The EU's medicines regulator on Thursday said it advised medics inject a second dose of the Pfizer/BioNTech coronavirus vaccine three weeks after the first, amid debate over how much time to leave between the two jabs.

"Previously, the product information stated that the interval should be 'at least 21 days'," the European Medicines Agency (EMA) said in a statement.

The vaccine developed by US pharma company Pfizer and Germany's BioNTech, like that produced by rival Moderna, requires two separate doses in order to give the recipient long-term protection against Covid-19.

Britain, the first country to authorise the Pfizer-BioNTech jab, has extended the gap between doses to up to 12 weeks as part of a strategy to quickly vaccinate more people with a first dose.

But that decision has been controversial, with the British Medical Association calling for a maximum of six weeks between the jabs.

In Malta the second dose is given after three weeks. 

The World Health Organization has recommended a four-week gap between doses -- to be extended to a maximum of six weeks in exceptional circumstances. 

The EMA noted that during clinical trials, the vaccine was tested using gaps between doses of up to six weeks.

"There are currently no clinical data on the efficacy of the vaccine when administered beyond intervals used in the clinical trial," it said.

Of those who took part in trials, "93.1 percent of these participants received the second dose 19 to 23 days after the first dose," it said.