Some of the world’s leading vaccine makers are developing bird flu shots to prepare for possible outbreaks of the sickness in humans. Bird flu, or avian flu, is officially known as H5N1. The virus usually spreads among different kinds of poultry. Human infections remain very rare. But most human cases have been linked to direct contact with infected birds. Now, several large vaccine developers are working on bird flu vaccines to prepare for the possibility that the virus could one day jump to humans. One current outbreak involves an avian flu version that has already killed record numbers of birds and some other infected animals. Officials at three major vaccine manufacturers told Reuters news agency they are already developing and preparing to test vaccines to protect against new bird flu variants. The companies include Britain’s GSK and CSL Seqirus and U.S.-based Moderna. Other vaccine makers told Reuters they “stand ready” to begin production efforts for bird flu if necessary. In addition, there has been a push among companies to develop a vaccine to be given directly to poultry. Such a market could be much larger than the one for humans. Vaccine company officials told Reuters that hundreds of millions of bird flu shots could be manufactured within months if a new variant of the virus starts spreading among humans.