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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.

What are the symptoms of swine flu? Symptoms of swine flu are similar to most influenza infections: fever (100 F or greater), cough, nasal secretions, fatigue, and headache, with fatigue being reported in most infected individuals. Some patients may also get a sore throat, rash, body aches, headaches, chills, nausea, vomiting, and diarrhea. In Mexico, many of the initial patients infected with H1N1 influenza were young adults, which made some investigators speculate that a strong immune response, as seen in young people, may cause some collateral tissue damage.The incubation period from exposure to first symptoms is about one to four days, with an average of two days. The symptoms last about one to two weeks and can last longer if the person has a severe infection. Some patients develop severe respiratory symptoms and need respiratory support (such as a ventilator to breathe for the patient). Patients can get pneumonia (bacterial secondary infection) if the viral infection persists, and some can develop seizures. Death often occurs from secondary bacterial infection of the lungs; appropriate antibiotics need to be used in these patients. The usual mortality (death) rate for typical influenza A is about 0.1%, while the 1918 "Spanish flu" epidemic had an estimated mortality rate ranging from 2%-20%. Swine (H1N1) flu in Mexico had about 160 deaths and about 2,500 confirmed cases, which would correspond to a mortality rate of about 6%, but these initial data were revised and the mortality rate worldwide was estimated to be much lower. Fortunately, the mortality rate of H1N1 remained low and similar to that of the conventional flu (average conventional flu mortality rate is about 36,000 per year; projected H1N1 flu mortality rate was 90,000 per year in the U.S. as determined by the presidents advisory committee, but it never approached that high number).


The Centers for Disease Control and Prevention recommends flu vaccination for all people older than 6 months of age. An H1N1 virus is one component of the seasonal flu shot for 2014-15. The flu shot also protects against two or three other influenza viruses that are expected to be the most common during the flu season. The vaccine will be available as an injection or a nasal spray. The nasal spray is approved for use in healthy people 2 through 49 years of age who are not pregnant. The nasal spray is not recommended for people who are older than 50, younger than 2, pregnant or allergic to eggs, or people who have asthma or a compromised immune system, or those who use aspirin therapy. These measures also help prevent swine flu (H1N1 flu) and limit its spread:

Stay home if you are sick. If you have swine flu (H1N1 flu), you can give it to others. Stay home for at least 24 hours after your fever is gone.

Wash your hands thoroughly and frequently. Use soap and water, or if they are unavailable, use an alcohol-based hand sanitizer.

Contain your coughs and sneezes. Cover your mouth and nose when you sneeze or cough. To avoid contaminating your hands, cough or sneeze into a tissue or the inner crook of your elbow.

Avoid contact. Stay away from crowds if possible. And if you are at high risk of complications from the flu for example, you are younger than 5 or you're 65 or older, you're pregnant, or you have a chronic medical condition such as asthma consider avoiding swine barns at seasonal fairs and elsewhere.

Reduce exposure within your household. If a member of your household has swine flu, designate only one household member to be responsible for the ill persons personal care.