One flu subtype, H1N1, lies behind three pandemics across a century
The label H1N1 simply names the two surface proteins a flu virus carries. Yet strains of this one subtype caused the 1918 pandemic, the 1977 Russian flu and the 2009 swine flu, each thought to have arisen when viruses swapped gene segments, and one H1N1 strain has gone into every seasonal vaccine since 1999.
Influenza A subtypes are named after two proteins studding the viral envelope: hemagglutinin, the H, and neuraminidase, the N. H1N1 carries type 1 of each. Some strains circulate permanently in people as part of seasonal flu, while others live in pigs or birds. The genome comes in separate RNA segments, and occasionally two strains infecting the same host trade pieces. That reshuffling, called reassortment, can hand a virus new traits that dodge existing immunity or let it leap into a new species. Each year the WHO's surveillance network chooses three strains for the coming season's vaccine.
The 1918 strain was exceptionally lethal, killing somewhere from 17 to 50 or more million people during roughly a year between 1918 and 1920. One explanation is a cytokine storm, in which infected lungs trigger such a violent immune response that the tissue itself is damaged. The Spanish tag stuck only because Spain's newspapers reported the outbreak openly while wartime governments elsewhere kept it quiet to protect morale.
Two later episodes remain puzzling. In 1976 a new swine-type H1N1 made 13 soldiers at Fort Dix, New Jersey, seriously ill and killed one; blood tests later showed up to 230 had been infected, yet it never spread beyond the base and no contact with pigs was found. The 1977 Russian flu, strain A/USSR/90/77, mainly struck people under 23, because older adults had met a similar virus during 1947–57. It had apparently circulated for about a year before being spotted in China and Russia.
The 2009 virus was a patchwork, carrying genes from North American swine, North American bird, human and Eurasian swine flu. Cases surfaced in Mexico and the United States in April, the CDC posted the genetic sequences to the GISAID database, and confirmed infections reached 2,099 by 7 May. On 11 June 2009 the WHO raised its alert to phase 6, the first pandemic declaration since the 1968 Hong Kong flu. By 30 May 2010 more than 214 countries and territories had reported laboratory-confirmed cases and over 18,138 deaths. In August 2011 northern sea otters off Washington state turned up carrying the same strain.
Source: Influenza A virus subtype H1N1
