The Hong Kong flu (also known as 1968 flu pandemic) was a category 2 flu pandemic whose outbreak in 1968 and 1969 killed an estimated one million people all over the world.  It was caused by an H3N2 strain of the influenza A virus, descended from H2N2 through antigenic shift, a genetic process in which genes from multiple subtypes reassorted to form a new virus.
The first record of the outbreak in Hong Kong appeared on 13 July 1968. By the end of July 1968, extensive outbreaks were reported in Vietnam and Singapore. Despite the lethality of the 1957 Asian Flu in China, little improvement had been made regarding the handling of such epidemics. The Times newspaper was the first source to sound alarm regarding this new possible pandemic.
By September 1968, the flu reached India, the Philippines, northern Australia, and Europe. That same month, the virus entered California from returning Vietnam War troops but did not become widespread in the United States until December 1968. It would reach Japan, Africa, and South America by 1969. The outbreak in Hong Kong, where population density is greater than 6,000 people per square kilometre, reached maximum intensity in two weeks, lasting six months in total from July to December 1968. However, worldwide deaths from this virus peaked much later, in December 1968 and January 1969. By that time, public health warnings and virus descriptions were issued in the scientific and medical journals.
In comparison to other pandemics, the Hong Kong flu yielded a low death rate, with a case-fatality ratio below 0.5% making it a category 2 disease on the Pandemic Severity Index. The pandemic infected an estimated 500,000 Hong Kong residents, 15% of the population.
The same virus returned the following years: a year later, in late 1969 and early 1970, and in 1972. The CDC currently estimates that, in total, the virus killed 1 million people worldwide and around 100,000 people in the U.S.
Fewer people in the U.S. died during this pandemic than in previous pandemics for various reasons:
Flu symptoms lasted four to five days (some symptoms lasted up to two weeks).
In the 1968 pandemic, vaccines became available one month after the outbreaks peaked in the US.
Both the H2N2 and H3N2 pandemic flu strains contained genes from avian influenza viruses. The new subtypes arose in pigs coinfected with avian and human viruses and were soon transferred to humans. Swine were considered the original "intermediate host" for influenza because they supported reassortment of divergent subtypes. However, other hosts appear capable of similar coinfection (e.g., many poultry species), and direct transmission of avian viruses to humans is possible. H1N1 may have been transmitted directly from birds to humans (Belshe 2005).
The Hong Kong flu strain shared internal genes and the neuraminidase with the 1957 Asian Flu (H2N2). Accumulated antibodies to the neuraminidase or internal proteins may have resulted in much fewer casualties than most pandemics. However, cross-immunity within and between subtypes of influenza is poorly understood.