Countries are warned to expect further cases and deaths as the spread of pandemic H1N1 swine flu virus speeds up, and there will come a point when case numbers will seem to explode, a World Health Organization director said on Friday.

Shin Young-soo, Director of the WHO Western Pacific region, told an assembly of health delegates meeting in Beijing that we can be certain there will be more cases and more deaths, reported the Associated Press, who quoted the director as saying:

"At a certain point, there will seem to be an explosion in case numbers."

Shin said governments must prepare their health systems, educate the public and protect the most vulnerable and they must act quickly because "we only have a short time period" to ensure that communities are made aware of how to reduce spread and how to get early treatment for severe cases.

He said developing countries are under the greatest threat because they don't have enough resources.

According to the latest WHO figures, as of 13 August, nearly 1,800 people have died from the 2009 swine flu, which reached pandemic status in June this year.

Individual cases are no longer tracked, so the total number of lab-confirmed infections, which has reached over 182,000 worldwide is likely to be a gross underestimate, said the WHO.

The Americas have reported 105,882 cases and 1,579 deaths; Europe 32,000 cases and 53 deaths; Western Pacific 27,111 cases and 50 deaths; South East Asia 13,172 cases and 106 deaths; Eastern Mediterranean 2,532 cases and 8 deaths; and Africa 1,469 cases and 3 deaths.

The WHO predicts that nearly one third of the world's population will be infected over the next two years, that is around 2 billion people.

The vast majority of cases are expected to be mild, but because of the large numbers involved, where cases are severe they are likely to overwhelm hospitals and health infrastructures, especially in poorer nations.

Experts are watching the pattern that the virus is following in the southern hemisphere in the hope of being able to anticipate how it might spread in the northern hemisphere when the flu season starts.

Unlike seasonal flu, where cases drop significantly during the summer and then resurge in the winter, the novel H1N1 swine flu has proved more resilient and is still spreading in the northern hemisphere.

However, northern countries are gearing up for a significant resurge as students and workers return from summer vacations. Vaccine makers are fast tracking production in an effort to get people vaccinated in time. Some are saying the first batches will be ready in October, but it is not clear how many doses will be available.

WHO Director-General Dr Margaret Chan said earlier this year that pregnant women and people with underlying medical conditions are two of the groups at higher risk of severe infection.

One study, by researchers in the US and Japan, has suggested that the novel H1N1 pandemic strain is more virulent than previously thought. It found that the virus infects cells deep inside the lungs, which can lead to pneumonia and in more severe cases, death, whereas seasonal flu viruses tend only to infect cells in the upper respiratory tract.

They found that the new pandemic swine flu strain spreads much more efficiently in the respiratory system than the seasonal flu virus, causing severe lesions in the lungs, more like the damage caused by other pandemic strains.

WHO and other experts have warned that the virus might also change quite suddenly, as Chan explained in June, when she announced the virus had reached pandemic status:

"The virus writes the rules and this one, like all influenza viruses, can change the rules, without rhyme or reason, at any time."

The last flu pandemic was the Hong Kong flu which globally killed around 1 million people in 1968 and 1969. It was the first known outbreak of the H3N2 strain. Seasonal flu kills around 250,000 to 500,000 people every year worldwide.

Sources: Associated Press, WHO, MNT archives.

Written by: Catharine Paddock, PhD