| The Food and Drug Administration plans to say this week that the single-dose vaccine has been linked to a serious but rare side effect called Guillain-Barré syndrome, according to four people who spoke on the condition of anonymity to describe internal discussions. The incidence is rare — fewer than 100 cases among the 12.8 million people who received the shot in the United States — and health officials plan to emphasize that the benefits of the vaccine outweigh potential risks. | 
