The definition: "occurring over a wide geographic area and affecting an exceptionally high proportion of the population. The following information suggests influenza pandemics have probably happened during the last 100 year.
1918: Spanish Flu: The Spanish Influenza pandemic was catastrophe. It is estimated that approximately 20 to 40 percent of the worldwide population became ill and that over 50 million people died. Between 1918 and 1919, approximately 675,000 deaths from the flu occurred in the U.S. alone.
Many people died from this very quickly. Some people who felt well in the morning became sick by noon, and were dead by nightfall. Those who did not die from the disease within the first few days often died of complications from the flu (such as pneumonia) caused by bacteria. The most unusual aspects of the Spanish flu was its ability to kill young adults. The mortality rates were high among healthy adults as well as the usual high-risk groups. The attack rate and mortality was highest among adults 20 to 50 years old. The severity of that virus has not been seen again.
1957: Asian Flu: The Asian influenza pandemic was first identified in the Far East. Immunity to this strain was rare in people less than 65 years of age, and a pandemic was predicted. Preparation for vaccine production began in late May 1957, and health officials increased surveillance for flu outbreaks. Unlike the virus that caused the 1918 pandemic, the 1957 pandemic virus was quickly identified, due to advances in scientific technology. Vaccine was available in limited supply by August 1957.
When U.S. children went back to school in the fall, they spread the disease in classrooms and brought it home to their families. Infection rates were highest among school children, young adults, and pregnant women. Most influenza-and pneumonia-related deaths occurred between September 1957 and March 1958. The elderly had the highest rates of death. By the end of December 1958 about 69,800 people in the U.S. died.
1968: Hong Kong Flu: The Hong Kong influenza pandemic was first detected in Hong Kong. The first cases in the U.S. were detected as early as September of that year, Illness become widespread in the U.S. in December, 1968 and January 1969. Those over the age of 65 were most likely to die. The same virus returned in 1970 and 1972. The number of deaths between September 1968 and March 1969 for this pandemic was 33,800, making it the mildest pandemic in the 20th century.
1976: Swine Flu Threat: when a novel virus was first identified at Fort Dix, it was labeled the "killer flu." Experts were extremely concerned because the virus was thought to be related to the Spanish flu virus of 1918. The concern that a major pandemic could sweep across the world led to a mass vaccination campaign in the United States. In fact, the virus--later named "swine flu"--never moved outside the Fort Dix area. Research on the virus later showed that if it had spread, it would probably have been much less deadly than the Spanish flu
1977: Russian Flu Threat: In 1977, influenza A/H1N1 viruses isolated in northern China, spread rapidly, and caused epidemic disease in children and young adults under the age of 23 years worldwide. Because of the timing of the appearance of these viruses, persons born before 1957 were likely to have been exposed and developed immunity. Therefore, primarily younger people became ill from A/H1N1 infections.
By 1978, the virus had spread around the world, including the United States. Because illness occurred primarily in children, this event was not considered a true pandemic. Vaccine containing this virus was not produced in time for the 1977-78 seasons, but the virus was included in the 1978-79 vaccine.
1997: Avian Flu Threat The most recent pandemic "threats" occurred in 1997 and 1999. In 1997, at least a few hundred people became infected with the avian A/H5N1 flu virus in Hong Kong and 18 people were hospitalized. Six of the hospitalized persons died. This virus was different because it moved directly from chickens to people, rather than having been altered by infecting pigs as an intermediate host. In addition, many of the most severe illnesses occurred in young adults similar to illnesses caused by the 1918 Spanish flu virus.
To prevent the spread of this virus, all chickens (approximately 1.5 million) in Hong Kong were slaughtered. The avian flu did not easily spread from one person to another, and after the poultry slaughter, no new human infections were found.
In 1999, another novel avian flu virus - A/H9N2 - was found that caused illnesses in two children in Hong Kong. Although these viruses have not gone on to start pandemics, their continued presence in birds, their ability to infect humans, and the ability of influenza viruses to change and become more transmissible among people is an ongoing concern.
An epidemic of influenza is different from the dreaded pandemic that scientists and world health officials fear is nigh. We might see an epidemic of seasonal influenza during any given year. In fact, we just had one.
Flu reached epidemic levels in the U.S. for 10 weeks in a row during the 2004-2005 season. Records kept by the CDC show that during the week ending March 5, 2005, 8.9% of all deaths reported in 122 U.S. cities were due to influenza and pneumonia (a common complication of the flu).
Seasonal flu epidemics may sicken millions, but those who die are typically a small number of the elderly, very young children, and people with weak immune systems. That's not the case during the worst influenza pandemics. There are two main features of an influenza pandemic.
1. the virus is a new strain that has never infected people before.
2. it's on a global scale. Sometimes it's also unusually deadly.
Influenza pandemics have struck about three times every century since the 1500s, or roughly every 10-50 years. There was one in 1957-1958 and one in 1968-1969. An estimated 40 million people died in less than a year, and what made it so different from seasonal flu epidemics is that it killed primarily young people, those aged 20-45.
The Next Pandemic
The world is closely watching a virus known as avian influenza H5N1, or "bird flu." Don't confuse it with pandemic flu. It isn't one. At least, it isn't one yet. The H5N1 (Bird) flu virus is an influenza A virus subtype that is highly contagious among birds. Rare human infections with the H5N1 (Bird) flu virus have occurred. The majority of confirmed cases have occurred in Asia, Africa, the Pacific, Europe and the Near East. Currently, the United States has no confirmed human H5N1 (Bird) flu infections, but H5N1 (Bird) flu remains a serious concern with the potential to cause a deadly pandemic.
More information on who declares a Pandemic, how it is tracked and what actions are taken in the US can be at the following web site http://www.flu.gov/individualfamily/about/index.html.
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