LONDON — A global study of more than 12,000 coronavirus mutations has found that none of them appear to have made the virus that causes Covid-19 spread more rapidly.

"Recurrent mutations currently in circulation appear to be evolutionary neutral and primarily induced by the human immune system via RNA editing, rather than being signatures of adaptation," researchers of the study said.

The results of the study come as drugmakers and research centers scramble to deliver a safe and effective vaccine to help bring an end to the coronavirus pandemic.

British pharmaceutical giant AstraZeneca said Monday that an interim analysis showed its coronavirus vaccine had an average efficacy of 70%.