The flu shot

The leaves are turning yellow, the nights are getting colder, and that can only mean one thing: a new season is upon us. Specifically, flu season, which last year, according to Health Canada, kicked off right here on the prairies at just about this time.

With the advent of this new season comes a growing chorus of public service announcements urging people to get flu shots.

Influenza is a viral disease characterized by fever, cough and muscle aches that typically last for three to seven days and are sometimes followed by several more days of fatigue and depression.

In an average year in Canada, 10 to 20 percent of the population contracts influenza. Tens of thousands of people are hospitalized; between 500 and 1,500 die. The elderly face the highest risk of death; others at risk include people with heart or lung problems or a weakened immune system.

In 1940 the U.S. Army, worried about epidemics decimating its ranks, formed the Board for the Investigation and Control of Influenza and Other Epidemic Diseases (later renamed the Armed Forces Epidemiological Board). The Board’s head was Dr. Thomas Francis, Jr., who had been researching the possibility of an influenza vaccine since 1935.

Because of his work, scientists already knew that a vaccine made with weakened live virus was unlikely to be effective–it tended to give the volunteers in the study the disease it was meant to prevent. So, through the 1940s, the Army board focused on developing an influenza vaccine made from killed viruses. A series of tests resulted in an effective vaccine, and in October and November of 1945, a large-scale vaccination program was carried out within the Army. The new vaccine soon became available for civilian use

The vaccine is made by injecting flu virus into chicken eggs. The virus replicates in the eggs, and can then be purified from them and killed with a chemical (such as formaldehyde). Because the vaccine is made with a killed virus, it cannot cause the disease. However, the immune system doesn’t distinguish between a killed virus and a live one: it produces antibodies to the killed virus. (The antibodies latch on to the viruses and signal to white blood cells to eat them.) With antibodies already in place, the body is much better prepared to ward off an infection with live flu virus later on.

The biggest challenge faced by influenza vaccine makers is that the virus comes in many different, constantly changing flavors.

To begin with, influenza viruses are divided into types A, B and C. A and B are the only ones that cause disease epidemics; C only produces a mild, cold-like illness, or sometimes no symptoms at all.

Within the A and B types are subtypes, determined by the specific characteristics of two antigens (proteins that generate an immune response in the body) that stick out from the virus’s fatty coat: hemagglutinin (H) and neuraminidase (N). The viruses that cause disease can have any combination of three different H types and two N types. Strains are further distinguished by the geographic area in which they first arose–i.e., Hong Kong, Fuji or Texas.

Both influenza A and B undergo antigenic drift, a gradual evolution. This kind of mutation slightly alters the H and N antigens, which in turn means the body only has to alter its usual flu-fighting antibodies slightly to fight them. Occasionally, influenza A viruses also undergo something called antigenic shift, an abrupt change in the antigens. This takes the immune system by surprise. As a result, it takes it much longer to generate antibodies, giving the virus more time to replicate in the body–and making the disease much more severe. Antigenic shifts are thought to produce the big flu pandemics. How this happens is unclear, but one leading theory is that human and animal strains of the influenza virus combine to form a new strain, which is why recent outbreaks of avian flu, which shows some ability to infect humans, are of great concern.

Every February experts at the World Health Organization recommend the composition of the vaccine for the northern hemisphere for the coming winter; in September, a recommendation is made for the southern hemisphere. The vaccine typically contains antigens from three strains of the virus, two type A and one type B. Because of the influenza virus’s tendency to mutate, even in the best years the vaccine is only about 75 percent effective at either preventing the disease or reducing the severity of its symptoms.

Except for those with severe allergies to the eggs used to make it, the influenza vaccine is very safe; it occasionally causes redness and tenderness at the site of the injection and sometimes a slight fever and muscle aches.

Considering the alternative, that doesn’t seem like much to put up with.

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