The World Health Organization 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."

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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, fueled 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.

Autumn Risk

Bhatiasevi earlier told a briefing the WHO was coordinating a network of independent institutions trying to project the total number of cases. Because no one currently has such an estimate, it is not possible to state the H1N1 mortality rate.

The WHO's latest update on July 27 said a total of 816 people had died from H1N1, while the total number of laboratory-confirmed cases, including deaths, was 134,503 — a figure well below the likely real total of infections which may already be in the millions, according to health experts.

As the northern hemisphere autumn approaches, and with it the onset of seasonal flu, the WHO is working with drug companies to ensure vaccines to cope both with H1N1 and seasonal flu will be available.

WHO spokeswoman Fadela Chaib said the agency hoped to give an update on its vaccine plans later this week. Leading flu vaccine makers include Sanofi-Aventis, Novartis, Baxter, GlaxoSmithKline and Solvay.

H1N1 rapidly became the most commonly isolated virus in flu cases in South America and Australia after seasonal flu started there, Bhatiasevi said. But that may be a distorted picture, because specimens for testing were often associated with events such as school closures or screening of travelers where H1N1 was suspected anyway.

In South Africa, where H1N1 arrived later, many cases of the seasonal flu virus H3N2 were reported, but as the flu season wanes and H3N2 cases decrease, H1N1 has begun to make up a much higher proportion of flu cases there.

The WHO has appointed Dr Mohamed Hacen as project manager for H1N1 in Dr Fukuda's influenza team, Bhatiasevi said.

A Mauritanian, Dr Hacen has served as WHO representative in several African countries and has extensive experience in the field and in communicable diseases.