"The only difference is that some of the stages overlapped so, for example, phase three of the trial - when tens of thousands of people are given the vaccine - started while phase two, involving a few hundred people, was still going on," says BBC Health reporter Rachel Schraer.
"ຄວາມແຕກຕ່າງພຽງແຕ່ວ່າບາງໄລຍະຊ້ອນກັນ, ດັ່ງນັ້ນ, ໃນໄລຍະທີສາມຂອງການທົດລອງ - ໃນເວລາທີ່ປະຊາຊົນຫລາຍພັນຄົນໄດ້ຮັບການສັກຢາວັກຊີນ - ເລີ່ມຕົ້ນໃນຂະນະທີ່ໄລຍະທີສອງ, ເຊິ່ງກ່ຽວຂ້ອງກັບປະຊາຊົນສອງສາມຮ້ອຍຄົນ, ຍັງ ດໍາ ເນີນຢູ່ຕໍ່ໄປ," ເວົ້າວ່າ ນັກຂ່າວ BBC Health Rachel Schraer.