The Sydney flu, which forced more than 45,000 people to take to bed over Christmas, was detected in a handful of cases last winter and incorporated into the flu vaccine that was given to the elderly and vulnerable in the autumn.
Dr Douglas Fleming, of the flu monitoring unit of the Royal College of General Practitioners, said: "If it wasn't in the vaccine strain, the outbreak of flu now would most certainly have been worse. But because we have good surveillance in place, it was picked up last winter."
His unit reported 93 cases of flu for every 100,000 people for the week to 27 December. The latest figures from the Public Health Laboratory Service show 103 cases per 100,000 in northern and central England with only 41 in the South. Fewer than 50 cases per 100,000 is commonplace in winter, between 50 and 200 is usual, and more than 200 is above average. Above 400 is "exceptional" and regarded as an epidemic.
Dr Fleming said he expected to see the figures rise significantly as the Sydney flu and more cases of the Peking flu, which caused problems last winter, hit the South.
"We have seen it rise in the North and central regions, but up to now not in the South. I cannot think that's a situation that can continue. I expect a sharp increase in the South," he added.
Stephen Thornton, of the NHS Confederation representing health authorities and trusts, said: "Every year, this is the busiest week of the health service year . Everybody puts off their illness and then rushes to the doctor after the holiday period."Reuse content