The swine flu pandemic has grown "unstoppable" and all nations will need access to vaccines, a WHO official has said, as 12 new deaths were reported and a study raised fresh concerns.
Britain, Brazil, Colombia, Mexico, the Philippines and Thailand all reported deaths on Monday, while Saudi Arabia shut an international school after 20 students were diagnosed with the A(H1N1) virus.
As the death toll increased, the World Health Organisation official said a swine flu vaccine should be available as early as September and all countries would need to be able to protect themselves.
A group of vaccination experts concluded after a recent meeting that "the H1N1 pandemic is unstoppable and therefore all countries would need to have access to vaccines," said Marie-Paul Kieny, WHO director on vaccine research.
Health workers should be at the top of the list for vaccination since they will be in high demand as people continue to fall sick, she added.
Countries would be free to decide on their national priorities, but other groups should include pregnant women and anyone over six months old who has chronic health problems, the WHO official said.
Particular attention would have to be paid to children since they are considered "amplifiers" of the spread of the virus, especially when gathered in schools, Kieny added.
More than 90,000 swine flu cases have been reported worldwide, including 429 deaths, the most recent WHO numbers from last week show.
While most cases have been considered mild, a study released on Monday said the virus causes more lung damage than ordinary seasonal flu strains but still responds to antiviral drugs.
Virologists tested samples of the virus taken from patients in the United States as well as several seasonal flu viruses on mice, ferrets, macaque monkeys and specially-bred miniature pigs.
They found that A(H1N1) caused more severe lung lesions among mice, ferrets and macaques than the seasonal flu viruses.
But it did not cause any symptoms among the mini-pigs, which could explain why there has been no evidence that pigs in Mexico fell sick with the disease before the outbreak began among humans.
The team also found that the virus was highly sensitive to two approved and two experimental antiviral drugs, including Tamiflu, now being hurriedly stockpiled around the world.
This confirms the drugs' role as a "first line of defence" against the flu pandemic, they said.
The worry about the present strain of A(H1N1) is that it could pick up genes from other flu strains that would enable it to be both highly virulent and contagious, and these warnings are spelt out in the new study.
"Sustained person-to-person transmission might result in the emergence of more pathogenic variants, as observed in the 1918 pandemic virus," it says.
Another concern is that the virus could acquire mutations enabling it to be resistant to Tamiflu.
"Collectively, our findings are a reminder that (strains of swine flu) have not yet garnered a place in history, but may still do so."
Most of the deaths reported on Monday were in Asia, with Thailand reporting three fatalities and the Philippines two.
Thailand's death toll has now reached 21, while the Philippines has three deaths.
Authorities in Britain announced the deaths of a six-year-old girl and a doctor who had contracted swine flu, bringing the number of fatalities there linked to the virus to 17.
Brazil on Monday reported the death of a nine-year-old boy, taking the number of fatalities in the country to three. A young woman died in Colombia and three new deaths were reported in Mexico, taking the toll to 124.
In Europe, Italy's health ministry reported 38 new cases over the course of the last four days, taking its total well over the 200 mark.
The school closure in Riyadh came with officials there especially concerned over swine flu with upwards of two million people expected over the next five months on pilgrimages to the Muslim holy cities of Mecca and Medina.