What does it mean for a disease to be “endemic”?
This does not mean that the disease has disappeared. When epidemiologists use the word “endemic”, they mean that a disease occurs “at an expected level in a place over a period of time”, explains epidemiologist René Najera, editor-in-chief of The history of vaccinesan online resource from the College of Physicians of Philadelphia.
Nor does ‘endemic’ mean that a disease has ceased to be harmful. Malaria, tuberculosis and influenza are serious and potentially fatal endemic diseases that occur every year. Since the 1940s, countries have built strong international health networks that identify strains of influenza in order to control them and avoid the kind of devastation that occurred in the 1918 pandemic. That’s something that epidemiologists and virologists have argued is necessary for COVID-19, and will continue to be necessary if and when the virus becomes endemic.
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WATCH: The 1918 flu was deadlier than World War I
Influenza early detection systems
In 1946, the United States established the Communicable Disease Center, or CDC, in Atlanta, Georgia. Now known as the Centers for Disease Control and Prevention, the CDC’s original goal was to prevent the spread of malaria. Two years later, the newly formed United Nations – created in the aftermath of World War II – created the World Health Organization, whose constitution states that “Governments are responsible for the health of their peoples”.
One of the earliest concerns of the World Health Organization, or WHO, was influenza, says Wenqing Zhang, director of the WHO’s Global Influenza Program. The 1918 flu pandemic resulted in approximately 50 million deaths worldwide. During World War II, the US military, remembering how the flu had devastated troops in World War I, began funding research for a flu vaccine. In the early 1940s, an Army-backed research team at the University of Michigan led by Thomas Francis Jr. and Jonas Salk (of polio vaccine) developed the first viable influenza vaccine. In 1945, flu shots became available to civilians.
In 1952, WHO created the Global Influenza Surveillance and Response System to collect data on influenza in different countries and coordinate global efforts to fight influenza. It was a time when the US military – which was stationed around the world – and the armies of other countries were monitoring outbreaks of infectious diseases, and the WHO’s flu control program was trying to use country monitoring stations to get an overview. Within a few years, the CDC’s Influenza Division became a collaborating center with the WHO program.
Initially, the WHO flu program was looking for signs and symptoms of influenza and apparent spikes in influenza cases, Najera says. Finding the right vaccine to treat these cases was a more difficult affair. Bivalent influenza vaccines could vaccinate against two flu strains at once, but finding out what kind of flu strains were circulating and what kind of vaccine composition would best treat an outbreak was still something scientists were learning to do. During the 1950s and 1960s they began to make progress.
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Identification of new epidemics
The first influenza pandemic after World War II occurred relatively early in the existence of the WHO influenza program. Yet according to Maurice Hilleman, the microbiologist who sounded the alarm on the outbreak, the WHO missed the first signs of the pandemic. In April 1957, Hilleman heard of an influenza epidemic in Hong Kong that had started a few months earlier in East Asia. After obtaining samples of the virus from a US Army General Medical Laboratory in Zama, Japan, he realized it was a new strain capable of developing into a pandemic.
That’s what he did. The 1957 pandemic killed an estimated 1.1 million people worldwide, and most likely would have killed many more had it not been for Hilleman, who initiated work on a vaccine that became available that year. Hilleman went on to develop more than 40 vaccines and received the National Medal of Science for his contributions to public health.
When the next influenza pandemic hit in 1968, WHO’s influenza surveillance program was more effective at detecting it. In July 1968, the National Influenza Center at the University of Hong Kong identified a strain of influenza that was spreading in the region. Scientists from the World Influenza Center in London and the CDC in Atlanta received samples of the virus. The scientists who analyzed it found that the virus, like the 1957 virus, was a unique strain that could cause a pandemic. They relayed this information to the WHO, which in August issued a warning that a new flu pandemic was beginning.
Thanks to this information and this warning, vaccine manufacturers were able to develop vaccines specific to this viral strain. Like all pandemics, the 1957 pandemic left a devastating toll, killing an estimated one million people. Yet global information sharing has led to the development of strain-specific vaccines that have helped prevent infection and serious illness in many people.
Identifying the right vaccine for the world
During the 1970s and 1980s, scientists began to better understand influenza. They found that different strains predominated in different flu seasons, meaning that vaccines would have to specifically target circulating strains to be effective. In 1973, the WHO made its first formal recommendation on which strains new flu vaccines should target, Zhang says.
In 2007, an international resolution gave WHO more authority to tell collaborating countries which influenza vaccines they should distribute for that year based on circulating strains, and also what to do if WHO detects a growing flu pandemic – things like increased vaccine production, increased testing, enforcement of travel restrictions, etc. (Zhang notes, however, that the WHO cannot force a country to use certain vaccines).
The H1N1 outbreak in 2009 “was the first test of these international health regulations,” says Najera. Although the virus has killed an estimated 151,700 to 575,400 people, early detection and the relative availability of tests and vaccines have kept the pandemic from getting worse. (In comparison, the WHO estimates that there are 290,000 to 650,000 flu-related respiratory deaths each year.) Ironically, many people who avoided the flu that year may not have realized how much they have benefited from the work of global health systems.
“It’s the public health problem,” says Najera. “When things are going well, nobody notices.”