“And there's a good chance many will become sick,” she said. She emphasised that empirical findings from a study of the outbreak in China – where most of the illness and deaths linked to the virus have occurred – showed that more than 80 per cent of people who were infected had only mild flu-like symptoms.

For now, Messonnier said, most places in the US are not seeing community transmission. While Washington state and California may begin adopting some “mitigating” measures such as reducing mass gatherings to deal with likely community transmission, not all parts of the country need to follow their example, she said.

Containment, or efforts to prevent community transmission through rigorous contact tracing of infected patients, remains a vital tool for public health officials, she said.

She reiterated earlier advice by the likes of Vice-President Mike Pence – Trump’s point man in dealing with the virus – that the people most vulnerable to the Covid-19 disease were the elderly with pre-existing conditions. These “people at higher risk” should take precautions such as avoiding crowds in poorly ventilated spaces and deferring travel on cruises.

Messonnier’s comments coincided with a separate briefing by the WHO during which the UN agency’s director general, Tedros Adhanom Ghebreyesus, said that with the virus now having a foothold in “so many countries, the threat of a pandemic has become very real”.

“But it would be the first pandemic that could be controlled. The bottom line is we are not at the mercy of the virus,” he said.

The US is among the countries where infections have surged. The CDC said at noon that 34 states plus the District of Columbia have reported a total of 423 cases. Nineteen people have died of the virus in the US.

Earlier on Monday, Trump continued to play down the contagion as he compared it with the annual flu outbreak, and also used higher case and fatality numbers than those reported by the CDC. In the case of the flu, “nothing is shut down, life [and] the economy go on”, he wrote on Twitter.

“At this moment there are 546 confirmed cases of CoronaVirus, with 22 deaths. Think about that!”

Trump remains defiant as White House is rebuked for coronavirus response

Epidemiologists have warned that such a comparison is deeply problematic. On average, seasonal flu strains have a fatality rate of 0.1 per cent. The WHO estimates that the figure may be 3.4 per cent for Covid-19.

While the rate may eventually be much lower – one study published on Friday put it at 1.4 per cent – it would still mean that the coronavirus is far more deadly compared with the seasonal flu, for which there is a vaccine.

In a display of the continued mixed messaging, Health and Human Services Secretary Alex Azar, in a Fox News interview on Monday that was broadcast just before Trump’s tweet, described the crisis as a “very serious health problem”.

He said Trump’s directive for a “whole-of-government approach” to the coronavirus showed that no one was trying to minimise the impact of the outbreak.

Discussions on extensive social distancing measures such as the cancellation of mass events – including political rallies – have come to the fore as the situation worsens in the US. Several universities, including Stanford and the University of Washington in Seattle, have cancelled in-person classes and moved lessons online.

On Monday. the number of infections worldwide stood at over 110,000, with the fatality count at 3,870.