Health officials warned Thursday that more manufacturers of the flu vaccine are needed, if a shortage is to be averted in the future. According to Martin G. Myers, head of the Centers for Disease Control and Prevention's National Vaccine Program Office, because there are now just three manufacturers of the flu vaccine, it is necessary to determine what it would take to interest other companies to start production. Myers, who was speaking at an immunization meeting sponsored by the CDC, also noted that it is still too early to know whether there will be shortages again this year, since the companies' production data is proprietary. Other conference participants added that physicians and manufacturers should work together to better determine who requires the vaccines first.