“[But] we are very worried about the US, where [new cases] are growing at about 20,000 every day … the world cannot control [the disease] and be at peace as long as there is an outbreak in any country,” he said on Friday during a video conference with South Korean experts, hosted by the Chinese embassy in Seoul.

In a separate interview the same day with China News Service, Zeng Guang, chief epidemiologist at the Chinese Centre for Disease Control and Prevention, and another member of the expert panel, said the fact that new infections continued to be reported overseas meant China remained under threat.

“I still don’t see any light for the global pandemic … because this is a new disease and we know so little about it.”

Zeng said unknowns like differences in medical supplies, hospital capacities and quarantine measures in different countries were among the factors making it difficult to say when the pandemic might end.

“And there is the question of how long it will take for developing a vaccine … and how are you going to organise production for everybody in the world to receive a jab?” he said.

Developing countries in Africa, South Asia and Latin America would be the battlefields in the next stage of the pandemic, Zeng said.

Covid-19 could be under control by end of April, respiratory expert says

Both Zeng and Zhong said that expediting the development of vaccines was key to changing the course of the coronavirus pandemic.

“Vaccination is the way to acquire immunity. Herd immunity won’t work – the cost and sacrifices are too huge,” Zhong said, referring to the situation when a large percentage of a population has become immune to an infection.

Based on the current spread of the coronavirus, some epidemiologists have estimated the critical threshold for achieving herd immunity at about 60 per cent.

But Zeng said that was too high a price to pay.

“How many lives do we have to sacrifice for that 60 per cent?”

Zhong said it was possible that the coronavirus, which had become very effective at infecting humans, would not be eradicated in the short term.

“This disease can’t be eliminated,” he said. “It will continue to spread for a long time, but probably will not cause a large scale outbreak.”

He cited studies in China that found that about 6 per cent of a population sample had antibodies to severe acute respiratory syndrome (Sars), the coronavirus that infected more than 8,000 people and killed close to 800 – mostly in mainland China and Hong Kong – during an outbreak in 2002-03.

“It means there have been infections [since 2003] but it did not spread on a large scale. It should be sufficient if we can control [Covid-19] at a similar level,” Zhong said.

How the Wuhan experience could help coronavirus battle in US and Europe

As of Saturday, almost 1.7 million people around the world had been infected with the new coronavirus, and more than 100,000 killed by Covid-19. Despite the huge toll, there have been signs that the outbreak might have reached its peak in some European countries and the US, which has led to discussions about when to relax control measures, such as mandatory lockdowns and social distancing.

In China, months of lockdowns and restrictions on people’s movements took a massive toll on the economy, prompting Beijing to seek a more sustainable and balanced strategy to fight the disease.

The number of Covid-19 patients in mainland China is now under 1,100, with a similar number of people infected but who have yet to show any symptoms. Of the confirmed cases, almost 70 per cent were imported, according to the National Health Commission.

Zhong said that unless China isolated itself from the rest of the world, it was unrealistic to think there would be no imported cases, especially in big cities.

To help keep those numbers down, close surveillance of people coming into the country should be maintained and anyone deemed to be infected should be swiftly quarantined, he said.

On the risk of asymptomatic carriers, Zhong said it was prudent to place them in quarantine for observation, though their number was “very small”.

Zeng said the demographic characteristics of the imported carriers had contributed to new symptom-free cases. Many are Chinese students studying abroad or young people working overseas – an age group that generally had lighter and fewer symptoms, he said.

It was important to help people return to work and schools, he said, adding that resuming normal hospital services and reopening the country to foreign travellers could be more pressing than managing asymptomatic carriers.

“We need a sustainable and efficient prevention and control strategy [for Covid-19] … it has to be cost-effective,” Zeng said.

“At critical moments we [fight the disease] at all costs. But in the long run, we must consider the costs and inputs.”