Influenza B: How Serious, When to Go In

Key takeaways

Influenza B is NOT the mild kind of flu. According to CDC, influenza B infection can be just as severe as influenza A infection — knowing which type you have does not tell you whether the illness will be mild or severe.
CDC notes that influenza B is more common in children, and that influenza B infection can be severe in children. So with a child, do not lower your level of watchfulness because you heard the letter B.
What to do at home, and the point at which you must go in, are THE SAME whether this is influenza A or influenza B. Do not wait to know the exact type before you start watching.
NO aspirin for children and teenagers who have flu: according to CDC, Reye syndrome occurs almost only in children taking aspirin and has been associated mainly with influenza B virus.
Difficulty breathing, pain or pressure in the chest, confusion, seizures, unable to drink, or a fever and cough that improved and then got worse again: go to emergency care, do not wait until morning.

The most important thing first: influenza B is not the mild kind of flu. Many people hear that “A is the severe one, B is the mild one” and then lower their guard — and that is where the danger lies. The name of the type tells you which group the virus belongs to; it does not tell you whether the illness will be mild or severe.

Recognising it: flu arrives suddenly

According to CDC, flu symptoms usually come on SUDDENLY, while a common cold comes on gradually. The usual ones: fever or feeling chilled, cough, sore throat, a runny or blocked nose, aching all over, headache, exhaustion. Young children may vomit and have diarrhoea more than adults do. And not everyone who gets flu runs a fever.

A common cold: worst in the nose and throat, and it rarely leaves you as wrung out as flu does.
“Viral fever”: a catch-all name used while nobody yet knows which virus it is, not a diagnosis — a fever that arrives suddenly could also be dengue.
Influenza A: the symptoms overlap almost entirely. Nobody tells them apart reliably by looking, not even a clinician — only a test can say which type it is.
A NEGATIVE rapid test does not rule flu out: according to CDC, you may still have flu even when a rapid test is negative.

Why “influenza B is milder” is a misreading

According to CDC, influenza B infection can be just as severe as influenza A infection, and influenza B should not be regarded as the milder type when treatment is being considered.
CDC notes that influenza B is more common in children, and that influenza B infection can be severe in children.
According to CDC, the common complications of flu such as pneumonia and bacterial co-infection have been found as often in children admitted with influenza B as in children admitted with influenza A.
Influenza B is less common than influenza A in most seasons — but “less common” does not mean “milder”.

Who needs to see a doctor early

Children under 5, with the highest risk in children under 2.
Adults aged 65 and over.
Pregnant women, and for the two weeks after a pregnancy ends.
Anyone with a long-term condition: asthma, lung disease, heart disease, diabetes, kidney disease, neurological disease, blood disorders; and anyone whose immune system is weakened.
If you are in one of those groups, call a doctor early rather than waiting to see whether it settles: antiviral flu medicines are prescription-only, work clearly best when started early, and do not conclude on your own that it is already too late.

What to do straight away at home

Write down the DATE and the TIME the symptoms began — that is the marker the doctor asks about, and the one most often misremembered.
Stay home and genuinely rest. Drink steadily, a little at a time: warm water, oral rehydration solution made up exactly as the sachet directs, rice water, milk.
Keep the person in their own room, with their own towel and cup, and a mask on when they leave it; everyone in the household washes their hands.
Watch what you can observe, not only the thermometer: is the breathing faster, can they still drink, are they passing urine, are they alert or drowsy; with a child, are they still playing.
If something is needed for fever or pain, ask a doctor or a pharmacist. Paracetamol (acetaminophen) is the usual choice, but the dose is a decision for the doctor or the pharmacist.

What must NOT be done

NO aspirin for children and teenagers who have flu — the most important prohibition in this illness. According to CDC, Reye syndrome occurs almost only in children taking aspirin and has been associated mainly with influenza B virus.
Do NOT buy antibiotics yourself: flu is caused by a virus, and according to CDC antibiotics do not work against viruses.
Do NOT buy antiviral flu medicine yourself, and do not reuse somebody else's leftovers — those are prescription medicines.
Do NOT take several over-the-counter cold remedies at once: many already contain something for fever, so adding more doubles up the same active ingredient.
Do NOT lower your watchfulness just because a test came back as influenza B, and do NOT wait overnight once any sign in the box below is present.

When you must go to hospital

GO TO EMERGENCY CARE NOW or call 115 (the emergency number in Vietnam) if ANY of the following is present. Adults: difficulty breathing or fast breathing; persistent pain or pressure in the chest or the abdomen; continuous light-headedness; confusion, hard to rouse; seizures; not passing urine; severe muscle pain; marked weakness or unsteadiness on the feet. Children: fast breathing or difficulty breathing; blue lips or face; the chest drawing in with every breath; chest pain; muscle pain so bad the child will not walk; no urine for several hours, a dry mouth, crying without tears; not alert, or not interacting when awake; seizures; a very high fever that medicine for fever cannot control. A baby under three months old with a fever goes in straight away, with no waiting. At any age: A FEVER OR COUGH THAT IMPROVED AND THEN GOT WORSE AGAIN, or an underlying condition that is worsening — go in, do not let it pass. — BE SEEN THE SAME DAY, do not wait until tomorrow, if: the person is in one of the high-risk groups above; a high fever is not settling; the cough is increasing, they are eating and drinking poorly, they are very tired. — If you are not sure, or the person is on several medicines, call and ask before deciding on your own.

When to call a doctor to the house

In these few days a family usually needs a clinician for three things: to see whether this really is flu or something else, to see whether the patient is in a high-risk group, and to listen to the chest so that pneumonia is not missed. A Y Te Xanh doctor comes to the house, takes the history, examines the patient, measures pulse, blood pressure, temperature and SpO₂, and says clearly at what point admission becomes compulsory; if nobody needs to come yet, there is a remote consultation first.

If more information is needed, the doctor can order tests at home — but not every test belongs at home: anything that must be run immediately, or that needs specialised equipment, gives more reliable results when the sample is taken at a medical facility. Severe flu needing oxygen or continuous monitoring belongs in hospital — if any sign in the box above is present, go in first and call afterwards.

The symptoms, the warning signs, the high-risk groups, the aspirin warning and the comparison of severity between influenza B and influenza A in this article follow US CDC material; this article passes that on and does not replace being examined. Citing CDC here does not imply that CDC endorses Y Te Xanh or our services.

Need personalised advice for your loved one? Call our hotline 094 368 81 15 (24/7) — calling to ask a question or to book costs nothing. GreenCare has doctors who come to examine, nurses who carry out doctor's orders (injections, infusions, dressing changes), remote consultation by video call, and a review of the medicines you are taking; an in-depth remote consultation, or any one after the first, is a paid service. For children we take examination and acute care only — not long-term care at home, and not newborns or expectant mothers. We confirm we have staff in your area and provide transparent pricing before we begin.

Sources

CDC — About Influenza

Frequently asked questions

Influenza B is milder than influenza A, isn't it?

No, and this is the commonest misunderstanding about this illness. According to CDC, influenza B infection can be just as severe as influenza A infection, and CDC states plainly that influenza B should not be regarded as the milder type when treatment is being considered. It is true that influenza B is less common than influenza A in most seasons, but “less common” is a fact about statistics, not a promise about how severe things will be for the person who is ill in your house. What matters more than naming the type correctly is this: is the patient in a high-risk group, and has any warning sign appeared yet.

Is there anything specific to watch for with influenza B in children?

Two things. First, CDC notes that influenza B is more common in children and that influenza B infection can be severe in children — so there is no basis for feeling more relaxed on hearing that a child has influenza B. Second, and this is the most important prohibition: never give aspirin to a child or teenager who has flu. According to CDC, Reye syndrome occurs almost only in children taking aspirin and has been associated mainly with influenza B virus; it is a severe complication affecting the liver and the brain. Many over-the-counter cold remedies contain aspirin, so read the label or ask a pharmacist before giving a child anything at all.

If a rapid test says influenza B, is there still any point in being seen?

Knowing that it is influenza B does not change what you need to do: still rest, still keep fluids up, still watch for the warning signs, and still see a doctor early if the patient is in a high-risk group. On top of that, according to CDC a rapid test can be negative while the person does have flu, so a negative strip is not a reason to stop watching. A doctor may also conclude that this is flu from the symptoms alone, without a test.

Does influenza B need antibiotics?

No. Flu is caused by a virus, and according to CDC antibiotics do not work against viruses — taking them does nothing about the cause, while you still get the full set of side effects and contribute to bacteria becoming resistant. Antiviral flu medicine is something else entirely: it is real and it does help, but it is prescription-only, shows its benefit clearly only when started early, and has to be ordered by a doctor after weighing up the underlying conditions and the other medicines being taken. If a bacterial infection does develop later, that is a doctor's conclusion after examining the patient, not a decision made at a pharmacy counter.

Does the flu vaccine protect against influenza B?

Yes. Current seasonal flu vaccines include a component aimed at influenza B of the Victoria lineage, alongside the components aimed at influenza A. The vaccine is not absolute protection, but people who have had it usually have a milder illness and fewer complications. Note that flu vaccine has to be repeated every year — last year's dose does not count for this year. Who should have it, and when, is a question for a doctor or a vaccination centre, because it depends on age, pregnancy and underlying conditions.

How long does influenza B take to clear?

A healthy person usually improves over about a week, but the cough and the tiredness can last some weeks longer. What is worth watching is not the total number of days already spent ill, but the direction of travel: if the fever or the cough improved and then got worse again, then according to CDC that is a sign to be seen straight away, not the moment to go back to work or school. The common complication after flu is pneumonia, and it tends to appear just when the family thinks the illness is over.