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The CDC a single dose of any FDA-approved respiratory syncytial virus (RSV) vaccine for all adults ages 75 years and older, as well as for 60- to 74-year-olds who are at increased risk of severe illness from the virus.

The CDC updated its adult RSV vaccination recommendations in June after real-world data confirmed how effective RSV vaccines were in their first season of use against hospitalizations, critical illness, and emergency department visits among adults ages 60 and older, including those who were immunocompromised.

The shift to a clear recommendation for vaccination goes beyond the prior guidance "just to encourage adults to talk to their provider to determine whether they should get an RSV vaccine," said the CDC's Amanda Payne, PhD, an author on a real-world effectiveness study published in . (The previous guidance simply recommended shared clinical decision-making about the vaccine for all adults age 60 and up.)

So what constitutes being at risk of severe disease in that 60- to 74-year-old age group? The , including chronic lung or respiratory disease, severe obesity, chronic cardiovascular disease, moderate or severe immune compromise, and living in a nursing home.

Other conditions include end-stage renal disease or dependence on hemodialysis, diabetes with comorbidities, certain neurologic or neuromuscular conditions, chronic liver disease, chronic hematologic conditions, and other chronic conditions determined risky by a healthcare provider.

Notably, individuals can self-attest to conditions that place them at high risk of severe illness from RSV, according to the CDC.

"Age most certainly is not the only risk factor," said Angela Branche, MD, of the University of Rochester Medical Center in New York, who authored a comment about RSV effectiveness published in .

"One thing that often gets missed is that RSV is associated with a lot of exacerbations of underlying health conditions," such as chronic heart and lung problems, Branche added.

Frequently, "what causes [an individual] to be hospitalized is not the virus itself," she said. For instance, most of the time when people end up in the hospital they are "starting to already clear the virus," but an underlying condition that has been exacerbated has led to the admission, she said.

For instance, an individual may be in heart failure or struggling to breathe due to a flare up of chronic obstructive pulmonary disease (COPD) or asthma following RSV infection, Branche noted.

All three RSV vaccines -- GSK's Arexvy, Pfizer's Abrysvo, and Moderna's mRESVIA -- are FDA-approved for all adults age 60 and up (and one RSV vaccine, Pfizer's Abrysvo, is FDA-approved and CDC-recommended for use in pregnant women to protect newborns.)

But some of the vaccines recently picked up expanded indications for use in younger populations at higher risk of severe disease from RSV. In June, Arexvy won expanded approval for use in at-risk adults ages 50 to 59, and in October, Abrysvo won an expanded approval for use in at-risk adults ages 18 to 59.

The CDC, however, has not yet included these younger age groups in its RSV vaccine recommendations. The agency's Advisory Committee on Immunization Practices (ACIP) in June " to inform an RSV vaccine recommendation in adults aged 50-59 years who are at increased risk for RSV disease," according to the CDC.

Sufficient evidence would require "updated RSV vaccine safety analyses among adults 60 and older, including results from the full 2023–2024 RSV season in the FDA-CMS partnership analysis incorporating chart confirmation of GBS [Guillain-Barré syndrome] diagnoses; additional data on duration of protection from RSV vaccination and immune response after revaccination; and immunogenicity data in adults with immunocompromise."

GBS following RSV vaccination among adults ages 65 and older was discussed during an October meeting of ACIP. Slides presented during the meeting noted that GBS risk following vaccination with Arexvy and Abrysvo was rare, with .

Branche said generating more real-world evidence on vaccine efficacy in younger groups will be beneficial, because clinical trials are unlikely in certain groups, such as vulnerable, immunocompromised younger adults, due to ethical concerns of knowing the vaccine would help some participants compared with a potential placebo group, she noted.

Payne confirmed that the CDC "does certainly intend to continue monitoring our RSV vaccine effectiveness in future seasons," garnering "hopefully more data to contribute to our understanding of vaccine effectiveness," including in areas like duration of protection.

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