Identifying the end of a pandemic isn't as straightforward as it may sound. Diseases may continue to spread long after people take steps to avoid transmission, while human and economic tolls of a pandemic can extend for decades. In some instances, diseases linger where medical interventions are unavailable, perpetuating the lifespan of the disease itself.
Historically, pandemics have stalled or leveled out, only to rebound with ferocity. Pandemics have also come to an end thanks to group immunity, viral adaptation, replacement by another virus, or vaccines developed by the scientific community. The so-called Spanish flu of 1918 and 1919, for all intents and purposes, underwent all of these phenomena.
Social distancing measures, masks, and containment efforts were all implemented to fight the Spanish flu, as millions of people worldwide succumbed to the virus. The timeline of how the Spanish flu pandemic "ended" spans nearly a century, with aspects of the disease continuing to come to light.
Historians and scientists recognize two "ends" to a pandemic - very much related, yet usually not concurrent. The medical end of a pandemic involves large-scale declines in patients affected by a disease and a drop in overall disease-related fatalities.
The social end of a pandemic is not determined by the disease, but rather by the populations it affects. When individuals stop being afraid of a disease, cease preventative action, and more or less decide the treatment of the illness no longer dictates their lives - that defines the social end.
The social end generally precedes its medical counterpart. Pandemic fatigue and adaptation to living with the disease, according to Harvard historian Allan Brandt, demonstrate how "the so-called end [of a pandemic] are determined not by medical and public health data but by sociopolitical processes.”
The so-called end of the pandemic of 1918 and 1919 varied by location. Cities across the United States handled the flu outbreak differently, leading to variability in the extent to which the disease affected residents.
In New York City, where the first casualty from the Spanish flu was reported in September 1918, quarantines, closures, and prohibitions against spitting were implemented. Public health officials in Chicago attempted to stop coughing and sneezing in public; closed "theaters, skating rinks, moving picture shows, night schools, and lodge halls;" and even called for dancing to stop due to "the close contact between dancers, the exercise of the dance, and the frequent chilling of the body that is to follow."
Philadelphia was slow to address the pandemic, even holding a World War I parade on September 28, 1918. As a result of the large public gathering, Philadelphia saw a huge uptick in Spanish flu cases, one that resulted in a citywide quarantine. In the end, nearly 12,000 residents of Philadelphia perished from the Spanish flu.
As social distancing measures and mask mandates were instituted across the United States, their efficacy in lowering cases of the flu proved successful. Furthermore, the earlier social distancing and other restrictions were put into place, the more successful the result. When steps to limit contact among groups were eased, second waves of the disease often occurred. In places like Oakland, CA, and St. Louis, MO, sharp spikes in flu-related fatalities meant a return to social distancing.
Although the Spanish flu pandemic is traditionally considered to have come to an end in 1919 or 1920, the virus itself was not eliminated. In fact, the virus (later identified as an H1N1 influenza A virus that could be carried and transmitted by animals) wasn't even visible to the human eye until the 1930s.
As science pushed ahead in its investigation and understanding of influenza, the H1N1 virus that caused the Spanish flu showed up seasonally for nearly four decades. In 1957, H1N1 essentially disappeared, however, when the H2N2 virus (the so-called Asian flu) triggered a pandemic of its own.
The H2N2 virus that resulted in the pandemic of 1957 and 1958 was also an influenza A virus. The first reports of the flu came from Singapore and Hong Kong in February and April 1957, respectively. It was present in the United States by the summer of 1957 and ended the lives of roughly 1 million people worldwide.
The H2N2 virus was active for 10 years until it was supplanted by the H3N2 virus and flu outbreak of 1968 (this time referred to as the Hong Kong flu). This H2N2 form of influenza still circulates today.
The initial outbreak of the Spanish flu wrought misery during the early months of 1918. The first wave of the Spanish flu was especially prevalent in the United States and Europe as WWI soldiers interacted and traveled, moving the virus with them as they went.
Compared to the first wave of the Spanish flu, the second wave from September to November 1918 proved more deadly, perhaps the result of a mutation of the virus. In the United States, the majority of the lives lost due to the Spanish flu took place during this second wave.
By late 1918 and into 1919, the third wave of the disease gripped the world. The third wave of the pandemic lasted until the summer, bringing to a close months of global suffering. There were continued cases of the flu around the world, however, prompting some observers to identify a much smaller fourth wave that hit localized areas in countries such as England and Austria.