History

Jump to: 1918 Spanish Flu | 1957 Asian Flu | 1968 Hong Kong Flu | Avian Influenza Virus Subtype H5N1 | History Links

Overview

There were three influenza pandemics during the 20th century: the 1918 Spanish Flu, the 1957 Asian Flu, and the 1968 Hong Kong Flu (informally, pandemics are named after their presumed sites of origin). Influenza virus subtypes H1N1, H2N2, and H3N2 are the antigenic subtypes that, respectively, caused these influenza pandemics. Each successive pandemic virus strain had hemagluttinin (HA, which may be shortened to H) antigens that differed from the HA antigens of previous seasonal flu virus strains and previous pandemic flu virus strains. True human influenza pandemics are classified by changes in the HA subtypes that arise from genetic reassortment with animal influenza A viruses (genetic shift). Historically, influenza pandemics have differed from each other in etiology, epidemiology, and disease severity.

1918 Spanish Flu

In 1918, the Spanish Flu caused an estimated 50 million deaths worldwide, including at least 675,000 deaths in the U.S. Genetic material recovered from the preserved (frozen) bodies of people who had died from that flu have enabled scientists to identify the causative influenza virus subtype as having been H1N1.

This virus was exceedingly virulent, which meant that those who were infected became very sick, many dying the same day as the first symptoms presented. Of those who did not die in the first few days, a high proportion succumbed later to flu-related complications, such as pneumonia. High infection rates and mortalities were especially common among otherwise healthy adults aged 20-50. High-risk groups, such as the elderly and young children, also had high infection rates and mortalities. No subsequent influenza pandemic has been caused by a virus as virulent as the 1918 influenza A virus.

1957 Asian Flu

The Asian Flu pandemic occurred about 40 years after the Spanish Flu pandemic. It caused an estimated one to two million deaths worldwide, including at least 70,000 deaths in the U.S. By this time, science and technology had advanced significantly. Therefore, scientists were able to rapidly identify the H2N2 flu virus subtype. These HA and neuraminidase (NA) antigens were completely different from the antigens of the 1918 influenza virus. Science and technology advancements also enabled scientists to start developing an appropriate vaccine in May 1957, with a limited vaccine supply becoming available by August 1957.

During this pandemic, attack rates (greater than 50%) were highest among school children (aged 5-19), who spread the virus to their classmates. Those infected children carried the virus back to their families. Infection rates were also high among young adults and pregnant women. The elderly had the highest death rates. The worst seemed to be over by December 1957, but then a second wave of illness occurred among the elderly in February 1958. Prolonged illnesses led to reoccurring outbreaks and more deaths.

1968 Hong Kong Flu

This 1968 Hong Kong influenza pandemic of 1968 caused fewer deaths than the previous two pandemics. The virus subtype that caused this pandemic was somewhat analogous to the 1957 influenza virus because it had the same NA antigen, N2. but a different HA antigen.

The first case of Hong Kong flu was detected in September 1968, just 11 years after the last flu pandemic. The outbreak developed rather slowly, becoming widespread in early December, and with the mortality rate peaking in December and January 1969. Similar to the previous pandemic, schoolchildren suffered the highest attack rate. By the time the pandemic waned it had caused about 700,000 deaths worldwide, including about 34,000 deaths in the U.S. Many fewer people died during this pandemic than the other two pandemics for three reasons: (1) improved medical care that gave vital support to the very ill; (2) the availability of antibiotics that were more effective against secondary bacterial infections; and (3) the severity the illness probably was reduced among many people because they retained antibodies against N2 in their systems from the 1957 influenza pandemic.

Avian Influenza Virus Subtype H5N1

In May 1997, the Government Virus Unit in Hong Kong isolated an influenza A virus from a three-year-old child who was admitted to the hospital with a fever and respiratory symptoms. This child later died of acute respiratory distress and Reye's syndrome. Subsequently, the National Influenza Center in the Netherlands identified the virus that caused the child's death as an influenza A H5N1 subtype.

Six months after the virus's identification, 17 human H5N1 infections were documented in hospitalized patients in Hong Kong over a seven-week period. These patients ranged in age from one year to 60 years; seven cases were male and ten were female. Six of the 18 patients died and, with the exception of the first case, symptoms were found to be less severe in children than in adults.

Molecular analysis determined that the influenza A viruses were avian in origin and that genetic reassortment had not occurred. During this time, the same virus was isolated from asymptomatic ducks and geese in local live-bird markets. Further studies revealed that humans became infected as a result of direct human-bird contacts and not person-to-person contact, which, with respect to this virus subtype, occurs only rarely. However, because the influenza A virus strain did not need to pass through an intermediate host such as the pig, but could move directly from bird to human, it represented a novel outbreak.

Similar to the H5N1 virus spread, there have been a few human H9N2 infections, often in association with H5N1 infections. All H9N2 viral isolates are of avian origin. Previously, scientists believed that due to receptor specificity, avian influenza strains could not invade humans. However, since we now know that humans can become infected with H5N1 and H9N2 avian influenza viruses, albeit rarely, this notion has been disproved.

Pandemic Influenza History Links

Kreiser, C.M. 2006. "1918 Spanish influenza outbreak: the enemy within." American History Magazine, December. Accessed on December 19, 2006 from <http://www.historynet.com/magazines/american_history/4499781.html?featured=y&c=y>.

Potter, C.W. 2001. "A history of influenza." Journal of Applied Microbiology, 91(4):572.

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