Moderna has so far supplied 217 million doses, with 110 million more to be delivered by the end of this year and 90 million in the first quarter of 2022.
Brandenburg was skeptical of vaccines in general and the Moderna vaccine specifically, and that he communicated his beliefs to his co-workers for at least the past two years, it said.
Oxford/AstraZeneca is the most widely given vaccine, administered in 170 of the 215 countries where campaigns have begun.
It is followed by Pfizer/BioNTech, which is being used in at least 97 countries; Moderna (at least 46); Sinopharm (45); Sputnik V (40); Johnson & Johnson (29).
Two mass vaccination centres operated by the military in Tokyo and Osaka will open next week and are expected to administer the two-shot Moderna vaccine, initially to the elderly.
The EU currently has four vaccines currently authorised by its European Medicines Agency: from BioNTech/Pfizer, Moderna, AstraZeneca and Johnson & Johnson.
This is because available stocks amount to just 529,000 doses of Pfizer's vaccine and 381,000 doses of the AstraZeneca product, an agency official told Reuters.
Simao said the WHO's independent panel was assessing the Moderna vaccine and a vaccine from China's Sinopharm on Friday and was due to look at China's other main vaccine, made by Sinovac Biotech, next week.
"So, by the beginning of next week or the end of next week we will have the final assessment of these three vaccines out," she told a briefing.
Moderna's vaccine is based on the same messenger RNA technology that Sanofi hopes to use for its own Covid shots, after its initial efforts were called off because of disappointing trial results.