Dangerous new strains of whooping cough bacteria are evading Australia's vaccine against the disease and entrenching a four-year epidemic that could spread overseas, Sydney scientists have found. Microbiologists from the University of [New South Wales] have found [that] variants of the pertussis bacteria with a particular genetic signature have increased to 86 per cent of all samples taken from infected people after a continuing disease epidemic began in 2008. Although the strains were present in Australia as early as 2000, they accounted for only 31 per cent of all samples collected between 2000 and 2007 - suggesting they have flourished alongside the current vaccine against the potentially fatal respiratory infection. The strains have swept across Australia during the epidemic period" according to Ruiting Lan, from the school of biotechnology and biomolecular sciences. More than 13,000 whooping cough cases were diagnosed in 2011 - an all-time high. An acellular vaccine - introduced in Australia in 1997 after concerns about side-effects from the previous whole cell version - appeared to have promoted the spread of these variants, Dr Lan said, which overseas authorities had linked to "higher virulence on the basis of hospitalisation and case mortality data". He warned that other countries using similar vaccines should be alert for shifts in genetic features detected in the whooping cough bug.