Is herd immunity a realistic concept? Fauci calls it ‘elusive’ and ‘mystical’ Posted on Of all the moving goal posts in the COVID-19 pandemic, herd immunity may be the most difficult to pin down — and it may even be the wrong metric to track. Herd immunity is the concept that once a majority of the population has been vaccinated or has natural immunity to an infectious disease, in this case SARS-CoV-2, life will shift back to normal. But to listen to Dr. Anthony Fauci, President Joe Biden’s chief medical adviser and the head of the National Institute of Allergy and Infectious Diseases, is to hear him downplay the focus on this “elusive” concept and instead urge Americans to pay attention to other figures like vaccination rates.