A World Health Organization (WHO) panel of experts recommended on Tuesday that the Moderna COVID-19 vaccine be given in two doses at an interval of 28 days, which could be extended under exceptional circumstances to 42 days.

The WHO's Strategic Advisory Group of Experts on Immunization, known as SAGE, issued a series of recommendations on Moderna, several weeks after issuing their guidance on the rival Pfizer shot.

After reviewing Moderna data the independent experts said that that vaccine not be used on pregnant women unless they are health workers exposed to the virus or have medical conditions putting them at high risk.

"The main recommendation for the use of this vaccine is that based on the current elements we recommend it should be given in doses of 100 micrograms or 0.5 ml with an interval of 28 days," Alejandro Cravioto, panel chair, told a news briefing in Geneva.

"This interval might be moved to 42 days but the evidence we have does not go behond that time," he said, speaking from Mexico.

WHO director of immunization Kate O'Brien said that clinical trials of the Moderna vaccine were needed on pregnant women.

"There is no reason to think there could be a problem in pregnancy, we are just acknowledging the data is not there at the moment," she said.

The WHO was liaising with Moderna on data it submitted as part of its application for WHO emergency use listing and would make a decision soon, she added.