Active Ingredient in RSV Prophylaxis for Infants
The active ingredient in the RSV prophylactic product given to infants is nirsevimab, a long-acting monoclonal antibody that targets the prefusion conformation of the RSV F protein. 1
What Nirsevimab Is
Nirsevimab (brand name Beyfortus) is a monoclonal antibody, not a traditional vaccine - it provides passive immunity through direct administration of antibodies rather than stimulating the infant's immune system to produce antibodies 1
The antibody specifically binds to the Ø antigenic site on the RSV prefusion F protein, preventing the virus from entering cells and causing infection 2
It has been genetically engineered with an extended half-life through modifications to its Fc region, allowing a single dose to provide protection throughout an entire 5-month RSV season 2, 3
How It Differs from Previous Options
Nirsevimab replaced palivizumab (Synagis) as the primary RSV prophylaxis for most infants - palivizumab required monthly injections throughout RSV season, while nirsevimab requires only a single dose 4, 5
Unlike palivizumab which was limited to high-risk infants (<5% of all infants), nirsevimab is recommended for all infants <8 months entering their first RSV season 1
Pharmacokinetics and Duration
Following intramuscular administration, nirsevimab provides RSV neutralizing antibody levels >140-fold higher than baseline at day 31, remaining >50-fold higher at day 151 and >7-fold higher at day 361 6
The typical clearance in a 5 kg infant is 3.4 mL/day, with body weight and postmenstrual age being the primary factors affecting drug clearance 3
Weight-banded dosing is used: 50 mg for infants <5 kg and 100 mg for infants ≥5 kg 3
Important Clinical Distinction
There is also a maternal RSV vaccine (RSVpreF, Abrysvo) given to pregnant women at 32-36 weeks gestation that provides passive immunity to newborns through transplacental antibody transfer 1, 7
The maternal vaccine and infant nirsevimab are different products with different mechanisms - the vaccine stimulates maternal antibody production, while nirsevimab directly provides pre-formed antibodies 1
Most infants do not need both - if the mother received RSV vaccine ≥14 days before birth, nirsevimab is typically not needed unless the infant has specific high-risk conditions 1