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The H1N1 influenza pandemic that swept across the world from 1918 to 1919, involved a particularly virulent new strain of the influenza A virus. The first wave of infections in early 1918 resulted in mild illness, but a second wave later that year was more deadly.

This strain of influenza is estimated to have infected 500 million people worldwide, killing between 20 and 50 million. The resulting death rates were so high that life expectancy rates around the world dropped by several years, and more people are thought to have died as a result of the flu pandemic than over the course of the entire First World War.

In 1933, British researchers Wilson Smith, C.H. Andrewes, and P.P. Laidlaw at London’s National Institute for Medical Research (NIMR) made a breakthrough when they isolated and identified the influenza virus. They found no bacteria in throat washings from patients with influenza and discovered that the disease was caused by a virus.

The first inactivated flu vaccine was developed by Thomas Francis and Jonas Salk at the University of Michigan. The vaccine was tested for safety and efficacy on the US military before being licensed for wider use in 1945.

It was suspected that different types of influenza viruses existed, as the blood of some influenza patients did not develop antibodies to the strain isolated in 1933. During the testing period, scientists also discovered the existence of another strain of the virus: influenza B.

In 1942, a new bivalent vaccine was developed that protected against both the H1N1 strain of influenza A and the newly discovered influenza B virus.

Scientists could now manufacture vaccines based on the monitoring of virus strains in circulation around the world, updating the strains targeted by the vaccine in response.

Here is a vaccine timeline for the past 225 years:

  • 1796 - Dr. Edward Jenner collected bits of cowpox pustule—the animal variant of smallpox - from the arm of a milkmaid named Sarah Nelmes and scratched it into the arm of an 8-year-old boy.

  • 1881 - French Biologist Louis Pasteur developed a successful vaccine against anthrax. Pasteur exposed anthrax pathogens to heat and oxygen to weaken, but not kill them.

  • 1885 - Pasteur developed a successful vaccine against rabies. Pasteur used the same approach he did for the anthrax vaccine.

  • 1914 - Pertussis, or whooping cough, vaccine is developed.

  • 1926 - Diphtheria vaccine is developed.

  • 1938 - Tetanus vaccine is developed.

  • 1948 - Pertussis, diphtheria and tetanus vaccines are combined and given as the DTP vaccine.

  • 1955 - Jonas Salk develops a break-through polio vaccine based on a dead poliovirus.

  • 1963 - Measles vaccine is developed.

  • 1967 - Mumps vaccine is developed.

  • 1969 - Rubella vaccine is developed.

  • 1977- The smallpox vaccine is no longer recommended. Because of the vaccine's success, the disease is considered eradicated.

  • 1981 - Hepatitis B vaccine is developed.

  • 1996 - Chickenpox vaccine is developed.

  • 1998-1999 - Rotavirus vaccine is developed.

  • 2000 - Hepatitis A vaccine is developed.

  • 2000 - Polio vaccine is no longer recommended. Because of the vaccine's success, the disease is no longer considered a threat.

  • 2001 - Pneumococcal vaccine is developed.

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