The most useful measles instruction this summer is a subtraction problem. CDC tells international travelers to complete two doses of MMR vaccine at least two weeks before departure, because two doses give 97 percent protection against measles and one gives 93. [1] Count back fourteen days from a boarding pass, and the deadline is fixed.

The paper reported on June 28 that the CDC tied the measles year to a 92.5 percent kindergarten vaccination rate, leaving roughly 286,000 five-year-olds unprotected. The travel record is where that gap meets a runway — CDC reports 2,134 confirmed cases as of June 25, reported by 41 jurisdictions, 93 percent of them outbreak-associated, and notes that U.S. cases often begin with an unvaccinated international traveler. [1][2]

Summer is the mechanism. The virus is not seasonal, but it moves on high-travel windows — camps, family trips, flights home — and two of every three unvaccinated travelers who import it are Americans. [1][2] One infected person will infect up to nine of ten unprotected close contacts, and the agency tells travelers to check its global travel notices for where measles is currently rising. [1][3]

The record adds instructions for the edge cases. Infants 6 through 11 months should get an early dose before a trip, then resume the normal schedule at 12 to 15 months and again at 4 to 6 years; anyone unprotected with a trip under two weeks away should still get a dose. [1] After returning, watch for fever and rash for three weeks — the window in which measles surfaces. [1]

That is the divergence the argument obscures. X runs each case as proof of whose fault the year is; coverage runs the rising count. [2] Neither hands a family the two-week deadline, the infant exception, and the three-week watch — the parts of the record that change one traveler's odds and, with them, the outbreak math. [1][3]

-- NORA WHITFIELD, Chicago