Happily, I seem to be back to normal. But I was uneasy when I got my second vaccination shot three weeks ago, worried about how my body would respond. I sobbed as the nurse stabbed me with a syringe; the next day I curled up in a ball on my bed, overwhelmed with chills and fever. Researchers suggest that the vaccine may help the immune system fight off any lingering residual virus. But the truth is there is still so much we don’t know about Covid. This month a study tracking the health insurance records of nearly two million people in the United States who contracted the coronavirus last year found that almost one-quarter of them — 23 percent — sought medical treatment for new conditions, including nerve and muscle pain, high cholesterol, high blood pressure and fatigue. People of all ages were affected, including children, and problems occurred even among those people who showed no symptoms from the virus. Doctors are only beginning to study the virus’s long-term effects. In February, the National Institutes of Health announced a $1.15 billion initiative to identify the causes of long Covid, as well as protocols to prevent and treat individuals whose symptoms persist. Dr. Francis S. Collins, director of the N.I.H., said then that given the number of individuals who had been infected, “the public health impact could be profound.”