WHO maintains 2 bln estimate for likely H1N1 cases

By Jonathan Lynn

GENEVA (Reuters) - The World Health Organisation stuck on Tuesday to its statement that about two billion people could catch H1N1 influenza by the time the flu pandemic ends.

But the estimate comes with a big health warning: no one knows how many people so far have caught the new strain, known as swine flu, and the final number will never be known as many cases are so mild they may go unnoticed.

"By the end of a pandemic, anywhere between 15-45 percent of a population will have been infected by the new pandemic virus," WHO spokeswoman Aphaluck Bhatiasevi said in a statement.

"Thirty percent is a midpoint estimate and 30 percent of the world's population is 2 billion."

But she added: "We must remember however, that attempts to estimate infection rates can only be very rough."

Early in the outbreak, which was first detected in April, Dr Keiji Fukuda, acting Assistant Director-General of the U.N. agency, fuelled accusations the WHO was creating panic about the disease when he used the two billion figure.

But the WHO, which raised its global flu alert to the highest level on June 11, declaring a worldwide pandemic, has since said the strain is already spreading much faster than previous flu pandemics.

At the same time, because most victims suffer only mild symptoms, it has told countries they no longer need to try to report each case, but concentrate on monitoring suspicious concentrations of the disease and tracking deaths.