Doctors fear influenza is sweeping Hong Kong, with cases up 86 per cent in one month and young children most at risk. Of the 522 sufferers detected last month, 95 per cent were infected with the Sydney A H3N2 strain. Only 56 people were infected with the new strain in December. The trend appears to be continuing this month, with private doctors reporting nearly 42 in every 1,000 consultations involving patients with flu-like symptoms. In January last year, there were only 46 influenza cases detected, and 94 in February - though sample sizes were much smaller. Health authorities stepped up surveillance in December following the outbreak of bird flu. A Department of Health spokesman said the figure could be increasing because of increased awareness among doctors and a larger number of samples. Hong Kong Medical Association representative Dr Lo Wing-lok said children were most vulnerable as they had had less exposure to the H3N2 strain. If a child has contracted the virus, the most common course would be a fever lasting four days up to a week.