Spain is unlikely to make vaccination against the coronavirus compulsory, at least initially, health ministry sources said.

Under Spanish law, vaccination is voluntary, although in some cases, such as an epidemic, the government could make it compulsory.

"There are instruments to make it so. But it is not the plan at the moment," one source said, while another said all vaccination was likely to remain optional.

Prime Minister Pedro Sanchez said on Sunday that Spain would begin a coronavirus vaccination programme in January, covering a substantial part of the population within six months.

The sources said, however, there was still not enough information about the Phase 3 trials of the most promising vaccines to say which Spain would buy, how it would transport and store them or who would be vaccinated first.

They said that when all the data is available and vaccines authorised for use in the European Union, Spain can quickly vaccinate much of its population thanks to its "vast vaccination experience" via a network of public healthcare centres.

AstraZeneca said on Monday its vaccine could be as much as 90% effective. It is cheaper to make, easier to distribute and faster to scale-up than rivals from Pfizer Inc or Moderna.