Influenza A(H1N1): symptoms, the 2009 pandemic strain and what to do

The UK Health Security Agency green book explains that influenza is an acute viral infection of the respiratory tract, that there are three types of influenza virus, A, B and C, and that A and B are responsible for most clinical illness. It describes influenza as highly infectious, with a usual incubation period of 1 to 3 days.

WHO adds that influenza A viruses are classified into subtypes by the proteins on their surface, and that A(H1N1) and A(H3N2) currently circulate in humans. The A(H1N1) virus is written A(H1N1)pdm09 because it caused the pandemic in 2009 and replaced the H1N1 virus that had circulated before. Only influenza type A viruses are known to have caused pandemics. The 2009 pandemic strain was widely called swine flu in the media.

The NHS lists a sudden high temperature, an aching body, feeling tired or exhausted, a dry cough, a sore throat, a headache, difficulty sleeping and loss of appetite. Children can also have ear pain, sore eyes and swollen glands and appear less active. WHO notes symptoms usually begin around 2 days after infection and that the cough can be severe and last 2 weeks or more.

The NHS says flu tends to be more severe than a cold, appears quickly within a few hours, affects more than just the nose and throat, and makes you feel exhausted and too unwell to carry on as normal.

According to the NHS, if you have flu you should rest and sleep, take paracetamol or ibuprofen to lower your temperature and ease aches, and drink plenty of water. It says not to give aspirin to children under 16, that antibiotics do not work for viral infections such as flu, and that a pharmacist can advise on flu remedies. Do not take paracetamol and flu remedies that contain paracetamol together.

Flu is very infectious and the NHS says you are more likely to give it to others in the first 5 days after getting infected. Wash hands often, cover coughs and sneezes with a tissue, bin tissues quickly and stay at home while you have a high temperature or feel unwell.

WHO lists pregnant women, children under 5, older people, people with chronic medical conditions and people with immunosuppressive conditions or treatments as being at greater risk of severe disease or complications, and says people at high risk or with severe symptoms should be treated with antiviral medicines as soon as possible. In severe cases influenza can lead to pneumonia and sepsis.

The NHS says to ask for an urgent GP appointment or get help from NHS 111 if you have flu symptoms and you are worried about a baby or child, are 65 or over, are pregnant or recently gave birth, have a long-term condition such as diabetes or a heart, lung, liver, kidney or nerve condition, have a weakened immune system, feel very unwell, feel short of breath or 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 says vaccination is the best way to prevent influenza and that annual vaccination is recommended because immunity from vaccination goes away over time. Vaccines are updated routinely to contain viruses that match those circulating.

In the UK, the NHS says the flu vaccine is offered every year in autumn or early winter to some adults and children at higher risk. Eligibility rules change each season, so check current NHS guidance or ask a GP surgery or pharmacy.