The World Health Organisation has raised the spectre of Hong Kong's deadly bird flu outbreak of 1997 in an advisory urging people to seek vaccinations as the flu season enters its peak. The WHO released its advisory last week amid Hong Kong's third H5N1 outbreak, which has led to the slaughter of 244,000 chickens at seven farms and eight market stalls and an extra one-day market shutdown last Friday. Twenty-five farms remain under quarantine. The WHO advisory said the worst flu outbreak in the last century was the Spanish flu pandemic of 1918-1919, which claimed up to 40 million lives and was thought to have infected half the world's population. 'The possibility that this could happen again was exemplified by the H5N1 episode in Hong Kong in 1997 when one-third of infected patients died. Fortunately this outbreak did not develop into a global health emergency,' the WHO said. That was the first time the H5N1 flu, which used to affect only birds, had jumped the species barrier, making 18 people sick, six of whom died. Two other outbreaks of H5N1 strains have since occurred - in May last year, leading to 1.2 million birds being slaughtered, and last week. Neither led to human flu outbreaks. The WHO's executive director of communicable diseases, Dr David Heymann, said 100 million people a year in the northern hemisphere, including Hong Kong, fell ill from the flu. About 20,000 sufferers died in the US each year. 'The WHO strongly advises early vaccination as the best prevention,' Dr Heymann added. Dr Hitoshi Oshitani, regional adviser on communicable disease surveillance at the WHO Western Pacific region, which includes Hong Kong, said the H5N1 outbreak was confined only to the SAR. The mainland and Japan had reported an increased number of flu-like illnesses since early last month, most of them of the H3N2 strain, he added. Hong Kong Tourism Board chairwoman Selina Chow Liang Shuk-yee, also a Liberal Party legislator, last week said news of any outbreak of bird flu in the SAR adversely affected tourism. 'We don't like this sort of news, these headlines that appear from time to time about avian flu. We see that inevitably this gets caught up and reported elsewhere. 'It's not good for tourism either. But what we hope is we can avoid [another outbreak],' she said.