AS A NUMBER of governments begin mass vaccination programs against pandemic influenza, attention is focusing increasingly on a crucial issue for Asia?how to ensure that poor countries in the region get enough vaccine to avoid explosive outbreaks of infection in the weeks ahead.

The World Health Organization?s Western Pacific Region contains many countries with weak health systems and large populations of young people, pregnant women, the malnourished and the unwell?the groups most vulnerable to severe illness from the virus. In addition, many of these underprivileged communities live in crowded, unsanitary conditions, which can be fertile breeding grounds for influenza. WHO fears that if the virus takes a grip in this kind of environment, it may be unstoppable.

With the supplies of vaccine limited, the challenge over the next few weeks is to find ways for wealthy nations to share more of their production with poorer ones, thus helping prevent unnecessary deaths and the crippling social and economic effects of mass absenteeism from work due to illness.

WHO is working with its United Nations partners, donors, development banks and pharmaceutical companies to secure developing countries? access to vaccines and also to antivirals such as Tamiflu. It is also taking steps to facilitate cheaper purchase prices for the vaccine.

These efforts are progressing well, but more vaccine is needed. Nine countries have pledged some of their vaccine supplies to the developing world, while pharmaceutical companies Sanofi Aventis and GlaxoSmithKline have agreed to donate 150 million doses of vaccine to WHO for distribution.

In total, around 200 million doses have been pledged for donation, short of WHO?s initial goal of obtaining 300 million doses for distribution to more than 90 countries. WHO hopes to meet the shortfall with donations from other countries and manufacturers.

WHO recommends that health care workers should get priority access to flu vaccinations to ensure essential health infrastructure keeps functioning even if under stress. WHO has identified other target groups for vaccination, namely: pregnant women; people aged above 6 months with one of several chronic medical conditions; healthy young adults of 15 to 49 years of age; healthy children; healthy adults of 50 to 64 years of age; and healthy adults of 65 years of age and older. Decisions on which of these groups are prioritized for vaccinations, however, are the responsibility of national governments based on local needs and conditions.

Efforts to vaccinate as many people as possible have been boosted by the recent news that one dose of vaccine, rather than two, should be enough to offer immunity. This should effectively double the number of people who can be immunized.

Still, current global production capacity for pandemic vaccines is estimated at around 3 billion doses a year, compared to a world population of 6.8 billion people faced with a highly contagious disease. WHO has estimated that up to 30 percent of people could be infected over time.

Clearly, some difficult choices will have to be made in deciding which countries receive donated vaccine. These judgments will be influenced partly by a country?s readiness to accept and start distributing the vaccine, as the aim is to avoid vaccine languishing in warehouses.

Other considerations will involve assessments of whether the pandemic is becoming more or less severe, which countries are reporting sudden increases in infections, and how the disease is affecting various high-risk groups.

Clinical trials and tests conducted by major manufacturing companies on the vaccine show that it will offer significant protection from the Pandemic (H1N1) 2009 virus. Trials have shown that the new vaccine meets safety benchmarks set by previous influenza vaccines, which have an established record of safety going back 60 years.

The vast majority of people infected with the Pandemic (H1N1) 2009 virus experience mild illness and recover after a short period of convalescence, often without the need for medication. Many people, however, have become seriously ill and more than 4,000 have died.

Flu vaccine contains killed influenza virus, which is injected into muscles and stimulates the immune system to produce antibodies which recognize and fight disease-causing pathogens. When the virus enters a vaccinated person, the antibodies attack and kill the virus and prevent infection. Vaccines are generally given before exposure to prevent future infection.

Dr. Shin Young-soo is the World Health Organization?s regional director for Western Pacific.