Moderna Inc said on Thursday it was developing a single-dose vaccine that combines a booster dose against COVID-19 and a booster against flu.

"Our number one priority as a company right now is to bring to market a Pan-respiratory annual booster vaccine, which we plan to always customize and upgrade," Chief Executive Officer Stéphane Bancel said during Moderna's R&D day.

The company provided updates on its ongoing mid-stage trial testing its authorized COVID-19 vaccine in children aged 6 months to less than 12 years.

Moderna says its Covid-19 shot 93% effective six months after second dose

Moderna's vaccine, which received its emergency authorization for people aged 18 and older in the United States in December, is currently under an FDA review for use in adolescents.

As part of the pediatric trial, the company said it will test 50 micrograms of its vaccine in a study group of 4,000 children between 6 years and less than 12 years.

Moderna said dose selection studies for different age groups such as 2 years to less than 6 years, and 6 months to below 2 years age groups, were still underway.

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Rival vaccine from Pfizer Inc and German partner BioNTech was authorized for ages 12 to 15 years earlier this year.

Novavax Inc said on Wednesday it has initiated an early-stage study to test its combined flu and COVID-19 vaccine.