Hong Kong flu
The Hong Kong flu was a category 2 flu pandemic whose outbreak in 1968 and 1969 killed an estimated one million people worldwide. 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 1968–1969 pandemic 
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 fatality of the 1957 Asian Flu in China, little improvement had been made regarding the handling of such epidemics. The Times newspaper was actually 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 US until December 1968. It would reach Japan, Africa and South America by 1969. The outbreak in Hong Kong, where density is about 500 people per acre, reached maximum intensity in 2 weeks, lasting 6 weeks 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. In the United States, approximately 33,800 people died.[dead link]
The same virus returned the following years: a year later, in late 1969 and early 1970, and in 1972.
Fewer people died during this pandemic than the two previous pandemics for various reasons:
- some immunity against the N2 flu virus may have been retained in populations struck by the Asian Flu strains which had been circulating since 1957;
- the pandemic did not gain momentum until near the winter school holidays, thus limiting the infection spreading;
- improved medical care gave vital support to the very ill;
- the availability of antibiotics that were more effective against secondary bacterial infections.
Clinical data 
Flu symptoms lasted 4 to 5 days. Those over the age of 65 were most likely to die.
In the 1968 pandemic vaccine 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.
"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."
See also 
- Paul, William E. Fundamental Immunology. p. 1273.
- "World health group issues alert Mexican president tries to isolate those with swine flu". Associated Press. April 25, 2009. Retrieved 2009-04-26.
- Mandel, Michael (April 26, 2009). "No need to panic ... yet Ontario officials are worried swine flu could be pandemic, killing thousands". Toronto Sun. Retrieved 2009-04-26.
- Starling, Arthur (2006). Plague, SARS, and the Story of Medicine in Hong Kong. HK University Press. p. 55. ISBN 962-209-805-3.
- Jones, F. Avery (1968), "Winter Epidemics", British Medical Journal 1968 (4): 327, doi:10.1136/bmj.4.5626.327-c.
- Coleman, Marion T.; Dowdle, Walter R.; Pereira, Helio G.; Schild, Geoffrey C.; Chang, W. K. (1968), "The Hong Kong/68 Influenza A2 Variant", The Lancet 292 (7583): 1384–1386, doi:10.1016/S0140-6736(68)92683-4.
- U.S. Department of Health and Human Services, http://www.pandemicflu.gov/general/historicaloverview.html
- Chapter Two : Avian Influenza by Timm C. Harder and Ortrud Werner from excellent free on-line Book called Influenza Report 2006 which is a medical textbook that provides a comprehensive overview of epidemic and pandemic influenza.
- Campbell, Charles H.; Easterday, Bernard C.; Webster, Robert G. (1977), "Strains of Hong Kong Influenza Virus in Calves", Journal of Infectious Diseases 135 (4): 678–680, doi:10.2307/30107903, PMID 853249.
Further reading 
- Blakely, Debra E. (2006), Mass Mediated Disease: A Case Study Analysis of Three Flu Pandemics and Public Health Policy, Lanham: Lexington Books, pp. 148–196, ISBN 0-7391-1387-9.
- Influenza Research Database – Database of influenza genomic sequences and related information.