A COVID-19 vaccine may be rolled out by March 2021 to the most vulnerable, which along with other advances could fundamentally change the course of the pandemic, a senior World Health Organization (WHO) official said on Monday. Bruce Aylward told the WHO's annual ministerial assembly that interim results from Pfizer Inc's late-stage vaccine trial were "very positive". "There is still much work to be done, this is just interim results...but some very positive results coming today which should hold great promise hopefully for the entire world as we move forward," Aylward told the 194-member state forum.