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 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. The outbreak reached maximum intensity in two weeks, lasting six weeks in total. The virus was isolated in Queen Mary Hospital. Flu symptoms lasted four to five days. By July 1968, extensive outbreaks were reported in Vietnam and Singapore. By September 1968, it reached India, the Philippines, northern Australia and Europe. That same month, the virus entered California from United States troops returning from the Vietnam War. It reached Japan, Africa and South America in 1969. Fujian flu (2003–2004) Fujian flu refers to flu caused by either a Fujian human flu strain of the H3N2 subtype or a Fujian bird flu strain of the H5N1 subtype of the Influenza A virus. These strains are named after Fujian province in China. A/Fujian (H3N2) human flu (from A/Fujian/411/2002(H3N2)-like flu virus strains) caused an unusually severe 2003–2004 flu season. 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. A/Fujian (H3N2) was made part of the trivalent influenza vaccine for the 2004–2005 flu season. 2004–2005 flu season The 2004–05 trivalent influenza vaccine for the United States contained: an A/New Caledonia/20/99 (H1N1)-like virus an A/Fujian/411/2002 (H3N2)-like virus a B/Shanghai/361/2002-like virus. 2005–2006 flu season The vaccines produced for the 2005–2006 season used: an A/New Caledonia/20/1999-like(H1N1) an A/California/7/2004-like(H3N2) (or the antigenically equivalent strain A/New York/55/2004) a B/Jiangsu/10/2003-like viruses 2006–2007 flu season The 2006–2007 influenza vaccine composition recommended by the World Health Organization on 15 February 2006 and the US FDA's Vaccines and Related Biological Products Advisory Committee on 17 February 2006 used: an A/New Caledonia/20/99 (H1N1)-like virus an A/Wisconsin/67/2005 (H3N2)-like virus (A/Wisconsin/67/2005 and A/Hiroshima/52/2005 strains)