Johnson & Johnson said on Thursday the US Food and Drug Administration has extended the shelf life of its single-shot Covid-19 vaccine from 3 months to 4.5 months, as millions of unused doses nationwide are set to expire this month.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 to 46 degrees Fahrenheit, or 2 to 8 degrees Celsius, according to the drugmaker, Xinhua reported."Evidence from our Phase 3 ENSEMBLE study demonstrates the efficacy of our single-shot COVID-19 vaccine, including against viral variants that are highly prevalent. Regardless of race and ethnicity, age, geographic location and comorbidities, these results remain consistent," the company said in a statement.Of the 21.4 million Johnson & Johnson doses distributed nationwide as of Wednesday, only 11.2 million doses have been administered, according to data of the US Centers for Disease Control and Prevention. It means half of the doses produced for the United States sitting unused.