Maricopa County Department of Public Health confirmed three related cases of measles. One case is an adult and the other two are kids. One of them had been hospitalized, but all 3 are recovering.
Whether these cases transition from an outbreak to an epidemic will depend on how many contacts the cases had with unvaccinated folks in their friend groups or at school. No doubt that Maricopa County staff have been busy doing case investigations to figure out just that.
Assuming the cases cooperate, Maricopa will be able to figure out who those unvaccinated contacts are (if any) and urge them to quarantine for the remainder of their incubation period. Cooperation of the cases and contacts and the fidelity to isolation and quarantine are critical to the success of the interventions. They will also likely urge unvaccinated contacts to get the measles vaccine right away. If taken within 3 days of the exposure it can be effective at preventing an infection.
Teams will also be checking the vaccination status of the classmates of the pediatric cases (assuming they’ve been in school). Best practice procedures include excluding unvaccinated contacts from school for at least one incubation period (up to 21 days) and vaccinating susceptible contacts within 72 hours of exposure.
Determining which measles contacts will be quarantined or excluded (from school), according to R9-6-303, to prevent transmission; and arranging for immunization of each nonimmune measles contact within 72 hours after last exposure, if possible.
R9-6-303 gives the county health department the ability to issue isolation, quarantine and other control measures. The details of that authority and best practice procedures are on pages 20 and 21 of the communicable disease rules.
Because the average incubation period for measles is 11–12 days (range of 7–21 days) this investigation will be ongoing throughout September and most likely beyond. It’ll be at least a couple of weeks before we’ll know how much (or whether) this cascades into an epidemic.
Much will depend on how much contact the cases had with unvaccinated folks before they were symptomatic, how good case and contact cooperation is, and whether folks follow MCDPH recommendations & instructions.