Side by side
| Influenza A | Influenza B | |
|---|---|---|
| Causes seasonal flu | Yes[1] | Yes[1] |
| Subtypes now circulating | A(H1N1) and A(H3N2)[2] | B/Victoria; B/Yamagata has not been detected since March 2020[2] |
| How fast it changes | Changes faster[2] | Generally changes more slowly[2] |
| Can cause pandemics | Yes, the only type known to[2] | No[2] |
| Covered by this season's vaccine | One H1N1 and one H3N2 strain[2] | One B/Victoria strain[2] |
| Antiviral treatment | Same recommendations[3] | Same recommendations[3] |
Do symptoms differ?
Not in a way you can count on. CDC's guidance on who should be treated, and its emergency warning signs, do not depend on the flu type.[3] Use the warning signs and your risk group, not the letter, to decide what to do next. Symptom check.
Antivirals work for both
Oseltamivir (Tamiflu) is approved to treat illness from both influenza A and B.[4] Of more than 4,400 flu viruses CDC tested in the 2025-26 season, about 99% were susceptible to all four FDA-approved antivirals.[5] Compare the antivirals.
Getting flu twice in one season
Several flu viruses can circulate in the same season.[1][2] Catching flu twice is possible, for example A early in the season and B later, but it is uncommon. In 13 years of lab-confirmed flu reports from Queensland, Australia, the cumulative chance of a repeat infection was under 1% within a year, rising to about 10% over ten years. Young children and adults 65 and older were more likely to have repeat infections.[6]
If you feel better and then get worse again, treat it as a warning sign and get care.[8] Getting worse after getting better.
Reading a home test
Home combination tests show separate results for flu A, flu B, and COVID-19.[7] A positive A or B result leads to the same next steps. What to do with your result.