(Reuters) - A vaccine developed by Novavax Inc was shown to prevent a common respiratory viral infection in a mid-stage study, taking it one step closer to becoming the first vaccine for the virus that affects almost all American children.

A vaccine to protect against the respiratory syncytial virus, or RSV, has long eluded developers as deficits in the understanding of the virus' molecular structure and multiple trial failures have come in the way.

An RSV vaccine represents a $1 billion opportunity in the United States and potentially double that worldwide, Wedbush analyst Heather Behanna estimated.

For most older healthy children and adults, the seasonal infection causes little more than a common cold, but in high-risk groups - including those with weak immune systems, young infants and the elderly - it can lead to more serious lung and airway infections.

Novavax's vaccine was similar to or better than a number of respiratory vaccines tested in the elderly, data presented on Monday showed.

Armed with this data, Novavax hopes to begin a late-stage 8,000-10,000 patient study in the elderly later this year, coinciding with the RSV season in the United States.

Meanwhile, Novavax is also immunizing pregnant women in a separate mid-stage trial in an attempt to jumpstart infant immunity. Data from this trial is expected later this quarter.

The biotech is aiming for the regulator's fast-track and/or breakthrough therapy pathway, potentially shortening its path towards approval.

Even though vaccine developers have been frustrated since the virus' identification nearly 60 years ago, MedImmune, now part of AstraZeneca Plc, developed a drug that prevents lung infections caused by RSV, but not the RSV infection itself.

"If you look at the principle involved in this monoclonal antibody working, you knew if you could replicate that, your vaccine should work ... that's been our big breakthrough," Novavax's senior vice president of R&D, Gregory Glenn, told Reuters.

According to researcher Dr. Pedro Piedra of the Baylor College of Medicine, who also serves on Novavax's scientific advisory board, RSV kills fewer people than influenza.

Piedra said between 10,000 and 15,000 elderly Americans a year on average die from RSV, while 20,000-50,000 older adults succumb to the flu.

For the flu we know how antibodies relate to stopping the infection, for RSV we haven't drawn those correlations yet, Behanna said.

"Part of what Novavax is doing for the first time is getting us data to help understand that relationship."

(Reporting by Natalie Grover in Bengaluru; Editing by Saumyadeb Chakrabarty)