The new A-H1N1 swine flu virus continues to spread worldwide.  The World Health Organization (WHO) says the virus has sickened nearly 28,000 people in 74 countries and has killed at least 141. Most of the deaths have been in Mexico, but the United States is reporting more than half of all cases worldwide. Meanwhile, U.S. health officials are trying to calibrate their response to swine flu.

In Washington, D.C. government workers are closely watching and planning for a potential pandemic.  At one of the command centers they work around the clock collecting data, passing it along to other agencies, and formulating contingency plans to meet various possible scenarios if the situation worsens.   

Employees at the Department of Health and Human Services Strategy Center in Washington have been coordinating the federal response to swine flu since the government declared a public health emergency in April. Since then, the A-H1N1 virus has spread to every U.S. state, sickening hundreds of people.

President Obama says sharing information is critical to staying on top of the situation. "We are essentially ensuring that in the worst case scenario we can manage this (the swine flu) appropriately, government working with businesses and individuals, the private sector and containing an outbreak," said the president.

Tom Sizemore manages the day-to-day operations at the Strategy Center. He says they plan for all types of public health emergencies but they didn't anticipate a flu outbreak originating in Mexico. "Although the playbook said this would start overseas it didn't. But the plans help us determine what we will do next no matter what happens." 

To slow the spread of the epidemic, several communities have closed schools and told workers to stay home if they develop flu-like symptoms.  The government also imposed restrictions on travel to Mexico, which have since been lifted. Health care workers are also focusing on rapidly collecting and verifying new cases while tracking outbreak history. 

But critics say the measures are an overreaction, since the current strain of swine flu may be less lethal than earlier feared.  Sizemore disagrees. He says an aggressive response is needed to limit deaths. "Nobody wants to go to bed at night and say I could have done A, B and C and didn't because someone was going to criticize me for doing it ? and now people are dying."

World health officials are alarmed because humans have no apparent immunity to the new virus, a combination of avian, swine and human flu. With the numbers of cases on the rise, the head of the World Health Organization now says the swine flu outbreak appears to have reached pandemic proportions.

Back at the HHS command center, Sizemore says his colleagues are watching the spread of the virus in Chile, Brazil, and Peru. It is winter in those countries, a time when seasonal flu normally peaks. "As their winter picks up, they now have the seasonal influenza plus the H1N1,? says Sizemore. ?There exists the possibility that the H1N1 could mutate and become more severe. If that happens we want to watch that."

Meanwhile, researchers are rushing to develop a vaccine ? a process that could take five to six months. Once it's ready, the World Health Organization says various countries will decide who will get the vaccine first.