Will natural herd immunity protect us before a vaccine will? Herd immunity is reached when around 70% to 90% of a population becomes immune to a disease. Dr. Anthony Fauci says letting this virus reach those thresholds would lead to massive death tolls. Dr Tom Frieden (former head of US CDC, said “Herd immunity means 1 million dead Americans. That's what it would take to get to herd immunity. That's not a plan—that's a catastrophe”.