The World Health Organisation (WHO) has warned “the worst is yet ahead of us” in the coronavirus outbreak, with the vast majority of the public thought to remain at risk from the disease even in countries that are lifting lockdown measures.

Tedros Adhanom Ghebreyesus, the organisation’s director-general, said early data indicated fewer than 3 per cent of the population has had Covid-19 in the worst-affected regions, meaning there is little chance of widespread immunity to the virus.

More than 2.5 million coronavirus infections have been confirmed across the world, but antibody testing – which shows whether a person has recovered from the disease and is now probably immune – is expected to reveal there have been many more undiagnosed and asymptomatic cases.

Dr Ghebreyesus, speaking at a briefing on Monday, said: “Early data from some of these studies suggest that a relatively small percentage of the population may have been infected, even in heavily affected areas – not more than 2 to 3 per cent.”

Maria van Kerkhove, an infectious disease expert leading the WHO’s work on Covid-19, added the preliminary data suggested fewer people than expected had been infected.

She told reporters: “We absolutely must remain vigilant, because what we’re learning from these serologic studies – these early serologic studies, even with all of their faults and all of the limitations – a lower proportion of people are actually, it appears, infected. And that means a large proportion of the public remains susceptible.

“So that means that the virus can take off again.”

Dr van Kerkhove said the figures demonstrated it was vital that countries easing social distancing measures do so “in a slow and staggered, controlled way, and that the systems are in place and the people are there to detect these cases, find the virus, find contacts and have locations where they can be cared for safely”.

Countries including Germany, Denmark, New Zealand and India have begun easing their lockdowns or announced plans to loosen restrictions such as stay-at-home orders, school and business closures, and bans on public gatherings.

The WHO has previously warned that lifting lockdowns too early would spark a “deadly resurgence” of Covid-19.

Dr Ghebreyesus said on Monday: “We want to re-emphasise that easing restrictions is not the end of the epidemic in any country.

“Ending the epidemic will require a sustained effort on the part of individuals, communities and governments to continue suppressing and controlling this deadly virus.

“So-called lockdowns can help to take the heat out of a country’s epidemic, but they cannot end it alone. Countries must now ensure they can detect, test, isolate and care for every case, and trace every contact.”

He acknowledged antibody tests were important for establishing the extent of outbreak, but stressed tests that find the virus in people still infected were “a core tool for active case finding, diagnosis, isolation and treatment”.

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More than 171,000 people with coronavirus have died across the world.

Dr Ghebreyesus warned many more people would lose their lives, telling reporters from the WHO’s headquarters in Geneva: “Trust us. The worst is yet ahead of us.”

He added: “We have been warning from day one: this is a devil that everybody should fight.”

The WHO warned last week that Africa, where many countries have fragile health systems, could become the epicentre of the Covid-19 outbreak.

There have been fewer deaths and infections in the continent compared to Europe and the US, but cases have been rising sharply in recent days.