 Although it was initially thought that two injections would be required , clinical trials showed that the new vaccine protected adults " with only one dose instead of two " , and so the limited vaccine supplies would go twice as far as had been predicted . Health officials worldwide were also concerned because the virus was new and could easily mutate and become more virulent , even though most flu symptoms were mild and lasted only a few days without treatment . Officials also urged communities , businesses and individuals to make contingency plans for possible school closures , multiple employee absences for illness , surges of patients in hospitals and other effects of potentially widespread outbreaks . 
