Respiratory Syncytial Virus (RSV) is a common respiratory pathogen causing significant illness, especially in vulnerable populations like infants and older adults. It frequently leads to lower respiratory tract infections, including bronchiolitis and pneumonia. The body’s immune response to RSV, whether from infection or medical interventions, generates antibodies. Understanding their persistence is important for prevention.
Immunity After Natural RSV Infection
When an individual contracts RSV, their immune system produces antibodies. These antibodies help neutralize the virus and can reduce the severity of subsequent infections. However, immunity from natural RSV infection is not long-lasting.
Antibody levels increase after infection but are not maintained. Research indicates that anti-RSV antibodies can return to pre-infection levels within six months, and reinfection is possible even within two months. This rapid decline explains why people can experience multiple RSV infections throughout their lives.
Subsequent RSV infections in healthy individuals are often less severe than the initial one. The body retains some partial immunity, which helps mitigate symptoms and reduce the likelihood of severe complications. However, this partial protection is not complete, and individuals remain susceptible to reinfection.
Studies show that within two months after a natural infection, about half of individuals could be reinfected, and by eight months, two-thirds could. Many experienced two or more infections within 26 months. The virus’s ability to evade the immune response contributes to this recurrent pattern.
Maternal Antibodies and Infant Protection
Newborns receive temporary immunity against RSV through the transfer of maternal antibodies. This process, known as passive immunity, primarily occurs during the final trimester of pregnancy. Antibodies cross the placenta from the mother to the developing fetus.
Most antibody transfer happens in the third trimester, so premature babies may receive lower levels. These maternally derived antibodies defend against RSV infection immediately after birth, when infants are particularly susceptible to severe disease.
The protection from these transferred antibodies is temporary and gradually wanes. The half-life of RSV-specific maternal antibodies in infants is estimated to be around 40 days, meaning their concentration significantly decreases over the first few months of life.
This decline in passive immunity leaves infants vulnerable to severe RSV infection by around six months of age. Strategies protecting infants often focus on enhancing or extending this early passive immunity.
While placental transfer is the primary route, breast milk also provides some supplemental antibodies. The main protective effect in the early months stems from antibodies received before birth.
Duration of Protection from Vaccines and Monoclonal Antibodies
Adult RSV Vaccines
Two RSV vaccines, Abrysvo and Arexvy, are available for older adults. A single dose of either vaccine is recommended for individuals aged 60 years and older. These vaccines stimulate the body to produce its own antibodies.
Clinical trials show these vaccines offer protection against severe RSV disease. For Arexvy, protection against symptomatic RSV lower respiratory tract disease has been demonstrated for approximately 23 months, with some waning over time. Abrysvo has shown durable protection for about 18 months, also with some decline.
These vaccines provide protection for at least one full RSV season and may extend into a second. The need for further doses beyond a single administration has not yet been definitively established, as studies monitor long-term durability.
Abrysvo is also approved for pregnant individuals, administered between 24 and 36 weeks of gestation. This maternal vaccination boosts the mother’s antibody levels, which are then transferred across the placenta to the developing baby. This provides passive immunity to the infant, protecting them from severe RSV disease from birth through their first six months of life.
Infant Monoclonal Antibody Immunization
Nirsevimab, marketed as Beyfortus, is a monoclonal antibody product used to protect infants. It is not a vaccine that stimulates the infant’s own immune system to produce antibodies; instead, it involves a direct infusion of laboratory-made antibodies.
This single injection provides immediate, passive protection against RSV. Nirsevimab offers protection for approximately five to six months after immunization. This duration covers a single RSV season, when infants are most susceptible to severe illness.
Nirsevimab is recommended for babies younger than eight months born to mothers who did not receive an RSV vaccine during pregnancy. It provides a direct and rapid way to offer protective antibodies, especially for those at high risk or during peak RSV season.
Factors Affecting Antibody Longevity
The duration and strength of an individual’s antibody response to RSV can vary due to several factors. Individual immune system differences play a role.
Age is a factor, as the immune systems of very young infants and older adults are less robust than those of healthy young adults. Neonates have delayed and lower antibody responses that persist for less time, reaching adult-like function after about six months. Older adults may also exhibit weaker immune responses to infection and vaccination, making them more susceptible to severe illness.
The severity of the initial RSV infection can also influence the subsequent antibody response. While a more severe illness might theoretically lead to a stronger immune response, the virus itself can interfere with immune system functions, affecting immunity’s durability.
An individual’s overall health status is another determinant. Immunocompromised individuals, such as organ transplant recipients or those with immune disorders, develop fewer antibodies following RSV vaccination compared to healthy individuals. Their adaptive immune response can be attenuated, leading to shorter-lived natural immunity and a weaker response to vaccines.