Winter flu in retreat as number of cases falls to record low

Flu is on the wane. The infection that has wreaked havoc every winter for generations appears to be losing its grip. The level of flu dropped to its lowest point for 35 years last winter and so far this winter it is running at low levels more usually associated with summer months, the Public Health Laboratory Service (PHLS) says.

The record low last year is the culmination of three decades of a steady decline in the impact of flu, punctuated by the occasional epidemic. The reason for the drop is thought to be less crowding and less pollution, causing fewer respiratory infections which provide less opportunity for the flu virus to spread. Flu vaccination, offered to the over-65s, may also have played a part.

A report on last winter's flu season, from October 2001 to May 2002, on the PHLS website, says the number of people infected with "influenza-like illness" each week never rose above the baseline level of 50 per 100,000 population. "This is the first reported influenza season since 1966 where the baseline level has not been breached."

Latest figures for this season, up to 27 December, show the level of flu is running at 20.5 per 100,000 population. The figures are based on the number of people consulting their GPs with a new infection of "influenza-like illness".

But the PHLS warns that the danger of a new flu virus which could cause much more serious and widespread illness remains. A new strain, influenza A (H1N2), was identified last winter but it proved to be harmless compared with those which triggered the three pandemics of the last century, which claimed millions of lives.

The report says: "The emergence of this new influenza subtype illustrates the high mutability of the influenza virus and emphasises the continued need for virological surveillance."

Douglas Fleming, head of the Royal College of General Practitioners' flu-monitoring system, which gathers the figures on flu consultations, said the 30-year decline was probably due to changes that had produced a "more congenial" environment. Less crowding and smaller families meant people were "not breathing other people's air as much". Less pollution meant lower rates of bronchitis and similar illnesses. The result was less chance for the flu virus to grow and be transmitted.

"The rates of all respiratory infections are falling and that is linked with environmental changes. There is infinitely less smog. Car pollution has certain problems but I don't think it is important in respiratory infections. There is far less smoking, and passive smoking is a factor in respiratory infections."

Dr Fleming, who has monitored trends in flu and other illnesses treated by GPs since the 1960s and has submitted a paper to the British Medical Journal on his latest findings, said the decline in respiratory infections could explain the falling incidence of asthma attacks, down by a third since 1993. "Outside the hay-fever season, most asthma attacks are triggered by a cold or flu," he said.

Despite the low level of flu this season, a surge in cases is expected after Christmas, because socialising over the holiday increases infection rates. But Dr Fleming said. "You rarely get really big flu outbreaks after Christmas."