The company hopes to eventually add vaccines it is working on for respiratory syncytial virus (RSV) and other respiratory diseases as an annual shot.

"We believe this is a very large opportunity that is ahead of us, if we could bring to market a high efficacy pan-respiratory annual booster," Moderna Chief Executive Officer Stéphane Bancel said during a presentation to update investors on its drugs in development.

"We believe Moderna could be first to market in this important new opportunity," he said.

The company is conducting clinical trials for an RSV vaccine in older adults.

Moderna and Pfizer Inc and German vaccine partner BioNTech SE are already expected to reap billions of dollars from COVID-19 booster shots, analysts and healthcare investors said. Adding influenza and other diseases could further boost those profits.

Moderna shares were up 6.2% on Thursday.

The drugmaker already had several influenza vaccine candidates in development. The new vaccine combines the experimental flu shot that is furthest along with its COVID-19 vaccine.

Oppenheimer & Co analyst Hartaj Singh said the news that clinical testing of combination vaccines was likely to begin over the next 6-12 months was a positive surprise for investors.

"The question becomes that after the pandemic is over, how much do total vaccine sales become and how much can Moderna expand that market," Singh said.

Novavax Inc, which does not yet have U.S. authorization for its COVID-19 vaccine, said on Wednesday it has initiated an early-stage study to test its combined flu and COVID-19 shots.

Moderna also provided updates on its ongoing mid-stage trial testing its authorized COVID-19 vaccine in children aged 6 months to 11 years old. It is testing a 50-microgram dose of the shot in the pediatric trial involving 4,000 children.

Moderna's vaccine, which received an emergency use authorization for two 100-microgram doses for people aged 18 and older in the United States in December, is currently under an FDA review for use in adolescents.

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 old, were still underway.

The Pfizer/BioNTech vaccine was authorized for those aged 12 to 15 earlier this year.