Influenza B: how it differs from influenza A and what to do

Influenza is an acute viral infection of the respiratory tract. UKHSA’s Green Book explains that there are three types of influenza virus, A, B and C, that A and B are responsible for most clinical illness, and that influenza is highly infectious with a usual incubation period of 1 to 3 days. WHO lists a fourth type, D, which mainly affects cattle and is not known to cause illness in people.

What distinguishes type B is how it is classified. WHO says influenza A viruses are divided into subtypes according to surface proteins, such as H1N1 and H3N2, while influenza B viruses are not divided into subtypes but into two lineages, B/Yamagata and B/Victoria.

The clinical picture in individuals is broadly similar. The NHS describes flu symptoms as coming on very quickly, with a sudden high temperature, an aching body, tiredness, a dry cough, a sore throat and a headache, and does not distinguish between virus types. Laboratory tests are needed to say which type or lineage caused an infection.

WHO notes that only influenza A is known to have caused pandemics. Influenza A and B viruses both circulate and cause seasonal epidemics of disease, so a winter flu season can involve either or both.

WHO says symptoms usually begin around 2 days after infection and include a sudden onset of fever, usually dry cough, headache, muscle and joint pain, severe malaise, sore throat and runny nose. Most people recover from fever and other symptoms within a week without medical attention, although the cough can be severe and last 2 weeks or more.

The NHS says children can also have ear pain, sore eyes and swollen glands and appear less active. It describes flu as making you feel exhausted and too unwell to carry on as normal, in contrast to a cold, which comes on gradually and mainly affects the nose and throat.

The NHS advice for flu is to rest and sleep, take paracetamol or ibuprofen for aches and temperature, drink plenty of water and ask a pharmacist for advice on flu remedies. It says antibiotics do not work for viral infections such as flu, and that aspirin should not be given to children under 16. You are most likely to pass it on in the first 5 days after infection.

It advises staying at home if you have a high temperature or feel unwell, washing hands often with warm water and soap, covering your mouth and nose with a tissue when you cough or sneeze and binning tissues quickly.

WHO lists pregnant women, children under 5, older people, those with chronic medical conditions and those with immunosuppression as being at greater risk of severe disease or complications, and says that in severe cases influenza can lead to pneumonia and sepsis.

The NHS says to ask for an urgent GP appointment or call NHS 111 if you have flu symptoms and are worried about a baby or child, are 65 or over, are pregnant or have recently given birth, have a long-term medical condition or a weakened immune system, feel very unwell, feel short of breath or your symptoms do not improve after 7 days. Call 999 or go to A&E for sudden chest pain, severe difficulty breathing or coughing up blood.

WHO adds that vaccination is the best way to prevent influenza and that annual vaccination is recommended because immunity goes away over time.