The WHO says the new virus could still mutate and it recommends the use of antivirals only for high-risk groups or the group of people at increased risk, depending on the availability.

Dr. Nikki Shindo from WHO says it is important to keep the drugs working well in case the swine flu becomes more dangerous - there is concern that the swine flu could mix with other viruses, including H5N1 bird flu, and become worse in the coming months.

The United States has had the most confirmed cases affecting 45 states, but only 116 have been hospitalized and Dr. Anne Schuchat of the U.S. Centres for Disease Control and Prevention (CDC) says pregnant women appeared especially vulnerable to H1N1, just as they are to seasonal flu, and should get prompt treatment as pregnant women risk dehydration and premature delivery when they have flu.

The CDC says seasonal influenza kills up to 500,000 people in an average year - the WHO has predicted the new H1N1 virus could eventually infect a third of the world's population.

The WHO says the overall severity of a pandemic is affected by their tendency to encircle the globe in at least two, sometimes three, waves and health officials are especially concerned because seasonal flu viruses are still circulating and could mix with the new H1N1 strain, either in people or in pigs - also the H5N1 avian influenza virus (bird flu) which has killed 258 out of 423 people infected since 2003, is still around.