Moderna Inc has said its Covid-19 shot was about 93 per cent effective through six months after the second dose, showing hardly any change from the 94pc efficacy reported in its original clinical trial, Reuters reports.

That compares favorably to data from Pfizer Inc and BioNTech SE last week in which they said their vaccine's efficacy waned around 6pc every two months, declining to around 84pc six months after the second shot.

Both the Moderna and Pfizer-BioNTech vaccines are based on messenger RNA (mRNA) technology.

"Our COVID-19 vaccine is showing durable efficacy of 93pc through six months, but recognise that the Delta variant is a significant new threat so we must remain vigilant," Moderna Chief Executive Stéphane Bancel said.