Global health chiefs warned against the "sneaky" swine flu virus but said a vaccine could be ready as early as June as Russia reported its first confirmed case of the disease.
"This is a subtle, sneaky virus, it does not announce its presence or arrival in a new country with sudden explosion of patients seeking medical care or requiring hospitalisation," said Margaret Chan, head of the World Health Organisation (WHO).
She warned that "countries especially in the developing world, where populations are most vulnerable, should prepare to see more than the present small number of severe cases."
WHO The spread in impoverished nations was one of the signals that the WHO was keeping an eye on before it declares a pandemic, instead of simply relying on geographical spread under its influenza rulebook.
But in some good news amid rising cases around the world, the WHO said the first vaccine against the A(H1N1) virus could be ready by the end of next month.
"We're hopeful that by the end of June, by the beginning July, this will be the time that commercial companies will be in a position of being able to make a vaccine," said WHO's interim Assistant Director General Keiji Fukuda.
The WHO's latest figures put the overall number of infections worldwide at more than 11,100, including 86 deaths.
But Fukuda added the daily count of the rising number of cases around the world was becoming irrelevant.
"The numbers themselves have become a little bit more irrelevant," interim assistant director general Keiji Fukuda told journalists. "They will increasingly not reflect what's going on.
"Countries such as the United States are moving away from large-scale testing of cases," he said.
Fukuda added though that experts were still mulling whether to give the go-ahead for production as this may reduce or halt the manufacture of vaccines for seasonal flu.
A study released Friday said the various A(H1N1) swine flu strains spreading across the globe react to antibodies in the same way, boosting the chances of a common vaccine for all of them.
The study, published in Science, says the virus has probably been circulating unnoticed in pig populations for some time, and it calls for more careful monitoring of swine populations.
It confirms that the new pathogen originated with pigs, and is a mix of a previously known virus that already contained avian, swine and human genetic segments with two other genes from Eurasian swine viruses never before detected outside Asia.
Understanding the origins of the novel A(H1N1) virus could help scientists prevent the pathogen from emerging in a new, and potentially more virulent form, the researchers said.
"These findings are critically important for our global public health," said Nancy Cox, chief of the Influenza Division of the US National Center for Immunization and Respiratory Diseases in Atlanta, Georgia and a co-author of the study.
Moscow Friday recorded its first case of the virus, as 11 Spanish soldiers were also diagnosed with the disease.
A Russian national on a flight from the United States was diagnosed with the A(H1N1) virus, the country's public health chief Gennady Onishchenko said.
"He feels normal and is receiving adequate medical care," Interfax quoted Onishchenko as saying. "He does not have a temperature. His condition is stable."
In Spain, 11 soldiers at a military school were diagnosed with swine flu, although their symptoms are mild, while in Rome two high schools were closed after four students back from the United States fell ill.
The United States said meanwhile it will invest one billion dollars to develop key components for a swine flu vaccine and conduct clinical studies into its efficacy.
US Health Secretary Kathleen Sebelius said the money will come from already existing funds and will be used to place orders with companies licensed to produce flu vaccine for antigens, the ingredient in a vaccine that causes the body to develop antibodies and adjuvants, which boost the body's immune response.