Beyfortus Provides Immediate Protection After Administration
Beyfortus (nirsevimab) is a monoclonal antibody that provides immediate passive immunity upon injection, not a vaccine that requires time for the immune system to respond. Unlike vaccines that stimulate active immunity requiring days to weeks for antibody production, nirsevimab delivers pre-formed antibodies directly into the infant's bloodstream, conferring protection essentially immediately after intramuscular administration 1, 2.
Mechanism and Timing of Protection
Nirsevimab works by directly providing antibodies rather than stimulating the infant's immune system to produce them, which is why it is classified as immunoprophylaxis rather than a vaccine 2, 3.
The antibody is immediately bioavailable after intramuscular injection, with protective serum concentrations achieved rapidly following administration 4, 5.
Clinical trials evaluated efficacy starting from the day of injection through 150 days post-administration, demonstrating that protection begins immediately and extends throughout an entire RSV season with a single dose 6, 4, 5.
Clinical Trial Evidence
In the phase 2b trial of 1,453 preterm infants and phase 3 trial of 3,012 late preterm/term infants, protection was measured from day 0 (injection day) through day 150, with consistent efficacy throughout this entire period 6, 4, 5.
The HARMONIE pragmatic trial demonstrated 83.2% efficacy against RSV-associated hospitalization when administered before or during RSV season, with protection evident immediately after administration 7.
Real-world effectiveness data from the 2023-2024 RSV season showed 90% effectiveness against RSV-associated hospitalization among infants who received nirsevimab ≥7 days before symptom onset, with a median time from receipt to symptom onset of 45 days 8.
Practical Implications for Administration
The CDC recommends administering nirsevimab shortly before or at the start of RSV season (typically October through March in most of the continental United States), not waiting for "peak" season, because protection is immediate 6, 3.
For infants with prolonged birth hospitalizations, nirsevimab should be given shortly before or promptly after hospital discharge to ensure immediate protection 6.
When comparing to maternal RSV vaccination, at least 14 days are needed after maternal vaccination for antibody development and transplacental transfer, which is why nirsevimab is recommended for infants born <14 days after maternal vaccination—the monoclonal antibody provides immediate protection that maternal vaccination cannot 6.
Important Distinction from Active Immunization
The extended half-life of nirsevimab (approximately 150 days) means a single dose provides protection throughout an entire RSV season, unlike palivizumab which required monthly injections 1, 3, 9.
Nirsevimab can be coadministered with routine childhood vaccines without interference, as it does not rely on the infant's immune response 6, 1.