Influenza A vs. B: What's the Difference?

Prevention,Flu Season,Physical Health,Trend,Chief Medical Officer

Influenza A vs. B: What's the Difference?

02.19.2025 | 4 min read

James D. Grant, M.D., is senior vice president and c...

One of the worst flu seasons the United States has seen in years has Americans wondering about the difference between influenza A and influenza B and what they can do to stay healthy for the rest of the winter.

Flu levels in the US are as high as they’ve been in 15 years, according to the Centers for Disease Control and Prevention (CDC). To put the magnitude of this year’s flu season in perspective, in the second to last week of January, a higher percentage of Americans died from the flu (1.7%) than from COVID-19 (1.5%) for the first time, according to weekly reports published by the CDC. Emergency room visits have spiked and schools are temporarily closing in as many as 10 states as we approach a second peak.

If you haven’t gotten sick yet, you should still be proactive. Washing your hands, avoiding close contact with people who are sick and getting vaccinated against the flu virus can help you avoid severe illness., As you protect yourself and your family from the flu, you can educate yourself by considering these important points.

What is the difference between influenza A and influenza B?

There are four types of influenza viruses: A, B, C and D. C causes only a mild respiratory illness, and D is not known to infect humans, narrowing the flu strains you need to protect yourself against to only A and B.

Influenza A is much more common than influenza B, making up roughly 75% of cases worldwide. These flu types are distinguished by key differences, the most important of which is type B only infects humans, while type A can infect humans and animals. This makes influenza A more dangerous because inter-species transmission can lead to new flu strains, which can then lead to seasonal epidemics and pandemics. Bird flu is an example of this, as it emanates from flu A.

Influenza A is more common in adults and infuenza B is more common in children, because they have underdeveloped immune systems and less immunity to the virus than adults. Type B can cause complications that sometimes lead to hospitalization or death in children, according to the CDC.

Timing is also a key way to differentiate flu A and flu B, which usually makes you sick toward the tail end of flu season, because it mutates slower than flu A. Because of slower mutation, Influenza B strains do not vary from year to year as much as Type A strains.

What do influenza A and influenza B have in common?

Influenza A and B do share a few characteristics, including:

They spread the same way and cause similar symptoms like:
- Diarrhea and vomiting (usually only in children)
- Fever/chills
- Headaches
- Muscle and body aches
- Runny of stuffy nose
- Sore throat
- Weakness and fatigue
Both flu strains are more likely to affect babies and older adults, causing more serious symptoms in these age groups.
Health care providers treat flu A and flu B the same way. So, if you’re asking yourself, “how do I know if I have influenza A or B?” it isn’t that important for recovery as the treatment is the same. For either virus, providers will recommend rest, drinking plenty of fluids, taking over-the-counter medications and getting vaccinated.

Get vaccinated to protect against the flu

Getting the shot is the best way to prevent a potentially deadly disease. Vaccines have been proven to lower the risk of severe illness and death resulting from the flu.

Flu shots are recommended every year for individuals ages 6 months and older. The best time to get a flu vaccine is September or October, but it’s certainly not too late to get one in February or March, especially considering how deadly this year’s flu season is. As we are seeing now, flu sicknesses often hit their second peak in February.

Flu shots are available at pharmacies, primary care offices, urgent care centers, health departments and clinics. Workplaces, schools and colleges often host vaccination events. You can find places to get a flu shot by visiting vaccines.gov.

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