-LRB- CNN -RRB- -- The World Health Organization raised the swine flu alert Thursday to its highest level , saying the H1N1 virus has spread to enough countries to be considered a global pandemic . Kindergarten students , some wearing masks , attend school in a residential estate in Hong Kong on Thursday . Increasing the alert to Phase 6 does not mean that the disease is deadlier or more dangerous than before , just that it has spread to more countries , the WHO said . `` This is an important and challenging day for all of us , '' WHO Director General Margaret Chan said in a briefing with reporters . `` We are moving into the early days of the first flu pandemic of the 21st century . '' The last previous pandemic occurred in 1968 . As of Thursday , the virus had spread to 74 countries , the health agency said . There were 28,774 confirmed cases and 144 deaths . The United States had 13,217 cases and 27 deaths , the Centers for Disease Control and Prevention said June 5 in its weekly update . Cases have been reported in all 50 states , the District of Columbia and Puerto Rico . See where H1N1 flu cases are in the U.S. '' The U.S. death toll is expected be higher when the CDC releases its latest figures Friday , said Anne Schuchat , director of the National Center for Immunization and Respiratory Diseases . New England -- particularly Massachusetts -- and the New York and New Jersey areas have been hit the hardest , Schuchat said Thursday at a CDC news conference . The Phase 6 pandemic designation had been widely expected for weeks . `` Further spread is considered inevitable , '' Chan said at a news conference at WHO headquarters in Geneva , Switzerland . `` The scientific criteria for an influenza pandemic have been met . '' The announcement came after a meeting of the WHO 's Emergency Committee , which has debated since April whether the spread of a novel H1N1 flu virus was fast and widespread enough to warrant a Phase 6 designation . Phase 6 , Chan said , is meant as a signal to countries to recalibrate their strategies to minimize the harm from swine flu . In countries where the virus and the response to it are already widespread , it is not likely to mean significant changes , but Chan urged countries that have not seen cases , or seen only limited cases , to get ready . `` The virus is not stoppable , '' she said . `` I would advise them to maintain vigilance , enhance surveillance and be prepared for the arrival of the novel H1N1 in their country . '' The U.S. Department of Health and Human Services and the Department of Homeland Security said those agencies have been acting for weeks as if Phase 6 were already in effect and no additional measures would be put into effect . The United States declared a public health emergency April 26 . `` WHO continues to recommend no restriction on travel and on border closures , '' Chan said . Discussions about shifting to Phase 6 have been under way for weeks . Chan indicated that a major factor in the decision was surveillance from countries in the Southern Hemisphere , where flu season is under way . In Chile and Australia , two countries with many flu cases , she said H1N1 appears to be the dominant strain , `` crowding out '' the seasonal influenza virus . Chan said she would recommend that vaccine manufacturers proceed with mass production of an inoculation against the new swine flu strain as soon as they finish production of seasonal vaccine , which she estimated would be complete in about two weeks . HHS spokesman Bill Hall said Thursday that no decision has been made on full-scale production because there is no need yet to make that decision . It is a step-by-step process that is moving forward . `` We 're doing as much as we can now , '' he said . The agency awarded a contract several weeks ago to five manufacturers to develop an H1N1 antigen . The companies are producing pilot lots . Clinical testing will take place over the summer , Hall said . At the end of the summer , HHS will decide whether to go into production . `` There 's no specific date on the calendar , '' Hall said . If officials decide to produce a vaccine , the first doses would be ready early in the fall . Hall noted that even full-scale production does n't mean there will be an immunization campaign . Federal officials may have a vaccine and decide not to use it , he said . Also Thursday , authorities in Hong Kong ordered the closure of all elementary schools , kindergartens and day care centers in the city after 12 students were found to be infected with the virus . Authorities have not determined the source of the infection , said Hong Kong 's Chief Executive Donald Tsang . This makes it the first cluster of swine flu cases in the city without a link to someone who had traveled overseas . The schools and day care centers were told to close for 14 days as investigators tried to identify the source of the infection , said Tsang , the chief executive . The health department will decide after two weeks whether to continue the shutdown . Also on Thursday , Israel 's health ministry announced that the number of people diagnosed with swine flu there was 68 . Health officials use the virus ' clinical name -- H1N1 -- to reflect that it 's actually a combination of several different types of flu and to reduce confusion about whether eating pork can spread the virus . It can not . CNN 's David S. Martin , Roya Shadravan in Hong Kong , and Michal Zippori in Jerusalem contributed to this report .