GENEVA : Astrazeneca's COVID-19 vaccine candidate is probably the world's leading candidate and most advanced in terms of development, the World Health Organization's chief scientist said on Friday.Soumya Swaminathan said that Moderna's COVID-19 vaccine candidate was also "not far behind" Astrazeneca's, among more than 200 candidates, 15 of which have entered clinical trials.The WHO is in talks with multiple Chinese manufacturers, including Sinovac, on potential vaccines, she said.Swaminathan, speaking to a news briefing, called for considering collaborating on COVID-19 vaccine trials, similar to the WHO's ongoing Solidarity trial for drugs.