The problems with influenza

Why do we keep getting the flu?

What is influenza?

Influenza (the ‘flu’) is a viral infection that attacks the respiratory system. If you’re young and healthy, you generally just need to rest and possibly treat your symptoms. Others, however, such as the very young, the elderly, or those with weak immune systems may require antiviral treatment.

We’ve beaten the virus, we’ve even had the vaccines, so why are we not immune? The thing is you are actually immune – at least to those particular strains that you have already had.

Types of influenza virus

Influenza viruses are divided into four types A, B, C, and D. Types C and D don’t usually cause problems in humans, whilst types A and B are the ones that circulate amongst humans causing seasonal epidemics.

Type B is divided into lineages which are then subdivided into strains. Influenza A is divided into subtypes according to two proteins on their surface, hemagglutinin (H) and neuraminidase (N), both of which come in many different forms, represented by numbers. These subtypes are then further divided into strains.

Spanish flu

Influenza A is the only type known to have caused pandemics. The Spanish flu of 1918 is one example, caused by a strain of the H1N1 subtype. This pandemic resulted in the deaths of anywhere between 21 and 100 million people (Jordan, 1927; Johnson & Mueller, 2002), or around 3 – 6% of the world population. Unlike seasonal flu, Spanish flu killed mostly young healthy adults.

This may have been due to circumstances related to World War I such as malnourishment and overcrowded hospitals, resulting in secondary bacterial infections (Brundage & Shanks, 2007). Another theory is that this strain caused the immune system to overreact, so those with healthy immune systems were more affected (Barry, 2004).

Genetic mutations resulting in virus changes

Viruses change in two main ways: antigenic drift and antigenic shift.

• Antigenic drift refers to the small genetic mutations in viral genomes that happen continuously as the virus replicates. These mutations generally produce viruses with similar surface proteins (antigens) that the immune system can still recognise and respond to (cross-protection). However, these mutations can accumulate over time, resulting in viruses that are antigenically different enough that the immune system no longer recognises, and must start all over again in order to recognise and kill the virus.
• Antigenic shift refers to the process in which two or more viruses combine genomes to form a new strain. For influenza A viruses, this can result in new hemagglutinin and neuraminidase forms and combinations. An example of an antigenic shift is the recent 2009 H1N1 ‘swine flu’ pandemic. Although not particularly deadly, the fact that this strain was originally exclusive to pigs until it combined with human influenza viruses, gaining the ability to infect and transmit among humans was concerning. It resulted in a completely new H1N1 strain, one that humans had almost no cross-protection for. This antigenic shift happens when two different viruses infect the same host at the same time and package up bits of each other’s genome when creating new viruses.

The image below shows you the difference between the antigenic drift and antigenic shift processes.

Animal flu strains infecting humans

Avian influenza (bird flu) is also of concern. Many species of birds are migratory so when strains of avian flu gain the ability to infect humans, they have the potential to spread extremely rapidly.

Luckily, so far, avian influenza strains have not been easily transmissible among humans. However, should this occur, the fact that birds are migratory would be irrelevant as we could spread the disease ourselves. This could certainly result in the next devastating influenza pandemic.

References

Barry, J.M. (2004). The great influenza: The epic story of the greatest plague in history. New York, NY: Penguin.

Brundage, J. F., & Shanks, G. (2007). What really happened during the 1918 influenza pandemic? The importance of bacterial secondary infections. Journal of Infectious Diseases, 196 (11), 1717–1718. doi:10.1086/522355.

Johnson, N. P., & Mueller J. (2002). Updating the accounts: Global mortality of the 1918–1920 “Spanish” influenza pandemic. Bulletin of History of Medicine, 76 (1), 105–115.

Jordan, E.O. (1927). Epidemic influenza: A survey. Chicago, IL: American Medical Association.