Dr Tedros Adhanom Ghebreyesus said he would reconvene the WHO's emergency committee this week for a review.
There have been five other global health emergencies: Ebola (two outbreaks), Zika, polio and swine flu.
More than 16m cases of Covid-19 have been reported since January, and more than 650,000 deaths.
"When I declared a public health emergency of international concern on 30 January... there were less than 100 cases outside of China, and no deaths," Dr Tedros said.
"Covid-19 has changed our world. It has brought people, communities and nations together, and driven them apart."
The total number of cases, he added, had roughly doubled in the past six weeks.
Although the world had made a huge effort in fighting the virus, there remained "a long hard road ahead of us", he said.
(Except for the headline, this story, originally published by bbc.com has not been edited by SLM staff)