Katherine "Kate" L. O'Brien (born 1963) is a Canadian American pediatric infectious disease physician, epidemiologist, and vaccinologist who specializes in the areas of pneumococcal epidemiology, pneumococcal vaccine trials and impact studies, and surveillance for pneumococcal disease. She is also known as an expert in infectious diseases (including pneumonia and diarrheal diseases) in American Indian populations. O’Brien is currently the Director of the World Health Organization's Department of Immunization, Vaccines and Biologicals.