How Maternal RSV Vaccination Protects Newborns

A Virus That Sends Millions of Babies to Hospital Every Year

Respiratory syncytial virus — RSV — sounds unremarkable. In healthy adults it causes little more than a cold. But in newborns and young infants, whose immune systems are still maturing, it can be devastating. The virus inflames the tiny airways deep in the lungs, causing bronchiolitis and pneumonia that can rapidly progress to breathing failure.

According to the World Health Organization, RSV causes more than 3.6 million hospitalizations and approximately 100,000 deaths in children under five each year. In the United States alone, the CDC estimates that 58,000–80,000 children under age five are hospitalized with RSV annually. It is the single most common cause of infant hospitalization in high-income countries.

For decades, there was no way to prevent it in the youngest infants — the very babies most at risk. That changed in 2023 with the approval of the first maternal RSV vaccine.

The Logic of Vaccinating the Mother, Not the Baby

Newborns cannot be vaccinated directly at birth: their immune systems are not yet capable of mounting a strong enough response to generate lasting protection. Nature, however, built a workaround into pregnancy itself.

During the final weeks of gestation, the placenta actively pumps immunoglobulin G (IgG) antibodies from the mother's bloodstream into the fetus. This process — called transplacental antibody transfer — means that a baby is born already carrying a copy of the mother's immune memory. It is why newborns have some protection against measles, flu, and other diseases their mothers have encountered.

Maternal vaccination exploits precisely this mechanism. By vaccinating the pregnant woman, doctors prompt her body to produce high levels of RSV-specific antibodies, which then cross the placenta and arm the newborn against the virus from the moment of birth — before the infant has ever encountered a single pathogen.

How the Vaccine Works

The only vaccine currently approved for maternal RSV immunization is Abrysvo (manufactured by Pfizer). Approved by the U.S. Food and Drug Administration in August 2023, it is a bivalent protein subunit vaccine, meaning it does not contain live virus. Instead, it presents the immune system with stabilized copies of the RSV fusion protein — the molecular key the virus uses to break into cells — in its prefusion form, which is the shape that triggers the strongest immune response.

A single dose is given intramuscularly between weeks 32 and 36 of pregnancy. This timing is critical: it must be early enough to allow the mother's body to produce antibodies and transfer them across the placenta, but late enough to minimize any risk of early delivery. Research from Mass General Brigham found that vaccination at least five weeks before delivery maximizes the efficiency of antibody transfer.

How Well Does It Work?

The pivotal evidence comes from the MATISSE trial — a randomized, double-blind, placebo-controlled Phase 3 study involving approximately 4,000 pregnant participants. According to Pfizer's published data and ACOG guidance:

• Efficacy against severe RSV-associated lower respiratory tract infection in the first 90 days of life: 81.8%
• Efficacy at 180 days (six months): 69.4%
• Efficacy against RSV hospitalization: approximately 57–68%

Protection lasts roughly the first six months of the infant's life — which is exactly the highest-risk window. After that, the transferred maternal antibodies naturally wane, but by then the child's own immune system is better equipped to handle infection.

Safety Considerations

No vaccine is without risk. The most common side effects in mothers — injection-site pain and fatigue — are mild and short-lived. More serious questions arose during post-marketing surveillance about a possible association with Guillain-Barré syndrome (a rare nerve disorder) and a signal of slightly elevated preterm birth rates in one trial arm. Regulatory agencies continue to monitor these signals closely. Both the FDA and the CDC maintain that for most pregnant women, the benefits of protecting newborns from severe RSV disease outweigh the risks.

Why It Matters Beyond the United States

Maternal RSV vaccination has the potential for an outsized impact in low- and middle-income countries, where 97% of RSV child deaths occur and where neonatal intensive care is scarce. A single vaccine dose given during routine antenatal care — a visit most pregnant women already attend — could prevent hundreds of thousands of infant deaths annually without requiring any additional clinic contacts for the baby. Global health agencies including the WHO have identified scaling up maternal RSV immunization as a priority for 2026 and beyond.

The Bigger Picture

Maternal RSV vaccination is part of a broader shift in how medicine thinks about protecting the very youngest patients. Alongside monoclonal antibody prophylaxis (nirsevimab, brand name Beyfortus) — which can be given directly to newborns at birth — it represents a new two-pronged strategy against a virus that has caused enormous suffering with surprisingly little public recognition. For parents, the key takeaway is straightforward: a single shot, timed carefully in late pregnancy, can give a baby its first and most important line of defense before it even takes its first breath.