The global spread of swine flu will endanger more lives as it speeds up in coming months and governments must step up preparations for a swift response, the World Health Organization said Friday.

There will soon be a period of further global spread of the virus, and most countries may see swine flu cases double every three to four days for several months until peak transmission is reached, said WHO's Western Pacific director, Shin Young-soo.

"At a certain point, there will seem to be an explosion in case numbers," Shin told a symposium of health officials and experts in Beijing. "It is certain there will be more cases and more deaths."

WHO has declared the swine flu strain a pandemic, and it has killed almost 1,800 people worldwide through last week. International attention has focused on how the pandemic is progressing in Southern Hemisphere countries such as Australia, which are experiencing winter and their flu season.

But it is in developing countries where the accelerated spread of swine flu poses the greatest threat as it places under-equipped and underfunded health systems under severe strain, Shin said.

Governments must act quickly to educate the public, prepare their health systems to care for severe cases and protect those deemed more vulnerable to prevent unnecessary deaths, he said.

"We only have a short time period to reach the state of preparedness deemed necessary," Shin said. "Communities must be aware before a pandemic strikes as to what they can do to reduce the spread of the virus, and how to obtain early treatment of severe cases."

Some at greater risk

Pregnant women face a higher risk of complications, and the virus also has more severe effects on people with underlying medical conditions such as asthma, cardiovascular disease, autoimmune disorders and diabetes, WHO chief Margaret Chan said in a video address.

WHO earlier estimated that as many as two billion people could become infected over the next two years — nearly one-third of the world's population.

Health officials and drugmakers, meanwhile, are looking into ways to speed up production of a vaccine before the Northern Hemisphere enters its flu season in coming months. Estimates for when a vaccine will be available range from September to December.

WHO has stressed that most cases are mild and require no treatment, but the fear is that a rash of new infections could overwhelm hospitals and health authorities, especially in poorer countries.

The last pandemic — the Hong Kong flu of 1968 — killed about one million people. Ordinary flu kills about 250,000 to 500,000 people each year.

Swine flu is also continuing to spread during summer in the Northern Hemisphere. Normally, flu viruses disappear with warm weather, but swine flu is proving to be resilient.