Flu

Flu is a small virus that is unique among human diseases. It is continually circulating in cool, dry areas of east Asia, but it spreads across the northern and southern hemispheres when each half of the globe enters winter every year.

The virus spreads very efficiently in exhaled droplets and can be picked up from contaminated surfaces, meaning that almost everyone will be exposed to it during a flu season.

There are several types of flu virus. Influenza A strains mostly infect water birds, but a few varieties have adapted to humans, pigs and other mammals. Influenza B strains only infect humans.

Each flu season, a mix of influenza A and B strains circulate together, infecting up to half of all people and causing disease in around 10 to 15 per cent.

Multiple strains

Influenza A strains are named after two proteins on the surface of the virus – haemagglutinin and neuramidase. There are 18 types of haemagglutinin and 11 types of neuramidase in bird viruses, and the combination of these is used to name the different strains. For example, H1N1, H2N2 and H3N2 are known to infect humans.

Mutations in the haemagglutinin protein help the virus evade your immune system, which is why having flu once isn’t enough to make you immune to the virus. Our immune system detects flu by recognising haemagluttinin, but it constantly mutates in seven different places. It mutates so quickly that it only takes a few years to look completely different and escape many of the antibodies you made the last time you had flu.

However, we are able to carry a degree of immunity to strains of flu that are relatively similar to each other. As a result, the flu strain that dominates each winter tends to be a virus that is sufficiently different to evade a population’s prior immunity, and this is why a new flu vaccine has to be generated each year.

A deadly virus

Influenza A is particularly concerning because new genes or strains sometimes jump from birds to humans, which can result in especially severe epidemics known as pandemic flu.

The worst pandemic was the Spanish flu of 1918, which killed as many as 100 million people – roughly 5 per cent of all humans living at the time.

Flu kills mostly by causing viral pneumonia, which damages the oxygen-absorbing membranes in our lungs. This is a particular danger in people with compromised immune systems, including elderly people and pregnant women.

Compared to common colds, flu usually feels worse and is more likely to have symptoms such as fever, muscle pain, a dry cough, sore throat, and headaches. Flu can also sometimes cause sudden fatigue and eye and joint pain. Severe symptoms like difficulty breathing require urgent medical attention.

Flu can cause vomiting and diarrhoea in children, and help should be sought immediately if children become very irritable, have fever with a rash, bluish skin, or difficulty breathing. Another severe symptom that requires immediate attention is difficulty waking. Babies with flu may have fewer wet nappies when usual, trouble breathing, and no tears when crying.

At any age, you should see a doctor if you seem to get better and then get sick again with a worse cough.

Because flu is a virus, antibiotics cannot be used as a treatment, but doctors do prescribe these drugs in cases where flu has caused a secondary bacterial lung infection.

Annual vaccines

Each year, a vaccine is developed for the flu strain that is predicted to be most dominant over the next winter, and vaccination is recommended for people in vulnerable groups or those who could develop nasty complications.

Nearly all flu vaccines are made of viruses grown in hens’ eggs. This process dates back to the 1940s and takes up to eight months. Globally, we have the capacity to make around 1.5 billion doses of vaccine each year, each protecting against three or four strains.

Contrary to what many people think, you can’t get the flu from a flu vaccine, because the virus used in the vaccine is dead and unable to replicate. However, the vaccine isn’t effective or popular enough to stop flu through herd immunity. This means that, even if you get the vaccine, you still have about a 40 per cent chance of catching flu.

Because virologists have to predict which A and B strains are likely to be dominant months in advance, they sometimes get it wrong, meaning that in some years, the flu vaccine isn’t quite as useful as in others.

It may be possible to develop better flu vaccines, particularly ones that are cheaper and faster to make. The ultimate goal would be a universal vaccine that can protect against all current and future strains of flu – a “universal flu vaccine” – but this is a long way from becoming a reality.

In addition to vaccination, you can help avoid spreading flu by frequently washing your hands, and not going to work or school if you feel sick.