The United Nations agency, which declared an influenza pandemic on June 11, revised its requirements so that national health authorities need only report clusters of severe cases or deaths caused by the new virus or unusual clinical patterns.
"The 2009 influenza pandemic has spread internationally with unprecedented speed. In past pandemics, influenza viruses have needed more than six months to spread as widely as the new H1N1 virus has spread in less than six weeks," it said in a statement on the new strain, commonly known as swine flu.
It has become nearly impossible for health authorities and laboratories to keep count of individual cases -- which have mostly been mild -- as the virus spreads, according to the 193 member-state agency.
The new flu strain can be treated by antivirals such as Roche Holding's Tamiflu or GlaxoSmithKline's Relenza, but many patients recover without medical treatment.
Flu experts say at least a million people are infected in the United States alone, and the WHO says the pandemic is unstoppable.
"It is very much agreed that trying to register and report every single case is a huge waste of resources," WHO spokesman Gregory Hartl said.
Such tracking has limited authorities' capacity to investigate serious cases and is no longer essential to monitor the level or nature of the risk posed by the virus, WHO said.
However, all countries should still closely monitor unusual clusters of severe or fatal infections from the pandemic virus, clusters of respiratory illness requiring hospitalization or unexplained or unusual clinical patterns.
"Signals to be vigilant for include spikes in rates of absenteeism from schools or workplaces, or a more severe disease pattern, as suggested by, for example, a surge in emergency department visits," it said.
Britain reported on Thursday that 29 people had died to date after contracting the virus. Health Minister Andy Burnham said this month the government was projecting more than 100,000 new cases a day of the flu in the country by the end of August.
The WHO will no longer issue global tables showing the numbers of confirmed cases for all countries -- which stood at 94,512 cases with 429 deaths as of its last update on July 6.
Instead, it will issue regular updates on the situation in newly affected countries, which should report the first confirmed cases, weekly figures and epidemiological details.
Countries should still test a limited number of virus samples weekly to confirm that disease is actually due to the pandemic virus and to monitor any virological changes that may be important for the development of vaccines, it said.
At least 50 governments have placed orders for vaccines against the new H1N1 strain or negotiating with drug makers, WHO vaccine chief Marie-Paule Kieny told Reuters.
The WHO does not report figures for cases of seasonal influenza, which it says is linked to 250,000 to 500,000 deaths a year globally.
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