Tedros Adhanom Ghebreyesus warned that nations around the world are doing too little to stop the virus, which has got more contagious over time.
The fittest variants are the best at transmitting, scientists say. Delta is more transmissible than any other variant so far.
Soumya Swaminathan, WHO chief scientist, said "we don't have the information that's necessary to make the recommendation" on COVID-19 booster shots.
Facebook's policy reversal comes after President Joe Biden asked the intelligence community to find a definitive answer on the virus' origins.
Hans Kluge said people should also "rethink or avoid" international travel, in a departure from the EU position.
A study found that working 55 hours or more every week - or 11 hours a working day - led to 745,194 people dying from a stroke or heart disease.
As wealthier countries stockpile vaccines, some poorer nations have yet to receive any doses. "The solution is more sharing," the WHO head said.
COVAX, the WHO-led project to vaccinate the world's poorest nations, aims to vaccinate 2 billion people by the end of 2021.
Working 55 hours or more a week was linked to a 35% increase in strokes and a 17% increase in deadly heart disease, according to the WHO report.
Despite excellent handling of the COVID-19 pandemic, Taiwan did not receive an invitation to the World Health Organization's annual assembly.
The coronavirus' origin remains a mystery. So the idea that it was accidentally released from a lab remains viable, according to 18 scientists.
The conclusion came in a report by The Independent Panel for Pandemic Preparedness and Response released Wednesday.
WHO Regional Director for Europe Hans Kluge revealed the data while speaking to reporters in Greece on Thursday.
WHO epidemiologist Maria Van Kerkhove says the US had 4.4 million new COVID-19 cases last week, compared to 500,000 new weekly cases a year ago.
The travel industry is hoping a COVID-19 passport will get people flying again, but opponents say there are issues with standardization and privacy.
China complained that the US is politicizing the origins of the COVID-19 pandemic as Beijing faces criticism over a lack of transparency.
WHO experts investigating the coronavirus' origins think it jumped from bats to an animal species like minks or pangolins before infecting people.