Hong Kong Flu(1968-1969)

The Hong Kong Flu was a category 2 flu pandemic caused by a strain of H3N2 descended from H2N2 by antigenic shift, in which genes from multiple subtypes reassorted to form a new virus. This pandemic of 1968 and 1969 killed an estimated one million people worldwide. The pandemic infected an estimated 500,000 Hong Kong residents, 15% of the population, with a low death rate. In the United States, approximately 33,800 people died.

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.

The Hong Kong flu was the first known outbreak of the H3N2 strain, though there is serologic evidence of H3N? 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. The outbreak reached maximum intensity in 2 weeks, lasting 6 weeks in total. The virus was isolated in Queen Mary Hospital. Flu symptoms lasted 4 to 5 days.

By July 1968, extensive outbreaks were reported in Vietnam and Singapore. By September 1968, it reached India, Philippines, northern Australia and Europe. That same month, the virus entered California from returning Vietnam War troops. It would reach Japan, Africa and South America by 1969.

"Three strains of Hong Kong influenza virus isolated from humans were compared with a strain isolated from a calf for their ability to cause disease in calves. One of the human strains. A/Aichi/2/68, was detected for five days in a calf, but all three failed to cause signs of disease. Strain A/cal/Duschanbe/55/71 could be detected for seven days and caused an influenza-like illness in calves."