[6][7][8] Seasonal influenza is an annually recurring outbreak of flu, which occurs during the cold half of the year in each hemisphere.

Most H3N2 virus isolates are triple reassortants, containing genes from human (HA, NA, and PB1), swine (NS, NP, and M), and avian (PB2 and PA) lineages.

Of the 97 recent H3N2 isolates examined, only 41 had strong serologic cross-reactions with antiserum to three commercial SIV vaccines.

[14][15][16] The pandemic infected an estimated 500,000 Hong Kong residents, 15% of the population, with a low death rate.

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.

[citation needed] The Hong Kong flu was the first known outbreak of the H3N2 strain, though there is serologic evidence of H3N2 infections in the late 19th century.

The first record of the outbreak in Hong Kong appeared on 13 July 1968 in an area with a density of about 500 people per acre in an urban setting.

That same month, the virus entered California from United States troops returning from the Vietnam War.

This was due to a reassortment event that caused a minor clade to provide a haemagglutinin gene that later became part of the dominant strain in the 2002–2003 flu season.