Twenty percent of the employees at acute care hospitals in Massachusetts rejected an offer for a flu vaccination last year, even though an annual vaccine is considered the most effective way of preventing people from getting and spreading the flu, according to state public health officials. With this year's flu season starting, the state Department of Public Health released a report this month that said 71% of hospital workers across the state during the 2010-2011 season received the flu vaccine and 20% turned it down. Vaccination could protect patients and reduce absenteeism among workers, the report said. "Patient safety should not be optional and vaccination compliance shouldn't be optional either," stated Lynn Nicholas, president of the Massachusetts Hospital Association, which supports mandatory flu vaccines for all hospital employees, something that is not currently required.