(Reuters) – The potential COVID-19 vaccines backed by the Trump administration’s Operation Warp Speed program are unlikely to receive a green light from regulators any earlier than November or December of this year, given the time needed for a large-scale clinical trial, National Institutes of Health director Francis Collins said in a call with reporters on Thursday.

Collins said he thinks that testing a vaccine in at least 10,000 people could potentially give enough evidence of safety and efficacy to clear it for wider use. The late-stage vaccine trials launched so far in the U.S. aim to recruit up to 30,000 people.