The biggest test yet of an experimental Covid-19 vaccine got underway with the first of some 30,000 Americans rolling up their sleeves to receive shots created by the US government as part of the all-out global race to stop the pandemic, AP reported.

Final-stage testing of the vaccine, developed by the National Institutes of Health and Moderna Inc., began with volunteers at numerous sites around the US given either a real dose or a dummy without being told which.

Another company, Pfizer Inc., announced that it had started its own study of its vaccine candidate in the US and elsewhere. That study also aimed to recruit 30,000 people.

It will be months before results trickle in, and there is no guarantee the vaccines will ultimately work against the scourge that has killed over 650,000 people around the world, including almost 150,000 in the US.