Following is a factfile on H1N1 influenza, a year after the novel "swine flu" virus first came to public notice on April 24 2009:

HOW MANY PEOPLE HAVE BEEN INFECTED?

The likely answer: hundreds of millions. A more accurate toll will never be known for sure as many people had only mild symptoms and their cases went unreported.

Based on three flu pandemics last century, epidemiologists initially predicted up to 30 percent of the world's population of 6.7 billion could be infected within the first year.

But a likelier estimate is 10-20 percent, says Christophe Fraser, an epidemiologist at Imperial College London. The US Centers For Disease Control and Prevention (CDC) estimated the total number of US cases in mid-January at approximately 57 million, or nearly 20 percent of the population.

Swine flu has been reported in 213 countries or territories, according to the World Health Organisation (WHO).

There is no sign of a surge in cases in the Southern Hemisphere, now in winter, says the European Centre for Disease Prevention and Control (ECDC), adding that the situation in many tropical countries was "unknown and unknowable."

HOW MANY PEOPLE HAVE DIED?

In its first year, H1N1 proved much less virulent than feared, with a mortality roughly equal to that of ordinary "seasonal" flu, although this varies according to location.

As of April 9, the WHO reported more than 17,700 swine flu deaths worldwide confirmed by laboratory tests.

"Seasonal" influenza claims about half a million lives each year. The pandemics of the 20th century killed between one or two million people each in 1957 and 1968, and up to 50 million in 1918-1919.

However, the actual number of swine flu deaths is probably far higher if deaths attributed to underlying conditions or other causes are taken into account.

In the United States alone, up to 17,100 people died from swine-flu related illness as of mid-February, according to the CDC.

WHO HAS BEEN HIT HARDEST?

People aged 65 or older have been less likely than young people to be infected, possibly because they were exposed to an older "H1" strain related to the 2009 virus and carry some degree of immunity to the new strain.

When the elderly do catch H1N1, though, they run a great risk of health complications or death due to a greater prevalence of underlying conditions, such as respiratory problems or heart disease.

Others at risk are the obese; pregnant women; children under two years old; and persons of any age with chronic asthma, lung disease or diabetes.

WHAT HAPPENED TO VACCINATION?

The world's wealthier nations ordered well over a billion doses of H1N1 vaccine by the end of 2009, but only about 300 million people worldwide have been given the swine flu jab so far, according to the European Society of Clinical Microbiology and Infectious Diseases (ESCMID).

Stocks worth hundreds of millions of dollars have gone unused and pending orders for the vaccine have been drastically scaled back.

About 20 percent of US adults and 37 percent of children under 18 had been vaccinated as of mid-February, about a quarter of the general population, according to the CDC.

The H1N1 vaccine has an "excellent" safety record, according to a study presented this month at the American Academy of Neurology. Post-vaccination cases of a neurological disorder known as Guillain-Barre Syndrome were 3.5 reported cases per 10 million, which is considered low.

WHAT DOES THE FUTURE HOLD?

Swine flu is currently in retreat in temperate zones in both hemispheres, although the situation in tropical regions is confused because of a lack of surveillance instrastructure.

It could continue to spread in a relatively mild form, effectively becoming the new seasonal flu, to which existing immunity and vaccines would be a useful shield.

But previous pandemics show that the H1N1 strain could re-emerge next season or even beyond in a newly virulent form, and there is a need to maintain vigilance.

"Everyone thinks this is a post-mortem, but unfortunately this virus is not dead yet. It is on a trajectory, and we don't know where it is going to end up," says John Oxford, a professor of virology at Queen Mary’s School of Medicine and Dentistry in London.

The new strain is a "Darwinian super virus" that has quickly pushed other strains to the sideline, Oxford says.

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