RSV symptoms in infants: what to watch for
RSV (respiratory syncytial virus) is so common that nearly all children have been infected at least once by age 2. For most, it's a cold. For infants — especially those under 6 months — it can become something much more serious.
The challenge for parents is that RSV starts exactly like a common cold. Knowing what the progression looks like, and which signs mean "call now," can make a real difference.
Days 1–3: The cold phase
RSV typically begins with upper respiratory symptoms that are indistinguishable from any other cold:
- Runny nose (often clear at first)
- Mild cough
- Sneezing
- Low-grade fever — typically under 101°F
- Decreased appetite
- Fussiness
Most babies with RSV will improve and recover during this phase. The concern begins if symptoms progress toward the lower airways.
Special rule for newborns: Any fever above 100.4°F (38°C) in an infant under 3 months requires a call to your pediatrician immediately — regardless of what's causing it. RSV or not, fever in a newborn is always a reason to call.
Days 3–5: When RSV moves lower
In some infants — particularly those under 6 months, premature babies, and those with heart or lung conditions — RSV descends from the nose and throat into the small airways of the lungs (bronchioles), causing bronchiolitis. This is when RSV becomes dangerous.
Signs that RSV may be moving lower:
- Worsening cough — getting deeper, more frequent, or producing a wheezing sound on exhale
- Faster breathing — count breaths per minute when your baby is calm and sleeping. Normal is 30–60 breaths/min for infants. Over 60 at rest is worth a call.
- Working harder to breathe — nostrils flaring, or a slight pulling-in of the skin above the collarbone
- Feeding less — eating less than half of normal; tiring during feeds
- Fewer wet diapers — sign of dehydration
The warning signs — act now
These symptoms mean call your pediatrician today or go to urgent care:
- Breathing rate above 60 breaths/minute at rest
- Wheezing or a high-pitched sound on exhale
- Audible grunting with each breath
- Cough that doesn't let up
- SpO₂ below 95% on a pulse oximeter
Emergency signs — call 911 or go to the ER immediately
- Ribs showing clearly with each breath (retractions)
- Nostrils flaring dramatically
- Skin or lips turning blue or gray (cyanosis)
- SpO₂ below 90%
- Baby is unresponsive, limp, or won't wake up
- Pauses in breathing (apnea) — especially in very young infants
Apnea (brief pauses in breathing) can occur with RSV in very young infants — sometimes even before other symptoms develop. If you notice your baby pausing in breathing, even briefly, call 911.
How to count breathing rate
Wait until your baby is calm or asleep. Watch their chest or belly rise and fall. Count the number of breaths in 30 seconds and multiply by 2. For infants under 12 months: under 50 is normal; 50–60 warrants monitoring; over 60 at rest warrants a call.
Using a pulse oximeter at home
A pulse oximeter clips to a finger or toe and measures blood oxygen saturation (SpO₂). Normal SpO₂ in a healthy infant is 97–100%. Most pediatricians use 95% as the threshold below which supplemental oxygen may be needed.
A home pulse oximeter won't replace clinical evaluation, but it gives you a concrete number to share with your pediatrician and can help you decide whether to go to the ER.
What RSV does not look like
RSV typically does not cause severe sore throat, muscle aches, or high fever (above 103°F) in otherwise healthy infants — those symptoms are more suggestive of flu. RSV also doesn't cause rash. If you're seeing those symptoms, call your pediatrician — another illness may be in play.
Not medical advice. This guide is for educational purposes. Always consult your pediatrician for medical questions about your child. In an emergency, call 911.