vapour and evaporate, along with a grid representing the space in front of a coughing person.

Each grid, the scientists said, holds information about variables like pressure, fluid velocity, temperature, droplet mass, and droplet position.

The study analysed the fates of nearly 1,008 simulated saliva droplets, and solved as many as 3.7 million equations.

"The purpose of the mathematical modelling and simulation is to take into account all the real coupling or interaction mechanisms that may take place between the main bulk fluid flow and the saliva droplets, and between the saliva droplets themselves," explained Talib Dbouk, another co-author of the study.

However, the researchers added that further studies are needed to determine the effect of ground surface temperature on the behaviour of saliva in air.

They also believe that indoor environments, especially ones with air conditioning, may significantly affect the particle movement through air.

This work is important since it concerns safety distance guidelines, and advances the understanding of the transmission of airborne diseases, Drikakis said.