Respiratory Syncytial Virus (RSV) is a common respiratory pathogen that causes cold-like symptoms in most people but can lead to severe lung infections, such as bronchiolitis and pneumonia, particularly in infants and older adults. The virus is a major cause of hospitalization for these vulnerable populations each year. Understanding how long the body remains protected against RSV is complicated because the immune response to this virus is generally transient. The duration of protection varies significantly depending on whether immunity is acquired through natural infection, active vaccination, or passive antibody transfer.
Immunity Duration Following Natural RSV Infection
Immunity acquired by recovering from a natural RSV infection is typically short-lived, which is why people can be repeatedly infected throughout their lives. Protection often wanes rapidly, leaving individuals susceptible to re-infection within a single RSV season or shortly thereafter. Studies indicate that high levels of protective antibodies decline quickly, with susceptibility potentially returning within four to eight months of the initial illness.
The virus possesses mechanisms that interfere with the development of a durable immune response, contributing to this transient nature of protection. For example, memory B cells that produce RSV-specific antibodies decline within four to six months after infection. Even when the body mounts a robust antibody response, that immunity fades over time, meaning full airway protection is rarely achieved. This limited baseline protection is what newer medical interventions are designed to overcome.
Duration of Protection from RSV Immunizations for Adults
RSV immunizations for adults, including those administered to older individuals, are designed to generate a robust and more durable response than natural infection. Vaccines approved for adults aged 60 and older stimulate the immune system to produce its own antibodies, a process known as active immunity. Clinical trial data for vaccines like Arexvy and Abrysvo suggest the protection is multi-seasonal.
For example, the Arexvy vaccine (RSVPreF3) demonstrated maintained efficacy against lower respiratory tract disease for at least three full RSV seasons following a single dose. Although efficacy declined from over 82% in the first season to approximately 48% by the third season, protection against severe outcomes remained significant. The Abrysvo vaccine has also shown durable protection against symptomatic disease through approximately 18 to 23 months post-vaccination. Since protection is designed to last beyond a single season, these RSV immunizations are not currently recommended as an annual shot for most eligible adults.
Maternal Vaccination for Infant Protection
Maternal vaccination involves the pregnant person receiving the Abrysvo vaccine. The goal is to boost the mother’s antibody levels, which are then passed through the placenta to the fetus during the third trimester. These transferred antibodies provide passive protection to the newborn during their first months of life, a period when they are most vulnerable to severe illness. This passive immunity has been shown to reduce the risk of the baby being hospitalized due to RSV by approximately 57% within six months after birth, covering the infant’s entire first RSV season.
Duration of Protection from Infant Prophylaxis
Infant prophylaxis provides temporary, direct protection against RSV through the administration of pre-made antibodies, which is a form of passive immunity. This differs from active vaccination because the infant’s immune system is not stimulated to create its own response; the antibodies are borrowed and immediately available. Protection duration depends entirely on the half-life of the administered antibody—how long the body takes to break it down and clear it.
The older monoclonal antibody treatment, Palivizumab (Synagis), has a relatively short half-life and provides protection for only about one month per injection. This shorter duration requires infants to receive monthly intramuscular injections throughout the typical five-month RSV season to maintain a therapeutic level. This regimen is typically reserved for high-risk infants due to the intensive dosing schedule.
A newer long-acting monoclonal antibody, Nirsevimab (Beyfortus), was engineered with an extended half-life for greater convenience and coverage. A single intramuscular injection of Nirsevimab is designed to provide sustained protection for at least five months. This single-dose approach covers the entire duration of an infant’s first RSV season. Once these antibodies are metabolized by the infant’s body, the protection ends, making the timing of administration before the start of the RSV season especially important.