The American company has shared the results of studies with about 350 test subjects. Six months after their second vaccination, they received a repeat dose of 50 micrograms, half of the first doses.
According to Moderna, the number of antibodies to the virus is significantly weakened six months after the first vaccinations. A third dose brings that back to normal.
In the coming days, Moderna also plans to share data with other competent regulators, including the European Medicines Agency.
Pfizer and BioNTech, who jointly provide a vaccine using the same mRNA technology, have already filed documents with US authorities seeking approval for their third dose.
In the US, some people with weakened immunity can already receive a third vaccine dose.
The administration of extra corona shots is sensitive, because vaccination in many developing countries is slow to get started due to a lack of vaccine doses.