According to the WHO, pharmaceutical companies plan to produce approximately 480 million doses of seasonal vaccine in 2009 - each one containing three separate vaccines against the three most common circulating flu strains - 430 million doses would be available by July 31st.

The U.S. Centers for Disease Control and Prevention (CDC) says the number of confirmed cases of the new flu and are 'the tip of the iceberg' and estimates suggest there were now 100,000 cases.

The CDC's Dr Anne Schuchat said she was worried the new strain may mix with seasonal flu strains, which are still circulating - H1N1 strain is now reportedly resistant to the most commonly used antiviral Tamiflu but the inhaled drug Relenza was still effective.

Dr Schuchat says there is the risk that the circulation of the new virus together with the seasonal strains, will allow a re-assortment event where two viruses meet and swap entire stretches of their genetic material - this is a particular feature of flu viruses and some pandemics have emerged because of such genetic mixing.

In update # 33 from the WHO on the influenza A(H1N1) situation, as of the 19th May, 40 countries have officially reported 9,830 human cases and there have been 79 deaths but in many of the fatal cases those affected had underlying conditions that put them at risk or had serious medical conditions.

Worst affected countries to date are Mexico with 3,734 cases including 74 deaths, the U.S. with 5,123 cases and 5 deaths, Canada with 496 cases and 1 death and Japan with 159 cases and no deaths.

Next are Spain 103, UK 102, Panama 59, France and Germany 14 each, Colombia 11, Costa Rica, New Zealand and Italy 9, Brazil 8, China and Israel 7, El Salvador 6, Belgium 5, Chile 4, Cuba 3, Guatemala, Republic of Korea, Netherlands and Sweden 3,Finland, Malaysia, Norway, Peru, Thailand and Turkey 2 each and Argentina, Australia , Austria, Denmark, Ecuador ,India, Ireland, Poland ,Portugal and Switzerland all have had 1 case each.