GENEVA

The work to develop a vaccine against COVID-19 is moving full throttle but coronaviruses are "tricky", the World Health Organization's (WHO) spokeswoman said Tuesday.

"The work to develop the vaccine is moving faster than in the history of science, and we have over 100 candidate vaccines, and the ways in which it's being tested are being accelerated," said Dr. Margaret Harris at an online media briefing.

"But coronaviruses are very tricky viruses -- difficult to produce vaccines against."

She said that is why a lot of the leading scientists are managing that expectation to stimulate the level of immunity.

"You need and to ensure that that immunity genuinely protects against the coronavirus. This area of science is the really sharp end of science right now.

"And that's something we don't yet have a clear answer to," she said.

Harris said that the US "is certainly driving the world epidemic".

"The country with the most cases and they're still the most cases in the world is the United States of America," said Harris.

She said: "What went wrong? Why have we got such a large global pandemic? All these questions are being looked at and being asked. And really, we won't have good answers until we have time to analyze."

WHO Director-General Tedros Adhanom Ghebreyesus said that until there is a vaccine against the novel coronavirus countries were urged to follow WHO's "comprehensive package of measures" to tackle it.

He said a clear understanding about current COVID-19 transmission and severity of the virus in children is needed.

As of Tuesday, there were 4.19 million cases of COVID-19 in the world with 286,615 deaths and 1.46 million recoveries. The United States had almost 1.35 million cases with 80,684 deaths and 232,733 recoveries confirmed, the Johns Hopkins University reported.