Flu - Simple as A B C

Flu, for most of us, is a self-limiting disease, cured with a few days in bed, so it’s easy to forget how dangerous it actually is. The seasonal flu causes 2-5 million severe cases per year and 250,000-500,000 deaths. In order to stop the spread of disease, and protect those immune-compromised patients, it is important to get a yearly flu jab.

Flu is caused by the influenza virus. There are three types: A, B, and C. Type A and B are very similar to each other, they have two particular proteins on their surface which are recognized by the immune system: hemagglutinin and neuraminidase. It’s these proteins that give the A type viruses their characteristic names e.g. H1N1. The seasonal flu vaccine has to give protection to at least three different types of flu: two types of influenza A (currently H1N1 and H3N2) and one type of influenza B.

Which exact strains go into the vaccine is decided yearly by a team of scientists and doctors at the WHO. Doctors all over the world swab the noses of patients who come to them with flu-like symptoms. These swabs are analysed in specialised labs called ‘National Influenza Centres’, where they determine the exact strains that are circulating in the population. There are 110 such centres in 80 different countries.

In temperate environments the flu season is in winter time, so monitoring the flu strains during the winter in the Southern hemisphere, helps to predict which strains will be circulating in the Northern hemisphere 6 months later. The WHO analyses the data provided by the National Influenza Centres and decides which strains go into the vaccine for next year. It takes roughly 6 months to have enough vaccines produced, this lag can be a problem when a pandemic breaks out and swift action is necessary. New technologies are therefore needed so vaccines can be produced faster.

Viruses can undergo antigenic drift and antigenic shift. The former is the reason why we need to get regular flu shots. The previously mentioned proteins hemagglutinin and neuraminidase undergo slight changes over time, this is because of selection pressure from the human population: as the virus circulates through the population, people become immune to it, so the virus has to adapt so it can infect more people. If antigenic drift is detected in the flu, while the flu vaccine is already being produced for next winter, it’s too late to do anything about it.

Antigenic shift, on the other hand, is rarer and more dangerous. It is only seen in type A influenza. This phenomenon is what causes major flu outbreaks, such as the 1918 Spanish influenza epidemic, which infected half the world’s population. There are several ways through which antigenic shift can occur, one of them being an intermediate host (for example a pig) being infected with both an avian and a human strain of the flu virus. In the pig, the viruses can then exchange genetic material, resulting in a brand new, never before seen virus that can infect humans. The fact that it is brand new means that no one in the population has built up immunity against it. Luckily antigenic shift is observed on average only 3 times per 100 year.

Gwendoline Deslyper