Flu virus

SCIENCE

“ A seasonal flu jab is developed every year, to match the flu that is circulating in the population”

Inside the flu

We reveal how this common winter bug stays one step ahead of our immune system

The influenza virus infects a staggering 5 million people worldwide every single year, travelling from person to person in airborne droplets, and causing chills, fever, sore throat, runny nose, headaches and muscle pain.

The flu virus changes gradually by a process known as antigenic drift. As the virus replicates, single nucleotide errors occur in the viral genome, causing minute changes to the proteins that coat the outside of the virus. The immune system recognises these proteins to detect and destroy the infection, so as they change, the ability of the body to recognise the virus decreases, preventing people from building up immunity.

Not only does the virus make continual, subtle changes to its genome and proteins, but it also occasionally develops huge mutations. If a host becomes infected by more than one strain of flu virus, and the two meet inside a single cell, there is a chance that their genomes will mix together, consequently producing new, mutant flu virus. This is a rather rare occurrence, but can form dangerous new strains of flu – the swine flu (H1N1) pandemic of 2009 was found to contain genetic information from four different viruses: one human, one avian and two swine influenza.

This is one of the reasons that a universal vaccine against all types of flu is such a challenge. Currently, a seasonal flu jab is developed every year, to match the flu that is circulating in the population. Each subsequent year, the virus has usually changed sufficiently that the vaccine is no longer effective.

However new research suggests that some cells of the immune system can recognise proteins from the core of the virus. These are essential to viral function, and mutate far more slowly, so developing a vaccine against these important proteins could help T-cells to develop long-term immunity to the bug.

The flu virus mutates each year so it’s hard for the body to build up immunity

Get to know your ABCs… The virus in focus

Take a closer look at the anatomy that makes up a single flu virion

RNA

The genetic material of the flu virus is stored on several strands of ribonucleic acid (RNA).

Haemagglutinin

Spiky protein haemagglutinin coats the outside of the virion, allowing it to stick to, enter and infect cells of the throat and lungs.

Influenza A

The natural hosts of influenza A are wild water birds. Transfer to domestic poultry exposes humans to the virus and can result in cross-species infection. The H1N1 Spanish flu of 1918 and the H5N1 bird flu of 2004 were influenza A.

Influenza B

Influenza B prefers a human host and is less common. It mutates slowly, enabling most to build up immunity, but it doesn’t last for ever. It rarely infects other species, preventing the creation of the new, mutant strains that cause pandemics.

Influenza C

This produces only mild disease, and most adults have been infected at some point in their life. It infects humans and pigs, but is far less common than influenza A and B. It can cause local epidemics, but does not lead to pandemic flu.

Matrix

Beneath the membrane is a protein shell, which provides the virion with strength and structure.

DID YOU KNOW?

Influenza A viruses are named after the two major proteins that cover their outer membrane

Neuraminidase

This surface protein is an enzyme, which helps the virus to eat through cell defences and breach the cell membrane. Drugs like Relenza and Tamifl u inhibit this process.

Lipid bilayer

The outer layer of the virion is a lipid membrane. It is not produced by the virus, but is stolen from the membrane of the host cell.

The H1N1 swine fl u virus caused a pandemic in 2009-2010, but it wasn’t as serious as many anticipated