U.S. prepares for possible swine flu epidemic as global cases riseStory Highlights

CNN) -- The United States stepped up preparations for a possible swine flu epidemic, and Canada confirmed its first cases on Sunday as researchers worked to determine how contagious the virus could be.Homeland Security Secretary Janet Napolitano briefs reporters at the White House Sunday. 1 of 3more photos » var CNN_ArticleChanger = new CNN_imageChanger('cnnImgChngr','/2009/HEALTH/04/26/swine.flu/imgChng/p1-0.init.exclude.html',2,1);//CNN.imageChanger.load('cnnImgChngr','imgChng/p1-0.exclude.html');Keiji Fukuda, the assistant director-general of the World Health Organization, called the outbreak "serious" on Sunday. Researchers are still trying to determine how easily the virus is transmitted person to person and it's too early to predict whether there will be a mild or serious pandemic, said Fukuda.Dr. Alejandro Macias, an epidemiologist and adviser to the federal Ministry of Health, told CNN en Español that the official death toll was at 86. Macias said five more deaths in Mexico City had been reported to federal authorities by local authorities in the city.Earlier, Mexican President Felipe Calderon said 81 deaths were suspected to be from the outbreak and 374 people remained hospitalized. Another 929 people have been examined and sent home, he said.