OBJECTIVES: Colorado has the lowest rate of measles vaccination in the entire country, partially due to the ease of obtaining non-medical exemptions. Our primary objective was to determine the magnitude and cost of a measles outbreak in Denver at current vaccination coverage levels. We then simulated the effect of changing the Colorado non-medical exemption complexity to medium or difficult. METHODS: An agent based transmission model simulated the transmission of the measles virus in Denver following the introduction of a measles case. We modeled public health response, including contact tracing and quarantine of cases. Model outputs included the number of secondary cases, hospitalizations and deaths. Four vaccination scenarios were modeled:  Colorado vaccination rate, national vaccination rate, Colorado vaccination rate under medium exemption regulations, and Colorado vaccination rate under difficult exemption regulations. RESULTS: At the Colorado vaccination rate, seven secondary cases followed the introduction of an index case, two of which required hospitalization. This outbreak ranged in cost from $111,048-$338,304, which is equivalent to the purchase of 5,580-17,000 measles vaccines. This could increase 2-dose vaccination coverage in Denver by 1.3%. If Colorado increased their difficulty in obtaining non-medical exemptions, the number of secondary cases reduced by 86%, alleviating costs related to public health interventions and the negative consequences of a measles outbreak. At the national vaccination level, no other individuals became infected due to herd immunity. CONCLUSIONS: Herd immunity is not established at the Colorado vaccination rate and thus an outbreak is likely following the introduction of an index case. There is an inverse relationship between non-medical exemption complexity and exemption rate. Increasing vaccination coverage by as little as 3.1% (from easy to medium complexity) would reduce the likelihood and magnitude of an outbreak.  Adding a required education component or a written statement of objection would be a prudent public health nudge toward measles herd immunity.