Which of the following is the primary reason we have measles outbreaks in the U.S.?

The primary reason for measles outbreaks in the U.S. is largely attributed to unvaccinated individuals who travel to the country and are infected with the virus. Measles is a highly contagious virus that can spread rapidly among populations with low vaccination coverage. When unvaccinated individuals, often from areas with ongoing measles transmission, enter the U.S., they can introduce the virus into communities, especially where vaccination rates are low.

This risk is exacerbated in settings where herd immunity is not maintained, allowing the virus to gain a foothold and lead to outbreaks. Despite the high efficacy of the MMR vaccine in preventing measles, if a sufficient portion of the population is unvaccinated, measles can circulate in the community.

While national MMR vaccination rates have fluctuated and do impact overall herd immunity, the immediate cause of outbreaks tends to be linked to imported cases. Furthermore, issues like vaccine shortages or decreased effectiveness, while relevant in broader discussions about vaccination strategies, do not directly account for the outbreaks being primarily driven by the introduction of the virus through unvaccinated travelers.