GENEVA • The number of reported Covid-19 cases has declined globally for the fourth week in a row, but "now is not the time for any country to relax measures or for any individual to let down his guard", the World Health Organisation (WHO) chief said.

In a briefing on Friday, director-general Tedros Adhanom Ghebreyesus said the decline in reported cases appeared to be the result of countries implementing public health measures more stringently.

"We should all be encouraged, but complacency is as dangerous as the virus itself," he said. "Every life lost now is all the more tragic as vaccines are beginning to be rolled out," he said, adding that alongside the traditional public health measures, the speed of the vaccines' distribution to all countries will determine how soon the pandemic will be controlled.

Meanwhile, the head of the European Union's disease control agency also urged caution, warning that the coronavirus could last indefinitely.

In an interview with Agence France-Presse, European Centre for Disease Prevention and Control chief Andrea Ammon urged European countries in particular not to drop their guard against a virus that "seems very well adapted to humans" and may require experts to tweak vaccines over time, as is the case with the seasonal flu.

"So we should be prepared that it will remain with us," said Dr Ammon.

Her comments on Friday came as some countries eased coronavirus containment measures.

South Korea said it was easing social distancing rules, letting nightclubs reopen and extending the operating hours of other businesses.

In Spain, several regions have begun to relax restrictions, with the 14-day infection rate down to 496 cases per 100,000 people from almost 900 late last month.

Iceland, which has Europe's lowest Covid-19 incidence rate, has gradually eased restrictions and allowed swimming pools, gyms and now bars to reopen, focusing instead on testing travellers at the border.

While vaccinations gather pace and countries focus on recovery, questions remain on the source of the virus.

The WHO has not ruled out any hypotheses about the roots of Covid-19, according to Dr Tedros, after a fact-finding mission to China rejected speculation that the coronavirus could have leaked from a lab.

All avenues of research remain open, Dr Tedros said. The investigation "has added important information that takes us closer to understanding the origins of the virus", particularly in the early days of the pandemic.

The comments appeared to pull back on the investigating team's remarks last week. The lab-leak theory was "extremely unlikely" and required no further study, according to Dr Peter Ben Embarek, a WHO food safety scientist.

The coronavirus most likely jumped to humans through an animal host or frozen wildlife products, he said, and those possibilities should continue to be probed.

Former US president Donald Trump suggested that the virus might have escaped from a high-security virology lab in Wuhan, the Chinese city where the virus was first detected.

Chinese state media and officials have promoted the theory that the virus did not start in China, but was brought in.

XINHUA, BLOOMBERG, REUTERS, AGENCE FRANCE-PRESSE