"The pandemic is a once-in-a-century health crisis, the effects of which will be felt for decades to come," he told a meeting of the WHO's emergency committee in Geneva.
Although knowledge about the new virus has advanced, many questions remained unanswered and populations remain vulnerable, he said.
"Early results from serology (antibody) studies are painting a consistent picture: most of the world’s people remain susceptible to this virus, even in areas that have experienced severe outbreaks," the WHO chief said.
"Many countries that believed they were past the worst are now grappling with new outbreaks. Some that were less affected in the in the earliest weeks are now seeing escalating numbers of cases and deaths."