Vaccines for RSV, the illness that's been sickening babies across the US, are poised to become available in 2023

• There is no vaccine available for RSV, a seasonal illness that can be deadly for babies and elders.
• Scientists tried out one RSV vaccine in the 1960s, but it made subsequent RSV infections worse.

Respiratory syncytial virus, or RSV, is ripping through pediatric hospitals across the country, with overflowing ERs struggling to deal with an unusually high influx of sick babies and toddlers who are struggling to breathe.

There is no approved vaccine or treatment available for RSV yet. The only preventative measure doctors can offer little ones at severe risk of infection is a pricey, monthly injection of monoclonal antibodies during the winter cold season.

As a result, RSV kills more than a hundred babies and 10,000 adults over the age of 65 in the US every year. Apart from the deaths, there are more than 100,000 hospitalizations, and more than 2 million clinic visits attributed to the disease, according to the Centers for Disease Control and Prevention.

But more than half a century ago, there was a vaccine for RSV. Four separate clinical trials of the vaccine FI-RSV in the late 1960s led to catastrophe. Instead of protecting babies from illness, the vaccine actually made their subsequent RSV infections worse. Roughly 80% of the children who tried that vaccine were later hospitalized with RSV. Two vaccinated babies later died of lung issues.

It's taken more than 50 years since then to made meaningful headway on RSV vaccines, but if all goes well with US regulators, two safe and effective RSV vaccine candidates are poised to be approved for use in pregnant women and elderly adults in 2023. A new one-time monoclonal antibody injection for healthy newborns may also be on the way next year.

Vaccine makers were spooked after an RSV vaccine failed in the 1960s

Dr. Wilbur Chen, an infectious disease expert at the University of Maryland School of Medicine who researches vaccines and serves as an independent advisor to the CDC, said the "leading hypothesis" is that the 1960s FI-RSV vaccine triggered the wrong kind of immune response in patients.

Babies who got the vaccine ended up with more severe disease, not less, when they were exposed to RSV after vaccination. Researchers think this is because the FI-RSV vaccine focused on the wrong stage of the virus' life cycle, or had the wrong parts of the virus in it.

"People were, after that first failure in 1960s, afraid to develop candidate vaccines because it takes many millions of dollars and no one wanted to invest in something that ultimately they thought might tank," Chen told Insider.

Scientific advances made over the past decade at the not-for-profit National Institutes of Health have propelled a new class of RSV candidates, which work differently from the vaccine of the 1960s. What are known as fusion protein vaccines focus on how RSV acts in the initial stages of an infection, before it binds to our cells, instead of after fusion, like FI-RSV did.

"There are certain proteins that are really keys to providing protection," Chen said. "So for coronavirus, it's the spike protein, or S-protein, and in this case, it's the F-protein or fusion protein."

By stabilizing the F protein in a pre-fusion state, instead of post-fusion, scientists have been able to develop vaccines that work better and more safely than the old ones.

RSV vaccines for pregnant people and elderly adults may come soon

In recent years, scientists at the pharmaceutical companies Pfizer and GSK have refined the NIH's pre-fusion technique further, developing vaccines now in late-stage clinical trials in thousands of people around the world.

If all goes well with the FDA and CDC, those vaccines could be available in doctors offices and pharmacies across the country next year. But, neither vaccine is going to be approved for babies.

GSK's vaccine is designed for people over the age of 60. The company said last month it was roughly 94% effective at preventing severe disease in older adults, in a controlled trial of approximately 25,000 people across 17 countries.

Pfizer's vaccine is designed to be given to pregnant mothers, who will protect their babies by passing on antibodies. Earlier this month, the company announced its vaccine was about 69% effective at preventing severe RSV in newborns during the critical first six months of life, in a trial of approximately 7,400 pregnant people across 18 countries.

Chen says that if the two new vaccines are approved for use in the US, something that could happen by the end of 2023, "we're going to see a significant decrease over time for RSV hospitalizations and deaths."

A new monoclonal antibody injection from AstraZeneca and Sanofi for healthy newborns was also just approved in the EU, and it could be available to infants across the US next year. Though the product isn't technically a vaccine, it's a one-time injection designed to protect newborns through their first RSV season in much the same way vaccination would. In a clinical trial of more than 1,400 infants, this formulation was more than 78% effective at preventing hospitalization.

But if you're young, not pregnant, and otherwise healthy, don't expect to line up for an RSV shot any time soon.

"For the rest of the population, RSV is basically an annoying common cold syndrome," Chen said. "It's really for the older adults, really young infants, immunocompromised — they are the ones that will actually progress into frequent hospitalization or respiratory failure," and death.