Johnson & Johnson’s COVID-19 vaccine will remain in limbo for a while longer after government health advisers declared Wednesday that they need more evidence to decide if a handful of unusual blood clots were linked to the shot — and if so, how big the risk really is. The reports are exceedingly rare — six cases out of more than 7 million U.S. inoculations with the one-dose vaccine. … At an emergency meeting, advisers to the CDC wrestled with the fact that the U.S. has enough alternative shots to vaccinate its population but other countries anxiously awaiting the one-and-done vaccine may not.