Some people who have spent months suffering from long-haul covid-19 are taking to social media to report their delight at seeing their symptoms disappear after their vaccinations, leaving experts chasing yet another puzzling clinical development surrounding the disease caused by the coronavirus. “The only thing that we can safely assume is that an unknown proportion of people who acquire SARS-CoV-2 have long-term symptoms,” said Steven Deeks, an infectious-disease physician at the University of California at San Francisco. “We know the questions. We have no answers. Hard stop.”