Novavax, a Maryland-based biotech company, aims to develop a vaccine to treat RSV.

If mid-stage trial data proves positive, Novavax Inc will be closer than any drug developer to a vaccine for respiratory syncytial virus, or RSV, which affects those most with compromised immune systems — including young infants and the elderly.

An effective vaccine would represent a $1 billion opportunity in the United States and potentially double that worldwide, said Heather Behanna, an analyst at Wedbush Securities.

But the complex molecular structure of the virus has proven to be an issue for drug developers in the nearly 60 years since its discovery. If the mid-stage trial data disappoints, Novavax will join a handful of other developers that have failed at this stage.

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