LA JOLLA, California (AFP) – A strain of swine flu that is resistant to treatment with the drug Tamiflu has been discovered near the US-Mexican border, the Pan-American Health Organization (PAHO) said on Monday.

“We have found resistance to Tamiflu on the border. We have observed some cases, few to be sure, in El Paso and close to McAllen, Texas,” said Maria Teresa Cerqueira, head of the local PAHO office.

Cases of A(H1N1) that were resistant to the anti-viral medicine have now been found in the United States, Canada, Denmark, Hong Kong and Japan.

Experts gathered in La Jolla, California on Monday to discuss the response to the outbreak, said the resistance was likely due to overuse of antivirals like Tamiflu.

“In the United States Tamiflu is sold with a prescription, but in Mexico and Canada it is sold freely and taken at the first sneeze. Then, when it is really needed, it doesn’t work,” said Cerqueira.

Roche, the manufacturer of Tamiflu, has said it expects a 0.5 percent rate of case resistance based on clinical trials.

Cerquiera said one patient diagnosed with a Tamiflu-resistant strain had been treated with Zanamivir — an anti-viral made by GlaxoSmithKline — and another was given no alternative medication. Both survived.

Since the swine flu outbreak emerged earlier this year it has killed 353 people in the United States and 146 in Mexico, according to authorities from each country.

A large proportion of nearly 1,000 recorded deaths around the world since the virus was first identified in Mexico in April have involved people with underlying medical conditions.

World Health Organization chief Margaret Chan has urged countries to boost their drug stockpiles amid warnings of the virus’s continued spread.

“Manufacturing capacity for influenza vaccines is finite and woefully inadequate for a world of 6.8 billion people, nearly all of whom are susceptible to infection by this entirely new and highly contagious virus,” Chan said on July 14.

A vaccine against the disease is currently being tested for safety and effectiveness, and is expected to be ready in the next two to four months, although the precise date is unclear.

US officials have recommended that children and pregnant women be among the groups first in line for swine flu shots, but said they were unlikely to have enough vaccine for everyone once the drug is ready to be rolled out.

Greece has announced it will vaccinate all its 11-million-strong population, the first country to introduce such a broad measure amid the current pandemic.

With such pledges emerging, developing nations have expressed concerns they will be left out, as richer nations race to build stock piles.

UN Secretary General Ban Ki-moon has said developing countries could need about one billion dollars by the end of the year to fund measures against the swine flu pandemic, especially vaccines.