Scientists at the US Food and Drug Administration (FDA) said on Tuesday that US pharmaceutical company Moderna Inc (Nasdaq:MRNA) had not met all of the agency's criteria to support use of booster doses of its COVID-19 vaccine, Reuters news agency reported on Wednesday.

This may be because the efficacy of the shot's first two doses has remained strong.

The FDA staff said in documents that data for Moderna's vaccine showed that a booster does increase protective antibodies, but the difference in antibody levels before and after the shot was not wide enough, particularly in those whose levels had remained high.

These documents were released ahead of a meeting later this week of the FDA's outside expert advisers to discuss booster doses of the vaccine.

Moderna is seeking authorisation for a 50-microgram booster dose, half the strength of the original vaccine given in two shots about four weeks apart. The company has asked US regulators to approve a third vaccination for adults aged 65 and over, as well as for high-risk individuals.

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