Health experts have not pinpointed the reason for the rise but have long warned that colder temperatures driving people inside, fatigue with COVID-19 precautions and students returning to schools and colleges, could promote the spread of the virus.

The United States has the most cases in the world at 8.5 million and the most fatalities with 224,000 lives lost. The United States has reported over the past week an average of 60,000 new cases per day, the highest seven-day average since early August.

WHO chief Tedros Adhanom Ghebreyesus had on Friday warned that “too many countries are seeing an exponential increase in Covid-19 cases and that is now leading to hospitals and intensive care units running close to or above capacity — and we’re still only in October.”

“We urge leaders to take immediate action to prevent further unnecessary deaths.”

AstraZeneca said that it is not unusual for some trial participants to fall ill during large scale vaccine trials but that the U.S. Food and Drug Administration has reviewed all safety data from the trials globally and deemed it safe to continue testing the vaccine.

AstraZeneca trials in the United Kingdom, Brazil and South Africa resumed last month even as the U.S. Food and Drug Administration continued its investigation into the case.