No survey asks adults whether they regret getting, or skipping, a measles booster during the 2025 resurgence. The few documented post-outbreak interviews run the other way: the parents of the Texas child who died in 2025 told reporters they stood by their decision not to vaccinate. So this pair is a proxy. The action side is anchored on the probability of a serious adverse event from the shot itself, and the inaction side on the share of the relevant cohort that carries no measurable measles immunity. Neither number is a regret rate; both are stand-ins, and the gap between them is wide.
The action downside is small and well-characterized. Anaphylaxis after an MMR-containing vaccine was reported to VAERS at 0.6 per million doses over 1990 to 2016, and the CDC’s Vaccine Information Statement describes a serious reaction as a “very remote chance.” The common outcomes — a sore arm, a brief fever, a mild rash — are not the kind of thing people later regret. The subtler point is that for the large majority of adults who turn out to have been immune already, the booster was redundant rather than harmful, and a redundant-but-protective dose tends to read as reassurance, not as a mistake. That is why the action proxy sits near the floor: the only genuinely regret-worthy action outcome is the rare serious event, on the order of one in a hundred thousand or rarer.
The inaction side is where the cohorts in the question matter. A US healthcare-worker serosurvey found 4% seronegative and 2% equivocal for measles IgG — a roughly 6% immunity gap — and a 2025 meta-analysis of 23,236 vaccinated people pooled seropositivity at 87.8%, falling to 84.3% among single-dose recipients. The 1963-67 killed-vaccine cohort is a separate, smaller group that ACIP explicitly tells to revaccinate, because the inactivated product conferred little durable protection. The catch is that an immunity gap is not the same as realized regret: with 2,288 confirmed US cases in 2025 against a population of roughly 340 million, the national attack rate stayed near 0.0007%, so most seronegative adults were never exposed at all. What gives the gap its weight is the severity tail — 11% of 2025 cases were hospitalized and there were deaths — so the rare intersection of “no immunity” and “exposed” is consequential. The honest reading: skipping leaves a measurable hole in protection for a minority of these cohorts, getting the dose closes it at trivial cost, and the realized-regret asymmetry favors acting even though almost no one in either group will ever face the outcome that would make the choice matter.