Three large studies, published on Friday by the Centers for Disease Control and Prevention, also highlighted the effectiveness of the shots at preventing infection and hospitalizations with the virus. The research underscored a deep conviction among scientists that vaccine hesitancy and refusal have prolonged the pandemic. The administration’s new plan should stem the flood of infections and return the country to some semblance of normalcy in the long term, several experts said in interviews. “It’s going to fundamentally shift the arc of the current surge,” said Dr. Ashish Jha, dean of Brown University School of Public Health. “It’s exactly what’s needed at this moment.” The new data also may help bolster confidence in the nation’s vaccines, which has eroded amid unexpected reports of breakthrough infections. One of the studies looked at more than 600,000 virus infections in 13 states, representing about one quarter of the U.S. population, between April and July. The researchers concluded that Americans who were not fully vaccinated were far more susceptible to infections, illness and death from the virus.