In a few years biotech company Novavax Inc's jab could become the first vaccine approved in the US against respiratory syncytial virus, or RSV.

Early research in older adults found an experimental vaccine prevented nearly two-thirds of serious cases of a common, seasonal respiratory virus that annually kills thousands of vulnerable Americans - babies and senior citizens.

If further testing by vaccine developer Novavax Inc. goes well, in a few years the biotech company's genetically engineered shot could become the first vaccine approved against respiratory syncytial virus, or RSV.

"It could be a major breakthrough," said Dr Andrew Pavia, a spokesman for the Infectious Diseases Society of America, cautioning that the preliminary study's results must be duplicated in more people.

"It's not time to break out the champagne."

There's no specific treatment for RSV, and drugmakers for the last half-century have unsuccessfully tried to create a vaccine.

In the US, it infects virtually every child by age two, is the top cause of infant hospitalisations and kills an estimated 11,000 to 14,000 people 60 and older each year.

Worldwide it causes more than 30 million lower respiratory infection episodes and kills roughly 175,000 children under age five annually.

The virus causes mild cold symptoms or none at all in healthy adolescents and adults, who are mostly protected by immunity built up from childhood exposure.

But for those at the beginning or end of life, whose immune systems are either revving up or sputtering out, it's a different story.

It's also a risk for those with weak immune systems, such as bone marrow transplant and cancer patients and people with uncontrolled HIV.

The virus only kills several hundred children each year in the US because of widespread availability of supplemental oxygen and other supportive medical care.

But by age 65 immunity to RSV has mostly worn off and the virus lands up to 180,000 people in hospital with pneumonia and other dangerous respiratory problems.

Novavax has been developing this vaccine for four years, testing it in animals and now people.

Past efforts by other companies didn't work because they were making vaccines either from killed RSV virus or a key protein on its surface, Dr Gregory Glenn, head of research and development at Novavax, said.

Those had membranes around them that made it very hard for the immune system to recognise the virus, allowing repeated infections over the years, including ones causing serious symptoms in about five per cent of seniors each year.