Two pharmaceutical companies are now working on a combination single-shot vaccine that could potential make them billions. American biotechnology companies Moderna and Novavax both announced this week that they are actively developing a vaccine that will be effective in preventing both COVID-19 and the common flu.

Moderna, which supplies the U.S. and large parts of the world with its mRNA vaccine against the coronavirus, said Thursday that it is already in the process of developing a new experimental flu shot.  The company said the vaccine should act as a booster dose against COVID-19, while also being effective against the common flu.

Moderna said it plans to make the shot effective against the respiratory syncytial virus, a common and contagious virus that causes mild cold-like symptoms. Moderna's Chief Executive Officer Stéphane Bancel said during a presentation to investors that the technology could be a "very large opportunity" for the company if it can bring it to market at a high efficacy rate.

Bancel told investors that Moderna could become the first company to bring this type of vaccine combo to the market. He said the company is now conducting clinical trials for an RSV and COVID-19 combo vaccine. Following the presentation, Moderna's stock price rose by more than 6.2%.

Novavax, which currently does not yet have U.S. regulatory approval for its COVID-19 vaccine, said it is also in the early stages of developing its own combination vaccine. The company said it is still conducting studies, but its flu and COVID-19 single-shot vaccine is looking promising.

Novavax mentioned the development of the combo vaccine while it was providing an update on the ongoing mid-stage trials for its COVID-19 vaccine for children below 11 years old. The company said it is now testing 50-microgram doses of the vaccine in a trial involving 4,000 children.

Unlike Novavax, Moderna had received an emergency use authorization for its two-shot vaccine in the U.S. last year. The vaccine is still under FDA review for use in younger patients.

So far, only Pfizer's vaccine has been authorized to be used for children aged 12 to 15. Moderna said it is still conducting dose selection studies for different age groups such as for children 6 months to 2 years old and 2 years old to 5 years old.