RSV vaccines contain a lab-made version of a protein found on the surface of respiratory syncytial virus, designed to train your immune system to recognize and fight the real virus. Three RSV vaccines are currently approved in the United States, and while they all target the same viral protein, they use different technologies and ingredients to deliver it.
The key protein in all three vaccines is called the prefusion F protein. RSV uses this protein to break into your cells, and it exists in two shapes: a “prefusion” form (before it attacks a cell) and a “postfusion” form (after). The prefusion shape is the better target because it exposes sites that trigger a stronger immune response. All three vaccines lock the protein in that prefusion shape so your body learns to produce antibodies against the virus’s most vulnerable form.
Arexvy (GSK): Protein Plus Adjuvant
Arexvy contains 120 micrograms of a lab-grown version of the prefusion F protein from RSV subtype A. What sets it apart from the other two vaccines is its adjuvant system, a combination of ingredients specifically designed to amplify your immune response to the protein.
The adjuvant, called AS01E, is the same type used in the shingles vaccine Shingrix. It contains two immune-boosting compounds: one derived from a detoxified bacterial molecule (from a strain of Salmonella) and another purified from the bark of a South American tree called Quillaja saponaria. Each is present at 25 micrograms per dose. These two compounds are packaged inside tiny fat bubbles called liposomes, which are made of cholesterol (125 micrograms) and a phospholipid called DOPC (500 micrograms). A small amount of a phosphate buffer (150 micrograms) keeps the solution at the right pH.
The adjuvant’s job is to create a stronger, longer-lasting immune response than the protein alone would produce. It essentially signals to your immune system that the protein is a serious threat worth remembering.
Abrysvo (Pfizer): Two Protein Subtypes, No Adjuvant
Abrysvo takes a different approach. Instead of using an adjuvant, it includes prefusion F proteins from both major RSV subtypes, A and B, at 60 micrograms each for a total of 120 micrograms per dose. The idea is to generate broad protection against both circulating strains of the virus.
The inactive ingredients are straightforward stabilizers and buffers: sucrose (11.3 mg) and mannitol (22.5 mg) act as stabilizers to keep the proteins intact during storage. Polysorbate 80 (0.08 mg) is a common surfactant that prevents the proteins from clumping together. Tromethamine and tromethamine hydrochloride maintain the correct pH, and a small amount of sodium chloride (1.1 mg) matches the solution to your body’s salt concentration. Abrysvo is the only RSV vaccine approved for both older adults and pregnant individuals (to protect newborns through transferred antibodies).
mRESVIA (Moderna): mRNA Technology
mRESVIA uses the same mRNA platform that became widely known through the COVID-19 vaccines. Instead of delivering the prefusion F protein directly, it contains 50 micrograms of modified mRNA, a set of genetic instructions that tells your cells to temporarily produce the prefusion F protein from RSV subtype A on their own. Your immune system then responds to that protein.
The mRNA is fragile and would break down quickly without protection, so it’s wrapped in a lipid nanoparticle, a tiny sphere made of four types of fats. These include SM-102 (the main structural lipid), a polyethylene glycol-linked lipid (PEG2000-DMG) that helps stabilize the particle, cholesterol, and DSPC (a phospholipid). The total lipid content per dose is about 1 milligram. These are the same categories of lipids used in Moderna’s COVID-19 vaccine, though the proportions differ.
How These Differ From the Infant RSV Product
If you’ve heard about RSV protection for babies, that product (nirsevimab, sold as Beyfortus) is not a vaccine. It’s a monoclonal antibody, a lab-made version of a single antibody injected directly into the infant. It provides ready-made protection that lasts a few months rather than training the baby’s immune system to make its own antibodies. The adult RSV vaccines work differently: they teach your immune system to recognize the virus so it can mount its own defense when exposed.
Who These Vaccines Are For
The CDC recommends a single dose of RSV vaccine for all adults 75 and older, and for adults 50 to 74 who have conditions that raise their risk of severe RSV illness. Those conditions include chronic lung disease, heart disease, severe obesity (BMI of 40 or higher), moderate to severe immune compromise, diabetes with organ damage, chronic liver or kidney disease, neurological conditions that weaken breathing muscles, and living in a nursing home.
RSV vaccination is not currently an annual shot. One dose is the complete series, and the CDC does not recommend a second dose at this time. For the best timing, getting vaccinated in late summer or early fall (August through October in most of the continental U.S.) offers peak protection heading into RSV season.
How Well the Vaccines Work
Protection from RSV vaccines is strongest in the first season after vaccination and gradually declines. In clinical trials of Arexvy, efficacy against RSV-related lower respiratory tract disease was 82.6% in the first season, 56.1% in the second, and 48.0% in the third, with a cumulative efficacy of 62.9% across all three seasons. This waning pattern is one reason researchers continue to study whether and when a booster might eventually be recommended.
Even with declining efficacy over time, the vaccines substantially reduce the risk of the most dangerous RSV outcomes, including hospitalizations and severe lung infections, particularly in older adults and those with underlying health conditions.