Influenza B FAQ: clear answers on symptoms, spread, and care

Influenza B is a viral respiratory infection caused by the influenza B virus, which mutates more slowly than influenza A and typically causes less severe pandemics but similar seasonal outbreaks.

The key differences lie in their genetic stability and pandemic potential. Influenza A undergoes more frequent antigenic changes, enabling it to cause global pandemics when novel strains emerge. Influenza B changes more gradually, primarily causing seasonal epidemics with predictable patterns.

Both viruses cause similar clinical symptoms including fever, cough, body aches, and fatigue. Laboratory testing is required to distinguish between them, though this differentiation rarely affects individual treatment decisions during routine seasonal outbreaks.

For more detailed comparisons, see the CDC influenza symptoms guidance which provides comprehensive information on both virus types.

Influenza B spreads through respiratory droplets when infected individuals cough, sneeze, or talk, and can also survive on surfaces for several hours.

The primary transmission route involves inhaling droplets containing the virus, typically when within 1-2 metres of an infected person. These droplets are produced during normal respiratory activities, with coughing and sneezing generating the highest concentrations of viral particles.

Surface transmission occurs when the virus survives on objects such as door handles, keyboards, or shared equipment. People can become infected by touching contaminated surfaces and then touching their nose, mouth, or eyes before washing their hands.

Crowded indoor environments with poor ventilation facilitate rapid spread, explaining why schools, offices, and healthcare facilities often experience outbreaks. Household transmission rates are particularly high, with 20-40% of family members typically becoming infected when one person brings the virus home.

Symptoms include sudden fever, cough, sore throat, body aches, fatigue, and headache, typically appearing 1-4 days after exposure.

The characteristic pattern involves rapid onset of systemic symptoms rather than the gradual progression seen with common colds. Many people describe feeling severely unwell within hours, often using phrases like "hit by a truck" to describe the sudden onset of fever and muscle aches.

Additional symptoms may include chills, runny or stuffy nose, and general malaise. Children often experience gastrointestinal symptoms such as nausea, vomiting, and diarrhoea more frequently than adults.

Warning signs requiring immediate medical attention include difficulty breathing, persistent chest pain, severe dehydration, confusion, or symptoms that improve then suddenly worsen. High-risk individuals including elderly adults, pregnant women, and those with chronic conditions should seek medical evaluation promptly.

The seasonal flu vaccine includes protection against influenza B strains and is typically 40-60% effective depending on the vaccine match and individual factors.

Vaccine effectiveness varies annually based on how well the selected vaccine strains match the viruses actually circulating during the flu season. When well-matched, vaccines provide substantial protection against illness, hospitalisation, and complications.

Quadrivalent vaccines include strains from both major influenza B lineages (Victoria and Yamagata), providing broader protection compared to older trivalent formulations. This comprehensive coverage helps maintain effectiveness even when one lineage predominates unexpectedly.

Even when breakthrough infections occur, vaccination typically reduces illness severity and duration. The WHO seasonal influenza fact sheet provides detailed information on global vaccination strategies and effectiveness monitoring.

Most people recover within 1-2 weeks, though cough and fatigue may persist for several weeks after initial infection.

The acute illness phase typically lasts 3-7 days, during which fever, body aches, and severe fatigue are most prominent. Fever usually resolves within 3-5 days, though some individuals may experience intermittent low-grade fever for up to a week.

Respiratory symptoms, particularly cough, often persist longest and may continue for 2-3 weeks after other symptoms resolve. This prolonged cough is usually dry and gradually diminishes in frequency and severity.

Recovery time varies based on individual factors including age, overall health status, and whether complications develop. Elderly adults and those with chronic conditions may require longer recovery periods, whilst healthy children and adults typically return to normal activities within 1-2 weeks of symptom onset.

Yes, antivirals like oseltamivir (Tamiflu) can reduce symptom duration and severity if taken within 48 hours of symptom onset.

Antiviral medications work by inhibiting viral replication, which can shorten illness duration by 1-2 days and reduce the risk of complications when started early. The 48-hour window is critical because viral replication is highest during the first few days of illness.

These medications are particularly recommended for high-risk individuals including adults over 65, pregnant women, young children, and people with chronic conditions such as asthma, diabetes, or heart disease. Hospitalised patients may benefit from antiviral treatment even when started beyond the 48-hour window.

Common side effects include nausea and vomiting, which are usually mild and resolve quickly. The comprehensive information on oseltamivir provides detailed guidance on usage, effectiveness, and considerations for different patient populations.