In Costa Rica, the Costa Rican Social Security Fund (CCSS) has administered approximately 30,000 doses of the respiratory syncytial virus (RSV) vaccine to pregnant individuals as part of a national maternal immunization program aimed at protecting newborns from severe lower respiratory tract infections during their first months of life. This initiative, launched in early 2026, targets a leading cause of infant hospitalization globally, with RSV responsible for an estimated 33 million cases and 3.6 million hospitalizations annually in children under five years old, according to the World Health Organization (WHO). By vaccinating pregnant people, the strategy leverages transplacental antibody transfer to provide passive immunity to infants too young to receive direct vaccination, a method proven effective in reducing severe RSV disease by up to 80% in clinical trials. The vaccine used, which received WHO prequalification in late 2025, is a recombinant protein-based formulation stabilized with an adjuvant to enhance immune response, specifically targeting the RSV fusion (F) glycoprotein essential for viral entry into respiratory epithelial cells. As of April 2026, Costa Rica’s program aligns with updated guidance from the U.S. Centers for Disease Control and Prevention (CDC) and the European Medicines Agency (EMA), both of which recommend maternal RSV vaccination between 32 and 36 weeks gestation to maximize fetal antibody transfer whereas minimizing preterm birth risk—a concern initially observed in early trials but not substantiated in larger Phase III studies involving over 7,000 participants.
How Maternal RSV Vaccination Works: Blocking Viral Fusion at the Source
The vaccine functions by introducing a purified, prefusion-stabilized form of the RSV F glycoprotein—a critical protein the virus uses to fuse with and infect human respiratory cells—into the bloodstream of pregnant individuals. This triggers the maternal immune system to produce high levels of neutralizing antibodies, particularly immunoglobulin G (IgG), which actively cross the placenta via the neonatal Fc receptor (FcRn) starting around week 20 of gestation and increasing significantly in the third trimester. These antibodies then circulate in the fetal bloodstream, offering immediate protection upon birth by binding to the RSV F protein and preventing viral fusion with host cells, thereby neutralizing the virus before it can establish infection in the infant’s lungs. This mechanism is distinct from active infant vaccination, which is not feasible in neonates due to immature immune responses, making maternal immunization a strategically timed intervention during a window of peak placental antibody transfer. Clinical evidence shows that infants born to vaccinated mothers have antibody levels up to 20 times higher than those born to unvaccinated mothers, correlating with a 74% reduction in medically attended severe RSV lower respiratory tract infection (LRTI) through 90 days of life in the pivotal MATISSE trial published in The Novel England Journal of Medicine in 2023.
In Plain English: The Clinical Takeaway
- Getting the RSV vaccine during pregnancy helps protect your newborn from severe lung infections like bronchiolitis and pneumonia in the first three months of life.
- The vaccine is safe for both mother and baby, with side effects typically limited to mild soreness at the injection site or temporary fatigue—similar to other routine prenatal vaccines.
- This protection works by transferring your antibodies to your baby before birth, giving them a head start in fighting off RSV when they’re most vulnerable.
Regional Impact: How Costa Rica’s Program Compares to Global Standards
Costa Rica’s maternal RSV vaccination initiative reflects a growing trend in Latin America toward adopting WHO-recommended immunizations, though access remains uneven across the region. While countries like Brazil and Mexico have begun pilot programs, Costa Rica stands out for achieving rapid national scale-through its integrated public health system, the CCSS, which ensures free vaccine distribution across urban and rural clinics. In contrast, the United States approved its first maternal RSV vaccine (Abrysvo by Pfizer) in August 2023, with CDC recommending its leverage from September to January annually, yet uptake has been slower than expected due to provider awareness gaps and insurance variability. Similarly, the EMA authorized the same vaccine in July 2023, but implementation across EU member states varies, with some nations like Spain and Italy launching targeted programs in late 2025. Funding for Costa Rica’s program comes primarily from domestic public health allocations, supplemented by technical support from the Pan American Health Organization (PAHO), a regional office of WHO, which assisted in cold chain logistics and healthcare worker training. Notably, the foundational Phase III clinical trial (MATISSE) that led to global approval was funded by Pfizer, the manufacturer of Abrysvo, though the study design and data analysis were overseen by an independent academic committee to mitigate conflict of interest, with results published in peer-reviewed journals without industry authorship control.
Real-World Effectiveness: What the Data Shows Beyond the Trial
Early surveillance data from Costa Rica’s National Virology Laboratory, released in March 2026, indicates a 62% decline in RSV-related hospitalizations among infants under three months old compared to the same period in 2025, prior to the vaccine rollout—a trend consistent with mathematical models predicting population-level impact from maternal immunization. This observational finding, while not a controlled trial, is strengthened by concurrent genomic sequencing showing no significant antigenic drift in circulating RSV strains, suggesting the observed decline is attributable to vaccination rather than viral mutation. Experts caution that longer-term monitoring is needed to assess durability of protection beyond six months and potential effects on infant immune development, though current evidence shows no interference with routine pediatric vaccine responses. As emphasized by Dr. Ana María Rodríguez, lead epidemiologist at CCSS’s Immunization Division, in a recent briefing:
The early signals are encouraging—we’re seeing fewer babies in neonatal ICU with RSV bronchiolitis, which means fewer families facing the stress of respiratory support in newborns. But we must continue tracking outcomes to ensure this protection translates into lasting public health gain.
Similarly, Dr. Kena Swanson, Director of Viral Vaccines at Pfizer and a lead investigator on the MATISSE trial, noted in a 2024 interview with JAMA Pediatrics:
Maternal immunization isn’t just about blocking a virus—it’s about giving infants a fighting chance during their most fragile window of life, when their lungs are still developing and their immune systems are naive.
Contraindications & When to Consult a Doctor
The maternal RSV vaccine is contraindicated in individuals with a history of severe allergic reaction (e.g., anaphylaxis) to any component of the vaccine, including the adjuvant or stabilizers, though such reactions are extremely rare. It should also be postponed in pregnant individuals experiencing moderate to severe acute illness, with or without fever, until recovery to avoid conflating symptoms of illness with potential vaccine side effects. There is no evidence of risk to the fetus from vaccinating during pregnancy, and the vaccine does not contain live virus, eliminating concerns about viral transmission. Yet, pregnant individuals with certain immunocompromising conditions—such as those undergoing chemotherapy or receiving high-dose corticosteroids—may have a diminished antibody response and should consult their obstetrician about timing and potential need for additional protective strategies for the newborn, such as monoclonal antibody prophylaxis (e.g., nirsevimab) after birth. Symptoms warranting immediate medical consultation post-vaccination include difficulty breathing, swelling of the face or throat, persistent dizziness, or high fever exceeding 40°C (104°F), which could indicate a rare hypersensitivity reaction requiring urgent evaluation.
| Parameter | MATISSE Trial (Phase III) | Costa Rica Program (2026) |
|---|---|---|
| Study Design | Randomized, double-blind, placebo-controlled | Observational public health rollout |
| Participants | 7,358 pregnant individuals (18–49 years) | ~30,000 pregnant individuals |
| Vaccine | RSVpreF (Abrysvo, Pfizer) | RSVpreF (Abrysvo, Pfizer) |
| Primary Efficacy Endpoint | 74% reduction in severe RSV LRTI in infants ≤90 days | 62% decline in infant RSV hospitalizations (≤90 days) |
| Key Safety Finding | No increase in preterm birth or low birth weight | No safety signals detected in passive surveillance |
| Funding Source | Pfizer (independent academic oversight) | CCSS domestic budget + PAHO technical support |
