The threat hasn't abated: The World Health Organization Tuesday gave the green light to drug makers to start manufacturing vaccines for the H1N1 flu, which will go to doctors, nurses, and other technicians first. Calling the illness "unstoppable," the WHO is pressuring all countries to administer vaccinations—as well as to continue vaccinating against seasonal flu, which kills anywhere between 250,000 and 500,000 people globally per year. Countries are asked to decide who gets priority after doctors, but new studies may help make the decision as the virus is found to affect the younger population and the severely obese. About nine out of 10 patients treated for the flu in an intensive care unit were obese, and had unusual symptoms such as blood clots in their lungs, making obesity a risk factor for severe reaction to the strain. Another study found that the blood of people born before 1920 carries the antibodies needed to fight off the 1918 flu strain (of the 1918 pandemic), adding a level of immunity to the swine flu, a distant cousin.