Respiratory Syncytial Virus (RSV) is a highly contagious respiratory illness that typically causes mild, cold-like symptoms. However, it can be severe in vulnerable populations, such as older adults, leading to serious complications like pneumonia, hospitalization, and death. The recent availability of preventative tools marks a significant advancement in protecting at-risk adults. This article clarifies the methods of RSV prevention and addresses the common question regarding dosage, specifically the unit of milligrams, often associated with these shots.
Vaccine vs. Antibody Understanding RSV Prevention Methods
The confusion over dosing units stems from the difference between active and passive immunization. Active immunization, typically via a vaccine, stimulates the body’s immune system to recognize a pathogen and create a protective response. The vaccine introduces an antigen, teaching the body to produce its own long-lasting antibodies, a process that takes a couple of weeks to become fully effective.
Passive immunization involves administering pre-made antibodies directly into the body. These are often monoclonal antibodies, laboratory-made proteins that mimic natural infection-fighting antibodies. Passive immunity provides immediate, but temporary, protection because the administered antibodies gradually break down over several months. Adults primarily utilize the active immunity approach through vaccination.
Approved Adult RSV Vaccines and Dosing
Adult RSV prevention uses vaccines, not monoclonal antibody shots, so dosing is not measured in milligrams (mg) of antibody. Instead, the dose is measured in micrograms (\(\mu\)g) of the active component or by volume. The U.S. Food and Drug Administration (FDA) has approved multiple RSV vaccines for adults, including Arexvy (GlaxoSmithKline), Abrysvo (Pfizer), and mResvia (Moderna).
Arexvy and Abrysvo are recombinant protein subunit vaccines focusing on the prefusion form of the RSV fusion (F) protein. A single 0.5 mL dose of Arexvy contains 120 \(\mu\)g of the recombinant prefusion F protein antigen, plus an adjuvant. A single 0.5 mL dose of Abrysvo also contains 120 \(\mu\)g of prefusion F protein, split evenly between RSV-A and RSV-B subtypes. The Moderna mResvia vaccine is an mRNA-based product, delivered in a single 0.5 mL dose, containing 50 \(\mu\)g of modified messenger RNA that encodes the prefusion F glycoprotein.
Recommendations for Adult Eligibility and Timing
The Centers for Disease Control and Prevention (CDC) recommends RSV vaccination for specific groups of adults to prevent severe illness. All adults aged 75 and older should receive a single dose of an FDA-licensed RSV vaccine. The recommendation also extends to adults aged 50 to 74 who are at an increased risk of severe RSV disease due to underlying health conditions.
Chronic medical conditions that increase the risk of severe RSV include:
- Chronic heart or lung disease.
- Weakened immune systems.
- Diabetes.
- Chronic liver disease.
- Chronic kidney disease.
For those aged 50 to 74 with risk factors, the decision to vaccinate is shared between the patient and the healthcare provider. The RSV vaccine is not currently an annual shot; one dose is generally considered sufficient protection for at least two years. Optimal timing for administration is typically in the late summer and early fall, before the RSV season begins.
Clarifying the “mg” Dosage Confusion
The inquiry regarding an “RSV antibody shot” measured in milligrams (mg) likely refers to the preventative treatment used for infants, not adults. The milligram dosage unit is specific to the passive immunization product, nirsevimab (Beyfortus). Nirsevimab is a long-acting monoclonal antibody indicated for preventing RSV lower respiratory tract disease in infants and certain young children.
This product is dosed in milligrams because it delivers the actual antibody, not an antigen. The recommended dosage for infants in their first RSV season is weight-based: 50 mg for those weighing less than 5 kg and 100 mg for those weighing 5 kg or more. Older children who remain at increased risk for a second RSV season may receive a 200 mg dose. This milligram-based, weight-dependent dosing is the source of the confusion, as it differs completely from the microgram-based antigen doses used in adult vaccines.