The Hong Kong flu was a global influenza pandemic that began in 1968 and killed an estimated 1 million people worldwide, including about 100,000 in the United States. It was caused by a new strain of influenza called H3N2, which first emerged in Hong Kong before spreading rapidly across the globe. It remains one of the three major flu pandemics of the 20th century, alongside the 1918 Spanish flu and the 1957 Asian flu.
How the Pandemic Started
The outbreak was first detected in Hong Kong in mid-1968, where the virus spread quickly through the densely populated city. Within months, it had reached Southeast Asia, India, Australia, Europe, and the United States. International air travel, which had expanded significantly by the late 1960s, accelerated the virus’s movement across continents in a way earlier pandemics had not experienced.
In the United States, the pandemic arrived in the fall of 1968 and peaked during the winter months. The virus hit in two distinct waves in many countries, with the second wave sometimes proving more deadly than the first, particularly in parts of Europe and Asia.
What Made the Virus Different
The Hong Kong flu was caused by an H3N2 influenza A virus. This strain arose through a process called antigenic shift, a major genetic change in which segments of different flu viruses recombine to create something new. In this case, the virus carried a new version of a key surface protein that the human immune system had never encountered before. Because most people had no pre-existing immunity to this protein, the virus was able to infect large numbers of people in a short time.
The H3N2 strain replaced the H2N2 virus that had been circulating since the 1957 Asian flu pandemic. This kind of replacement is a hallmark of pandemic flu: a virus with a fundamentally new surface structure displaces the old one and becomes the dominant strain in circulation.
Symptoms and Severity
The Hong Kong flu caused symptoms typical of influenza: fever, cough, sore throat, runny nose, muscle and joint pain, fatigue, and headache. Some people also experienced vomiting and diarrhea. The incubation period was roughly 1 to 4 days, and for most healthy individuals, recovery took anywhere from a few days to less than two weeks.
The pandemic was considered moderate in severity compared to the catastrophic 1918 flu, which killed tens of millions. Most deaths occurred among the elderly and people with chronic health conditions. The relatively lower death toll, while still staggering at 1 million globally, meant the pandemic received less public attention than might be expected. Life in most countries continued with relatively few social restrictions, a stark contrast to the shutdowns seen during COVID-19 more than 50 years later.
The Vaccine Came Too Late
Public health authorities moved faster in 1968 than they had during the 1957 pandemic. The CDC isolated and identified the virus and sent candidate vaccine strains to manufacturers more quickly than in the previous pandemic. Despite this improved preparation, the vaccine was not released until after the pandemic had already peaked in the fall of 1968. By the time doses were available in significant quantities, the worst of the first wave had passed through many communities.
This timing gap highlighted a challenge that persists today: manufacturing enough flu vaccine to meet demand during a fast-moving pandemic takes months, and the virus often outpaces production. The 1968 experience became an important case study for pandemic preparedness planning in the decades that followed.
Why H3N2 Still Matters
The 1968 pandemic didn’t simply end and disappear. The H3N2 virus settled into the human population and became one of the seasonal influenza strains that circulates every year. Descendants of the original 1968 virus continue to cause illness and death more than five decades later. H3N2 is one of the influenza strains included in the annual flu vaccine, and it tends to be one of the more troublesome seasonal strains, particularly for older adults.
H3N2 seasons are generally associated with higher hospitalization and death rates than years when other flu strains dominate. The virus mutates frequently, which makes it harder to match in the annual vaccine and helps explain why flu vaccine effectiveness varies from year to year. In seasons when H3N2 is the predominant circulating strain, the vaccine sometimes performs less well than in H1N1-dominant years.
How It Compares to Other Pandemics
The 20th century saw three major influenza pandemics, each caused by a different strain. The 1918 Spanish flu (H1N1) was by far the deadliest, killing an estimated 50 million or more people worldwide. The 1957 Asian flu (H2N2) killed roughly 1.1 million. The 1968 Hong Kong flu, at about 1 million deaths, was similar in scale to 1957 but milder than 1918.
One reason the 1968 pandemic was less severe than 1918 is that medicine had advanced considerably. Antibiotics could treat secondary bacterial pneumonia, which was a major killer in 1918. Better hospital care and supportive treatments also improved survival. Additionally, some portion of the population may have had partial immunity from exposure to the 1957 H2N2 virus, since the two strains shared one of their two key surface proteins.
The Hong Kong flu is often called the “forgotten pandemic” because it unfolded during a turbulent period in world history. The Vietnam War, political assassinations, and social upheaval dominated headlines, and the pandemic received comparatively little media coverage or public alarm despite its significant death toll.