The Food and Drug Administration on Thursday authorized an extension of the expiration date of Johnson & Johnson’s coronavirus vaccine, the company said in a statement, expanding the shelf life by six weeks shortly before millions of doses were set to possibly go to waste. “The decision is based on data from ongoing stability assessment studies, which have demonstrated that the vaccine is stable at 4.5 months when refrigerated at temperatures of 36 – 46 degrees Fahrenheit,” Johnson & Johnson said in a statement. The move gives states extra time to figure out how to use up supply of the single-dose vaccine, even as local officials have struggled to use up stockpiles of the shot, which has lately faced sagging demand. Since it was authorized by the F.D.A. in late February, it has been a critical resource in reaching more isolated communities and people who prefer to receive just one shot.