The mission has become politically charged, as China seeks to avoid blame for alleged missteps in its early response to the outbreak.
The researchers, who were required to complete 14 days in quarantine after arriving in China, could be seen leaving their hotel and boarding a bus on Thursday afternoon.
The WHO had granted emergency use validation for the Pfizer-BioNTech Covid-19 vaccine on December 31, 2020, opening the door for countries to expedite their own regulatory approval processes to import and administer the vaccine.
His first executive order was launching his 100-day masking challenge, asking Americans to mask up for 100 days.
Two members had already set out on their journey - one has now turned back and the other is in transit in a third country, the BBC reported on Wednesday.
While many countries rely on their own regulatory systems for vaccine approvals, some countries with weak system depend on WHO's nod to a great extent for rolling out their national vaccination programmes.
Awareness drive planned in fluoride-hit villages of Nalgonda district from this month
The overall number of global coronavirus cases has topped 61.5 million, while the deaths have surged to more than 1.44 million, according to the Johns Hopkins University.
The antiviral is one of only two medicines currently authorized to treat Covid-19 patients across the world.
WHO Director General Ghebreyesus made the announcement in a video message at an event in which PM Modi inaugurated two future-ready Ayurveda institutions via video conferencing.
To curb the spike in Germany, Chancellor Angela Merkel has ordered a round of shutdowns from Monday until the end of the month.