Moderna announced Thursday that it is developing a single-dose vaccine that combines a COVID-19 booster and a seasonal flu shot.

The mRNA shot would combine the drugmaker’s COVID-19 vaccine, which has received emergency use authorization from the Food and Drug Administration for people 18 and older, and its flu vaccine candidate.

“I am proud of the progress that the Moderna team has made in advancing our best-in-class mRNA pipeline while addressing the global COVID-19 pandemic,” CEO Stéphane Bancel said in a news release. “Today we are announcing the first step in our novel respiratory vaccine program with the development of a single dose vaccine that combines a booster against COVID-19 and a booster against flu.”

Moderna’s two-dose vaccine became the second to be available in the United States last December. The Massachusetts-based company last month completed its application for full FDA approval for people 18 and older. Last week, it asked the agency to grant it emergency use authorization for its booster shots. 

The company did not give an estimate for when the hybrid shot might be available to the public.

Moderna’s stocks had risen 7.2% after Thursday’s announcement.

It is not the only company developing a combination booster shot. Novavax announced Wednesday it has begun early-stage testing on its own two-for-one vaccine. 

Moderna said it also is working on four vaccine candidates against variants of concern, three of which have been administered in clinical trials. One specifically targets the beta variant, first detected in South Africa, while another targets the delta variant, which was first found in India and which has raged across the U.S. this summer. Another candidate combines the main vaccine with the beta-specific candidate, and the other combines the beta- and delta-specific candidates. 

The company also provided an update on its vaccines trials involving children younger than 12. It said it selected 50 micrograms as the dosage for expanded testing on kids 6 to 11, a trial that it says has reached its full enrollment of 4,000 participants. Studies are continuing to determine the proper dosage for children 2 to 5 years old, and for 6-month-olds to those younger than 2.

Moderna also said clinical trials on its vaccine for 12- to 17-year-olds found no COVID-19 cases and only mild to moderate side effects. The company requested FDA emergency use authorization for the shot for adolescents in June.