Remember the swine flu pandemic? It was the disease that swept the world last year, sparking a global health emergency and costing the UK alone at least £1bn.
Plans were made to cope with up to 65,000 deaths in Britain after the virus emerged in Mexico in April 2009 and began its journey round the world. The Government ordered 132 million doses of vaccine – two for every person in the country – and launched a National Flu Pandemic Service to hand out millions of courses of Tamiflu to ward off the killer threat.
Now experts say pandemic flu ended up saving lives rather than costing them – by driving out seasonal flu, which targets the more vulnerable elderly. Official figures published last week show there was a dramatic drop in deaths last winter, the coldest for 30 years. Excess winter mortality was 30 per cent lower in 2009-10 than in the milder winter of 2008-09.
The Office for National Statistics, which published the figures, said the chief reason for the low death rate was the low level of flu.
Last year's pandemic –officially called "H1N1 2009 influenza" but better known as swine flu – targeted the under 50s whose relative youth made them less vulnerable and thus less likely to die. Older people over 50 had immunity to swine flu from exposure to similar H1N1 strains decades earlier
The swine flu pandemic caused 474 deaths up to last April, mainly among younger people, pregnant women and children, compared to the 2,000 to 4,000 mainly elderly people estimated to die from seasonal flu in a mild flu winter. In a severe epidemic of seasonal flu, such as occurred in 1999-2000, the disease caused more than 20,000 deaths.
Experts say that overall deaths from flu last year were among the lowest on record – thanks to the pandemic H1N1 strain driving out the H3N2 seasonal strain, which was more lethal to the elderly.
John Oxford, director of Retroscreen Virology, London, said: "The swine flu virus was the fittest virus last year in the Darwinian sense. It dominated the whole scene and pushed out influenza viruses that would normally kill the elderly. So the elderly couldn't be killed by the normal epidemic viruses and they couldn't be killed by swine flu as they had some immunity. So the paradox is that in the first year of the pandemic there are fewer deaths than normal."
Angus Nicoll, head of influenza programmes at the European Centre for Disease Prevention and Control, said: "As you would expect in a pandemic the new H1N1 virus dominated during the first year. It pushed out H3N2. In the short term all the flu transmission that took place was H1N1."
But Douglas Fleming, of the Royal College of General Practitioners Birmingham flu monitoring unit denied the theory and said it was likely that last winter would have seen low levels of seasonal flu in any case.
"In most years one virus dominates. The low death rate among the elderly last winter was due to the prevalent [pandemic] strain and the immunity held by the population. But the idea that one sort of flu drives out another is a bit of a myth. If you go back over the last 20 years you often get two viruses circulating together. The H3N2 strain has had a long course over 30 years and has gradually exhausted itself," Dr Fleming said.
Swine flu in Britain
65,000 Number of deaths planned for
132m Vaccine doses ordered by the Government
28m Courses of antiviral drugs (Tamiflu, Relenza) were stockpiled in the UK
£1.2bn Cost of preventative measures
494 Actual deaths from the virus (to April 2010)
2-4,000 Estimated annual deaths from seasonal winter flu (in a mild year)