Recent studies have shown that the effectiveness of Covid-19 vaccines is decreasing, though experts say the shots still work well. WSJ explains what the numbers mean and why they don’t tell the full story. Photo illustration: Jacob Reynolds/WSJ WSJ Explains…

UPDATE: CDC guidance on reported rare blood clots: Dr. Dean Blumberg, Chief of Pediatric Infectious Diseases at UC Davis Children’s Hospital, explains how the new Johnson and Johnson COVID-19 vaccine works and answers common questions, including why it’s different from…