Swine flu basics

Behind the Headlines

Monday July 27 2009

Pandemic (H1N1) 2009 virus

recent article in the British Medical Journal discusses the basics of the Pandemic (H1N1) 2009 virus, including how it differs from ordinary seasonal flu and how mutations lead to these new strains.

Here we discuss these points in the context of other basic facts about the H1N1 strain that is causing the current global pandemic.


Influenza type A

There are three main types (genera) of influenza virus: type A, type B and type C. Type A is the most noteworthy as it is responsible for regular outbreaks of disease in humans. This virus also infects pigs, horses and other animals and its natural host is the wild bird. In fact, only type A can infect birds. Type B viruses also cause human infection, but mutate slowly and are less common. Influenza type C causes only mild respiratory symptoms and has not been linked to serious human epidemics.

Influenza A is further classified by the nature of some of the proteins that are embedded in its outer layer. Haemagglutinin (H) and neuraminidase (N) are two proteins with important roles in how effectively the virus can invade a host. Influenza A can have a number of different types of haemagglutinin but for human infections, H1, H2 and H3 are important. There are also a number of different neuraminidases, of which N1 or N2 are usually found in combination with one of the above H molecules.

Viruses are named after which complement of H and N are in their outer layer. For example, swine flu is a new H1N1 virus and therefore carries the proteins haemagglutinin 1 and neuraminidase 1. Although other H1N1 viruses have circulated before, this new strain differs substantially from previous strains.


What are the concerns with swine flu?

Swine flu is a new H1N1 virus. This means that before this year’s outbreak, this exact type of virus has never before circulated in humans. This is of concern at it means that the general population is not immune to the virus, and so it has the potential for a greater impact than ordinary seasonal influenza.

H1N1 viruses are fairly uncommon and every year H1N1 adds to the burden of disease associated with seasonal influenza. However, viruses change regularly, creating new strains. Even small differences in viral structure can affect the way a host responds to an infection. This is why flu vaccines need to be updated every year to keep up with these changes.


How do viruses change?

Viruses change by mutating in several different ways. Sometimes spontaneous mutations can happen in the genes of a virus. Alternatively, a process called reassortment can occur, when different strains of flu virus share genes with each other in the same host to make a new strain.

‘Drift versus shift’

The term ‘antigenic drift’ is sometimes used to explain the small mutations in a virus that happen over time. These lead to a gradual evolution of the virus and population immunity is more or less able to keep up with these changes. ‘Antigenic shift’ on the other hand explains bigger mutations that suddenly produce a new virus. These new viruses are usually responsible for serious outbreaks and pandemics because populations have little or no immunity to them.

The BMJ article explains that small changes in the structure of the genes responsible for haemagglutinin may alter the way the body responds. The origin of the new H1N1 strain currently circulating has been described by three virologists as a descendant of two unrelated pig viruses.


What will the impact of swine flu be?

It is difficult to predict the impact of swine flu. So far, most infected people recover after a period of illness that is similar to seasonal flu. However, in people with comorbidities (e.g. lung disease, heart disease, kidney disease and diabetes) or the elderly and very young, the infection can be more severe.

To ensure that services can cope with future demand, scientists work hard to predict how the virus may spread and how it will affect people.


Reasons to be optimistic

Geoff Watts, the author of the BMJ article, points out several important reasons to be optimistic about the current flu pandemic:

  • Antiviral treatments are available that can limit the spread of infection and reduce the impact of swine flu on populations.
  • A vaccine will soon be available and rolled out. The most vulnerable groups will most likely receive it first, both protecting them from potentially serious illness and helping to halt the spread of infection.
  • There is some evidence that pandemics are becoming less severe over time, although this is difficult to prove given that management of outbreaks is also improving with time and experience.

Analysis by Bazian

Edited by NHS Choices

Links to the science

A/H1N1 influenza virus: the basics. BMJ 2009, July 24

Further reading

Influenza viruses. Types, Subtypes, and Strains.  Centers for Disease Control and Prevention 2009


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