Protecting infants from RSV — the Beyfortus® infant antibody and the Abrysvo® maternal vaccine
Two ways to protect babies from RSV — an antibody given to the infant, or a vaccine given in pregnancy.
RSV (respiratory syncytial virus) is a very common respiratory virus. In older children and adults it's usually like a cold, but in babies — especially young infants — it can be more serious.
| Aspect | What parents might notice | How it spreads | Why it can be serious | How protection helps |
|---|---|---|---|---|
| RSV illness in babies | Runny nose, cough, reduced feeding, and sometimes fast or laboured breathing, wheezing, or pauses in breathing in very young infants. | Through respiratory droplets from coughing and sneezing, and by contact with contaminated hands and surfaces. It spreads very easily. | In infants, RSV can cause bronchiolitis (inflammation of the small airways) or pneumonia. It's one of the most common reasons babies are hospitalized, especially in the first months of life. | Both the infant antibody and the maternal vaccine substantially reduce the chance of severe RSV illness and hospitalization in babies. |
RSV protection for infants is unusual because there are two different approaches, and they work in different ways. Usually a baby only needs one of them, not both.
1. Beyfortus® (nirsevimab) — given to the baby. This is a monoclonal antibody, not a vaccine. Rather than prompting the baby's own immune system to make antibodies (which takes time), it provides ready-made antibodies directly, giving immediate protection. It's a single injection, and protection lasts at least about 6 months — covering the baby through their first, most vulnerable RSV season. Ontario's program (and NACI) prefer Beyfortus for infant protection, based on its effectiveness and duration.
2. Abrysvo® — given during pregnancy. This is a vaccine given to the pregnant parent, usually between 32 and 36 weeks of pregnancy, when the baby is due during RSV season. The parent's immune system makes antibodies that cross the placenta to the baby, so the newborn is born with protection. This protection is passed on and fades over the baby's first several months.
Timing: Both are timed around the RSV season (typically November through March). Beyfortus is given to eligible infants entering or born during the season; high-risk children may be eligible up to 24 months. Abrysvo is given in the third trimester for babies due during the season.
Can these be given with other vaccines? Yes. Abrysvo can be given alongside other pregnancy vaccines (like Tdap, flu, and COVID-19), and Beyfortus can be given around the same time as a baby's routine vaccines. Different injection sites are used.
Instead, RSV protection is offered through Ontario's Infant and High-Risk Children RSV Prevention Program, which provides Beyfortus and Abrysvo free to eligible families during the RSV season. It's about protecting babies when they're most vulnerable, not about school entry.
Reporting: Whether your baby received Beyfortus, or you received Abrysvo in pregnancy, it's worth keeping a note on your child's health record so future providers know what protection was given. The provider who administers it can record it.
High-risk children: Some children with certain medical conditions (such as those born prematurely, or with heart or lung conditions) may be eligible for RSV protection up to 24 months of age, and may have different recommendations. A healthcare provider can advise based on your child's situation.
The two RSV products work differently, so their ingredients differ. Beyfortus® (nirsevimab) is a monoclonal antibody given to the infant; Abrysvo® is a protein-based vaccine given in pregnancy. The information below reflects the general categories from their Health Canada product monographs. Neither contains a live virus, and neither can cause RSV.
| Ingredient / Component | Product | Category | Plain-English explanation |
|---|---|---|---|
| Nirsevimab (a monoclonal antibody) | Beyfortus | Active ingredient | A lab-made antibody — the same kind of protective protein the body itself makes — designed to recognize and block RSV. It provides protection directly, without needing the baby's immune system to build its own response. |
| RSV "F" proteins (stabilized, from two RSV subgroups) | Abrysvo | Active ingredient | Purified copies of a protein from the surface of RSV. They contain no live virus and can't cause RSV — they teach the pregnant parent's immune system to make protective antibodies that pass to the baby. |
| Sugars (e.g. sucrose) and amino acids (e.g. histidine, arginine) | Both | Stabilizers / buffers | Help keep the product stable and at a body-friendly acidity. Amino acids are the building blocks of proteins, found throughout the body. |
| Polysorbate (80 or 20) | Both | Stabilizer | A common ingredient in many foods and medicines that helps keep the product stable. |
| Salts (e.g. sodium chloride) | Both | Tonicity agent | Ordinary salts so the injection is gentle on the tissues. |
| Water for injection | Both | Base | Sterile water making up most of the injection. |
Expected / common side effects
Timing and duration: These reactions are usually mild and typically settle within a day or two.
Home care (baby): Keep your baby comfortable and well-fed. A mild fever or fussiness usually passes quickly. Acetaminophen or ibuprofen can be used for babies if needed and age-appropriate — check with a provider or pharmacist on dosing. Do not give ASA (Aspirin) to children.
When to call a provider: If your baby seems unusually unwell, has a high or persistent fever, or you're worried, it's always reasonable to call.
Many babies have little or no reaction, and that does not mean the protection didn't work.
Rare but important
What providers do to reduce risk: Providers ask about allergy history before giving the injection and are equipped to recognize and treat a severe allergic reaction.
Observation period: Staying nearby for a short time after the injection is recommended, as severe allergic reactions — though rare — typically occur shortly after.
A note on monitoring: Because RSV protection for infants is newer than many vaccines, it's under active safety surveillance — part of the ongoing monitoring described on the Safety & Ingredients page.
Get urgent help now if your baby has:
If any of these occur, go to the nearest emergency department or call 911. Some of these are signs of RSV illness itself (which protection reduces but can't always prevent), and some relate to a rare reaction to the injection — either way, they need prompt attention.
Speak with a healthcare provider before this protection if:
These usually do NOT prevent RSV protection:
If you're pregnant
Ask your pregnancy provider (doctor, midwife, or obstetrician) about RSV protection — whether the maternal vaccine (Abrysvo) at 32–36 weeks or planning Beyfortus for your baby suits you best.
If your baby is born / newborn
Beyfortus may be offered at the hospital after birth, or through your baby's provider during RSV season. Ask whether your baby is eligible.
Check eligibility for the season
Eligibility is set each season. Confirm current criteria with your provider, your public health unit, or Ontario's RSV program page.
Usually one option, not both
If you had Abrysvo in pregnancy, your baby usually doesn't also need Beyfortus. A provider confirms what's right for you.
High-risk children
Premature babies and those with certain conditions may be eligible up to 24 months and have specific recommendations — ask your provider.
Keep a record
Note whether your baby got Beyfortus, or you got Abrysvo in pregnancy, on the health record. See the Resources page.
This page is based on:
Please note: This is an independent website. It is not affiliated with, endorsed by, or operated by the Government of Ontario, the Ministry of Health, or any public health unit. The information here is for general education only and is not medical advice — always consult a licensed healthcare provider about your or your child's immunizations.