A Vaccine for the Mother, Protection for the Baby: What Families Should Know About RSV Immunisation in Pregnancy

Two 2025 medical reviews explain how well the RSV vaccine protects newborns, and what is still being learned about its effects on mothers

A vaccine given to a mother late in pregnancy can protect her baby from the most common cause of serious chest infections in infancy. Two medical reviews published in 2025 found that this protection is strong in the first three months of life and fades gradually afterwards, and that side effects for mothers are usually limited to a sore arm — though researchers say more study of mothers' own health is still needed.

Why a winter virus is so dangerous for the youngest babies

Respiratory syncytial virus, usually shortened to RSV, is a common respiratory virus that most children catch by the age of two. In older children and adults it feels like a bad cold. In a very young baby it can be far more serious, because a newborn's airways are tiny and easily blocked by swelling and mucus. The infection can move down into the smallest air passages, causing a condition called bronchiolitis, or into the lungs themselves, causing pneumonia. Babies who become this unwell may need oxygen, help with feeding, and sometimes a breathing machine in intensive care.

The scale of the problem is easy to underestimate. In children under five worldwide in a single recent year, RSV was linked to roughly 3.6 million hospital admissions and just over 100,000 deaths [1][2][3]. Most of those deaths were in babies under six months old, and most were in countries where intensive care is not easily available [1]. Even babies who recover fully may be affected later: children who were hospitalised with RSV in their first year are more likely to develop wheezing or asthma in early childhood [1].

How families and doctors used to face this problem

For most of the last thirty years, there was very little that could be done to prevent RSV. Doctors could treat the symptoms — oxygen, fluids, gentle suctioning of the nose — but there was no vaccine and no widely available preventive medicine. A protective antibody injection called palivizumab existed from 1998, but it had to be given every month through the winter and was reserved for a small group of babies at the highest risk: those born very prematurely, or with serious heart or lung conditions [1]. The great majority of babies who ended up in hospital with RSV had never been eligible for it.

The obvious solution — vaccinating babies directly — turned out to be extremely difficult. A vaccine tested in infants in the 1960s not only failed to protect but made the illness worse when those children later caught the virus naturally, a result that made researchers cautious for decades [1]. Even recently, a newer type of infant RSV vaccine was paused after more severe illness appeared in the children who received it [1]. The underlying obstacle is biological: RSV is most dangerous in the first few months of life, exactly when a baby's immune system is too immature to respond well to a vaccine.

That is what pushed researchers to a different idea, one already used successfully for tetanus, influenza and whooping cough. Instead of vaccinating the baby, vaccinate the mother. During pregnancy, protective proteins called antibodies pass from the mother's bloodstream across the placenta into the baby, so a baby whose mother was vaccinated is born already carrying protection [1].

What the research actually found

The key study is known as MATISSE, short for the Maternal Immunization Study for Safety and Efficacy. It was a large, carefully controlled trial run in 18 countries across four RSV seasons. Pregnant women between 24 and 36 weeks were given either a single injection of the RSV vaccine or a dummy injection, without knowing which — 3,682 received the vaccine and 3,676 the placebo — and their babies were followed for one to two years [1][4].

The results were clear. Among babies whose mothers received the vaccine, severe RSV chest infections needing medical care were reduced by about 82% in the first three months of life, and by about 69% over the first six months [1]. For RSV chest infections of any severity, the reduction was around 57% at three months and 51% at six months. In other words, the vaccine works best against exactly the kind of illness that lands a baby in hospital.

The protection fades, and this is worth understanding rather than being disappointed by. Antibodies borrowed from a mother do not last indefinitely. In one study of 291 babies, 95% carried protective RSV antibodies at birth, but only 31% still had them at two months and 8% at seven months [1]. The vaccine is designed to cover the window when the risk is highest, not to provide lifelong immunity.

Real-world experience has matched the trial. When Argentina introduced national maternal RSV vaccination in 2024, a study across 12 hospitals found that hospital admissions for RSV chest infections were reduced by about 79% in babies under three months and about 71% in babies under six months [1].

Is the vaccine safe for the mother?

This is where the second review comes in, and where the honest answer includes a caveat. Palmero and colleagues searched six large medical databases for every study that had reported on mothers' own health after receiving this vaccine. Out of 1,259 research papers screened, only five had actually measured maternal side effects [2]. Most research had focused on the babies.

What those five studies showed was reassuring. The commonest side effects were mild to moderate reactions where the injection was given — pain, redness and tenderness — which were more frequent after the vaccine than after a placebo. Symptoms affecting the whole body, such as fever or tiredness, were generally no more common in vaccinated women, although headache and muscle aches were slightly more frequent in the large MATISSE trial [2]. Serious problems were no more common in vaccinated women than in those given placebo, and no serious problem in the babies over two years of follow-up was judged to be caused by the vaccine [1].

The history behind this caution is real. An older maternal RSV vaccine trial in 4,636 women had already shown a similar pattern of sore arms without harm to pregnancies, though it ended early for unrelated reasons [5]. A different maternal RSV vaccine, made by another company, was tested and then stopped because more babies were born early in the vaccinated group [6]. Regulators approved the currently used vaccine only after the large trial found no such signal.

One study has raised a question that is not yet settled. Researchers looking back at records from 2,973 births at two New York hospitals found no link between the vaccine and premature birth, but one of their statistical analyses suggested a possible link with high blood pressure conditions in pregnancy [7]. The authors of the review describe this as something that needs further investigation rather than an established risk. Since then, a 2026 analysis that combined six clinical trials, involving 18,011 pregnant women and 17,769 babies, confirmed the reduction in severe RSV illness and hospital admission and found no signal for premature birth, low birth weight or birth defects [8].

What this means for your baby's care

If you are pregnant, the practical questions are when to have the vaccine and whether anything else is needed. Timing differs by country: in the United States the vaccine is recommended between 32 and 36 weeks during the RSV season, roughly September to January [9], while the World Health Organization and the United Kingdom recommend it from 28 weeks all year [1]. Your maternity team will know the local schedule.

There is also a second option, and sometimes both are used. A long-acting antibody injection called nirsevimab can be given to the baby directly, as a single dose that lasts through the RSV season. It reduced RSV hospital admissions by about 62% in babies born at or near term [10] and by about 78% in babies born between 29 and 35 weeks [11]. This matters particularly if a baby arrives very soon after the mother is vaccinated, or arrives prematurely, because there may not have been enough time for antibodies to cross the placenta [1]. In those situations the baby may be offered the antibody injection as well. Which approach a country chooses depends on cost, supply and how its health system is organised [1][12].

Breastfeeding also helps, and works alongside rather than instead of immunisation. Babies exclusively breastfed in their first two weeks were considerably less likely to be hospitalised with RSV [1].

What researchers are working on next

Three things are being actively studied. The first is whether the vaccine is equally safe in pregnancies that already carry higher risk, since the trials so far deliberately enrolled women with straightforward, low-risk pregnancies [2]. The second is the unresolved question about blood pressure conditions in pregnancy, which needs a study designed specifically to answer it. The third is long-term follow-up of mothers, which does not yet exist. Researchers have also noted that the women enrolled in these studies were mostly from wealthier countries and were more likely to be older and privately insured, so more diverse research is needed [2][13].

None of this undercuts the central finding. A single injection in late pregnancy substantially reduces the chance that a newborn will be hospitalised with a severe RSV chest infection during the months when that infection is most dangerous — a genuinely new option for families that did not exist a few years ago.

References

  1. Chantasrisawad N, Boonjindasup W, Puthanakit T, Chaithongwongwatthana S. Beyond the first breath: comprehensive respiratory syncytial virus prevention through maternal immunization and infant immunoprophylaxis. Asian Biomed (Res Rev News). 2025;19(3):116–130. doi:10.2478/abm-2025-0015 ↩
  2. Palmero M, Martin RA, Olson B, et al. Side effects associated with respiratory syncytial virus prefusion F (RSVpreF) maternal vaccination: a scoping review. Cureus. 2025;17(7):e88162. doi:10.7759/cureus.88162 ↩
  3. Li Y, Wang X, Blau DM, et al. Global, regional, and national disease burden estimates of acute lower respiratory infections due to respiratory syncytial virus in children younger than 5 years in 2019: a systematic analysis. Lancet. 2022;399(10340):2047–2064. doi:10.1016/S0140-6736(22)00478-000478-0) ↩
  4. Kampmann B, Madhi SA, Munjal I, et al. Bivalent prefusion F vaccine in pregnancy to prevent RSV illness in infants. N Engl J Med. 2023;388(16):1451–1464. doi:10.1056/NEJMoa2216480 ↩
  5. Madhi SA, Polack FP, Piedra PA, et al. Respiratory syncytial virus vaccination during pregnancy and effects in infants. N Engl J Med. 2020;383(5):426–439. doi:10.1056/NEJMoa1908380 ↩
  6. Dieussaert I, Kim JH, Luik S, et al. RSV prefusion F protein–based maternal vaccine — preterm birth and other outcomes. N Engl J Med. 2024;390(11):1009–1021. doi:10.1056/NEJMoa2305478 ↩
  7. Son M, Riley LE, Staniczenko AP, et al. Nonadjuvanted bivalent respiratory syncytial virus vaccination and perinatal outcomes. JAMA Netw Open. 2024;7(7):e2419268. doi:10.1001/jamanetworkopen.2024.19268 ↩
  8. Lopes JR, Martins Esteves I, Dias SO, et al. The impact of maternal respiratory syncytial virus vaccination on infant and perinatal outcomes: a systematic review and meta-analysis of randomized controlled trials. J Perinatol. Published online April 13, 2026. doi:10.1038/s41372-026-02675-0 ↩
  9. Fleming-Dutra KE, Jones JM, Roper LE, et al. Use of the Pfizer respiratory syncytial virus vaccine during pregnancy for the prevention of respiratory syncytial virus–associated lower respiratory tract disease in infants: recommendations of the Advisory Committee on Immunization Practices — United States, 2023. MMWR Morb Mortal Wkly Rep. 2023;72(41):1115–1122. doi:10.15585/mmwr.mm7241e1 ↩
  10. Hammitt LL, Dagan R, Yuan Y, et al. Nirsevimab for prevention of RSV in healthy late-preterm and term infants. N Engl J Med. 2022;386(9):837–846. doi:10.1056/NEJMoa2110275 ↩
  11. Griffin MP, Yuan Y, Takas T, et al. Single-dose nirsevimab for prevention of RSV in preterm infants. N Engl J Med. 2020;383(5):415–425. doi:10.1056/NEJMoa1913556 ↩
  12. Torres-Torres J, Villafan-Bernal JR, Martinez-Portilla RJ, et al. Maternal RSV vaccination for infant protection: a systematic review and meta-analysis of phase 3 trials with an integrated economic evaluation. Int J Gynaecol Obstet. Published online 2026. doi:10.1002/ijgo.70641 ↩
  13. Simões EAF, Center KJ, Tita ATN, et al. Prefusion F protein–based respiratory syncytial virus immunization in pregnancy. N Engl J Med. 2022;386(17):1615–1626. doi:10.1056/NEJMoa2106062 ↩