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Swine Flu
(Novel H1N1 Influenza and H3N2v)

Swine Flu History and Overview

Influenza viruses are small RNA viruses that infect many mammals, including humans, birds, and swine. Before 2009, swine influenza predominately affected swine and was not transmitted often or easily to people. Even in the isolated instances in which swine influenza infected people, it had very limited ability to spread from person to person. Most cases were directly linked to contact with swine through farming or at fairs. Since 2009, the interactions and understanding of the role of swine and flu viruses in human infections has been markedly changed.

Swine were first noticed to have influenza-like illnesses in 1918 during the human influenza pandemic. The term pandemic means that an infection has spread to many countries around the world, causing widespread human disease. Swine influenza did not cause the 1918 pandemic. Rather, pigs apparently acquired the infection from humans or from an undiscovered source. For decades, the swine virus remained relatively unchanged. In the 1990s, however, swine influenza viruses became more diverse and new strains appeared. The reason for this change is not clear but may have been related to overcrowding on large swine farms.

Before 2009, there was only one swine influenza outbreak in people that caused public-health concerns. This outbreak occurred in 1979, in soldiers at Fort Dix, N.J. One recruit died, and approximately 12 were hospitalized with influenza. Further testing showed that more than 200 recruits had acquired the virus, although most had few or no symptoms. The infecting strain was found to be strongly related to swine influenza virus, raising concerns that a new pandemic might occur. In response, public-health officials began a massive public vaccination program. Up to 25% of people in the United States were vaccinated. Unfortunately, the 1979 vaccine was associated with a small increased risk of Guillain-Barré syndrome, a serious neurological condition, with the risk estimated to be one to nine excess cases per million doses. Importantly, the 1979 strain did not spread easily from person to person and there was no epidemic. Human cases outside of Fort Dix were uncommon. Moreover, the 1979 vaccine was made using an old-fashioned process which is no longer utilized.

The lessons learned from the 1979 swine influenza event were applied in dealing with pandemic threats, including the severe acute respiratory syndrome (SARS) outbreak of 2003 and the 2009 influenza outbreak. Key lessons included ensuring adequate communication with the public, producing a rapid but measured response to potential threats, and ensuring that any new strain fulfills criteria to cause a pandemic before large-scale vaccination was undertaken.

In 2011-2012, another outbreak occurred with a variant influenza virus strain termed H3N2v. The "v" indicated the strain that only usually infected swine was found to infect humans. Fortunately, the strain is not transmitted easily from human to human, and since 2011, only about 329 individuals have been infected. Readers should note that another strain that is termed H3N2 causes flu, but this strain is not a "v" or variant strain and is different from swine flu-causing H3N2v.

Medically Reviewed by a Doctor on 9/25/2013

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