Measles is also one of the most contagious viruses around. On average, one person who has measles can infect 12-18 susceptible people who are exposed to them. By comparison, one person who is infected with SARS-CoV-2 (the virus that causes COVID-19) infects 2-3 people and a case of influenza infects 1-2 people. Epidemiologists refer to this as the basic reproductive number.
However, according to the World Health Organization, “herd immunity against measles requires about 95% of a population to be vaccinated.” The Ann Arbor community is a highly vaccinated area. For example, of the undergraduate students living on campus in Ann Arbor in fall 2024, 96.7% are fully vaccinated against measles.