University of Hong Kong researchers have developed a vaccine that may be the first in the world to offer a shield against many influenza viruses - including the deadly H7N9 bird flu strain - in one simple shot. The HKU team described it as a breakthrough for seasonal flu vaccines, which can usually only provide protection against one or a few specific viral subtypes each. The study found the new vaccine helped 80 to 100 per cent of mice fight different strains of flu viruses. "Current influenza vaccines target only specific and individual subtypes, but it is difficult for scientists to predict which types of a virus will cause the next pandemic," the lead researcher, associate professor of public health Dr Leo Poon Lit-man, told the South China Morning Post . "That is the reason why we should study whether there is another way to offer a broader spectrum of protection against different subtypes of viruses." But it might take years before the vaccine is tested in human clinical trials, Poon said. HKU led the study with technical help from the United States' National Cancer Institute. It published the findings in international scientific journal Proceedings of the National Academy of Sciences of the United States of America yesterday. The researchers used the vaccinia virus - the key ingredient in the smallpox vaccine - as a vaccine carrier. The vaccinia virus is known to be able to induce robust protection in mice against the H5 type of influenza A viruses. They tested hundreds of mice infected with various viral strains, including H3N2, H1N1, H7N7 and the most recent to surface among humans, H7N9. Of the mice that were injected with the vaccine, 80 to 100 per cent were protected from subsequent viral attacks. Among those that did not receive a jab, none survived. "An influenza pandemic caused by a novel strain of influenza A virus that is virulent and highly transmissible among humans will be of great global concern," HKU said in a statement. "This vaccine provides a promising strategy for universal protection against new and emerging influenza viruses." The H7N9 strain of bird flu first infected humans in the mainland's Yangtze River Delta and adjoining Anhui province in February last year. By February 28 this year, there had been 375 human cases, 115 of them fatal, according to the World Health Organisation. The development of the HKU vaccine for commercialisation is likely to be outpaced by a US study on an experimental H7N9 vaccine. That prospective vaccine is tipped to be the first to protect humans against the deadly virus. In November, the US vaccine underwent its first human test conducted by biopharmaceutical company Novavax, based in Maryland. The company said results were encouraging.