Europe's drug regulator has refrained from making any recommendations on mixing shots of Covid-19 vaccines from different drugmakers and said it was too early to confirm if and when an additional booster dose would be needed, Reuters reports.

The European Medicines Agency (EMA), however, did say both doses of a coronavirus vaccine are needed to protect against the fast-spreading Delta variant.

"Adherence to the recommended vaccination course, in line with the product information, is vital to benefit from the highest level of protection against the virus," EMA said in a statement.