As fourth doses of Covid vaccines roll out, some are questioning whether the general population needs them. At the center of this debate are mysterious T-cells. WSJ’s Daniela Hernandez explains T-cells’ role in Covid immunity and how they relate to antibodies. Illustration: Adele Morgan Nearly 60% of people in the U.S., including three in four children, exhibited signs of previous Covid-19 infection as of February, a Centers for Disease Control and Prevention report said. The estimated proportion of people in the U.S. with detectable, infection-induced antibodies jumped from 34% in December 2021 to 58% by February 2022, according to a study the CDC released Tuesday, highlighting the reach of the winter Omicron surge that washed over the country. “We do believe that there is a lot of protection in the community both from vaccination, as well as from boosting and from prior infection,” CDC Director Rochelle Walensky said. “Those […]