RSV vs Flu (Influenza): Key Differences
Content reviewed against published CDC, CDC, AAP guidelines
Last reviewed:
RSV and influenza are both common respiratory viruses in children, but they differ in their typical age groups, symptom patterns, and available treatments.
Key takeaways
- Flu has sudden onset with high fever; RSV starts gradually like a cold
- RSV primarily causes lower respiratory symptoms (wheezing) in babies; flu causes systemic symptoms (fever, body aches)
- Flu has antiviral treatment (Tamiflu); RSV has no specific antiviral
- RSV is most dangerous for very young infants; flu is dangerous across all pediatric ages
: Cause
RSV (Respiratory Syncytial Virus): Respiratory syncytial virus
Flu (Influenza): Influenza A or B virus
: Highest risk age
RSV (Respiratory Syncytial Virus): Under 6 months (premature babies especially)
Flu (Influenza): Under 5 years; also those with chronic conditions
: Onset
RSV (Respiratory Syncytial Virus): Gradual, starts like a cold
Flu (Influenza): Sudden onset of high fever and body aches
: Fever
RSV (Respiratory Syncytial Virus): Low-grade or absent
Flu (Influenza): High fever (often 102-104°F / 39-40°C)
: Key symptoms
RSV (Respiratory Syncytial Virus): Wheezing, nasal congestion, poor feeding in infants
Flu (Influenza): High fever, body aches, fatigue, cough
: Duration
RSV (Respiratory Syncytial Virus): 7-14 days, peak at days 3-5
Flu (Influenza): 5-7 days, though cough may linger
: Treatment
RSV (Respiratory Syncytial Virus): Supportive care only (suctioning, fluids)
Flu (Influenza): Oseltamivir (Tamiflu) if started within 48 hours
: Vaccine/prevention
RSV (Respiratory Syncytial Virus): Nirsevimab (monoclonal antibody) for infants; maternal RSV vaccine
Flu (Influenza): Annual flu vaccine (available from 6 months)
: Main complication
RSV (Respiratory Syncytial Virus): Bronchiolitis, pneumonia in infants
Flu (Influenza): Pneumonia, dehydration, febrile seizures
| RSV (Respiratory Syncytial Virus) | Flu (Influenza) | |
|---|---|---|
| Cause | Respiratory syncytial virus | Influenza A or B virus |
| Highest risk age | Under 6 months (premature babies especially) | Under 5 years; also those with chronic conditions |
| Onset | Gradual, starts like a cold | Sudden onset of high fever and body aches |
| Fever | Low-grade or absent | High fever (often 102-104°F / 39-40°C) |
| Key symptoms | Wheezing, nasal congestion, poor feeding in infants | High fever, body aches, fatigue, cough |
| Duration | 7-14 days, peak at days 3-5 | 5-7 days, though cough may linger |
| Treatment | Supportive care only (suctioning, fluids) | Oseltamivir (Tamiflu) if started within 48 hours |
| Vaccine/prevention | Nirsevimab (monoclonal antibody) for infants; maternal RSV vaccine | Annual flu vaccine (available from 6 months) |
| Main complication | Bronchiolitis, pneumonia in infants | Pneumonia, dehydration, febrile seizures |
When to Worry
- Rapid breathing or visible rib retractions
- Blue or gray color around lips or fingernails
- Baby under 3 months with fever over 100.4°F (38°C)
- Signs of dehydration: no wet diaper in 8+ hours, no tears when crying
- Child is lethargic, difficult to wake, or unresponsive
- Fever over 104°F (40°C) that does not respond to medication
Frequently asked questions
What is the main difference between RSV (Respiratory Syncytial Virus) and Flu (Influenza)?
How can I tell if my baby has RSV (Respiratory Syncytial Virus) or Flu (Influenza)?
When should I worry?
Sources
All content follows our editorial policy and is reviewed against published clinical guidelines.
2,705 evidence-based guides6 authoritative medical sources
Bottom line
Both RSV (Respiratory Syncytial Virus) and Flu (Influenza) are common in young children. The key differences in symptoms can help you identify what your child may have, but when in doubt, consult your pediatrician.
Trust your instincts. If something feels wrong, reach out to your pediatrician. Worrying about your baby means you care — that is a good thing.