What are the odds of an unvaccinated child getting measles?
Evidence quality 4.25/5
Eight-dimension review score against the quality rubric . Each dimension scored 1–5.
- D1 Source grounding
- 5/5
- D2 Source authority
- 5/5
- D3 Arithmetic
- 3/5
- D4 Uncertainty
- 3/5
- D5 Scope
- 5/5
- D6 Prose
- 5/5
- D7 Perception honesty
- 4/5
- D8 Caveat completeness
- 4/5
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≈ As likely as
Perceived
Measles occupies a peculiar position in public risk perception. Older generations remember it as a routine childhood illness -- unpleasant but survivable. Younger parents, raised in the post-elimination era, often underestimate its contagiousness while simultaneously hearing alarming outbreak headlines. Anti-vaccine communities sometimes frame measles as mild; pro-vaccine messaging emphasizes worst-case complications. The result is a bimodal perception where the actual transmission probability is rarely discussed in concrete terms.
Rough estimate: ~5-20% chance an unvaccinated child catches measles
Source: editorial intuition, not polled
Actual
>90% infection rate among unvaccinated children by age 15 (pre-vaccine era)
unvaccinated children in pre-vaccine-era United States
Show derivation
In the pre-vaccine era (before 1963), >90% of US children were infected with measles by age 15 (CDC Pink Book, Chapter 13). In the current US environment, herd immunity from high (though declining) vaccination rates dramatically reduces -- but does not eliminate -- exposure risk. Kindergarten MMR coverage has fallen from 95.2% in 2019-2020 to 92.5% in 2024-2025 (CDC SchoolVaxView), below the ~95% threshold needed for reliable herd immunity. Full-year 2025 data (updated 2026-06: superseding the May-2025 mid-year snapshot of 2,144 cases) records 2,288 confirmed US measles cases across 48 outbreaks and 3 deaths -- the worst US measles year since 1991-1992 and the first US measles deaths in over a decade, all among unvaccinated individuals, with 93% of cases unvaccinated. With R0 of 12-18, measles is extraordinarily contagious; 90% of susceptible contacts become infected upon exposure. We estimate a current-era cumulative probability of ~50% for an unvaccinated child contracting measles by age 18, reflecting the reality that herd immunity still provides substantial (but eroding) indirect protection, while pockets of low vaccination create significant outbreak risk. This is far below the pre-vaccine >90% but far above what most parents assume.
Caveats: The normalized 50% estimate is necessarily imprecise because it depends heavily …
The normalized 50% estimate is necessarily imprecise because it depends heavily on local vaccination coverage, which varies enormously by county and school district. An unvaccinated child in a 98%-vaccinated suburban school faces far lower risk than one in a 75%-vaccinated community. The pre-vaccine >90% figure represents the biological ceiling in a fully susceptible population. The current estimate will continue to shift as vaccination rates change. Measles case fatality in the US is roughly 1-3 per 1,000 cases, but rises significantly in malnourished populations and infants. Present-day exposure risk for unvaccinated children is rising: full-year 2025 closed with 2,288 confirmed US cases across 48 outbreaks and 3 deaths (all unvaccinated) -- the worst US measles year since 1991-1992 and the first US measles deaths in over a decade -- while kindergarten MMR coverage slipped to 92.5% (2024-2025) from 95.2% (2019-2020), below the ~95% herd-immunity threshold. These are national counts, not per-child incidence rates, so they support a higher present-day exposure caveat rather than a change to the headline cumulative estimate.
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Before the measles vaccine arrived in 1963, more than 90% of American children had been infected by age 15. The virus has an R0 of 12-18 — the highest of any common infectious disease — meaning 90% of susceptible contacts become infected upon exposure. In the current US environment, declining vaccination coverage (down to 92.5% among kindergartners, below the 95% herd-immunity threshold) has eroded indirect protection, and full-year 2025 closed with 2,288 confirmed cases across 48 outbreaks and 3 deaths (all unvaccinated) — the worst US measles year since 1991-1992 and the first US measles deaths in over a decade.
The perception gap runs in both directions. Parents in anti-vaccine circles often frame measles as a benign rite of passage, ignoring the 1-3 per 1,000 case fatality rate and the 1 in 1,000 risk of encephalitis. Meanwhile, many pro-vaccine parents assume herd immunity makes the question moot for any individual unvaccinated child — which was largely true when coverage sat above 95%, but is increasingly false as exemption clusters create pockets of susceptibility. The biology has not changed; what has changed is the social infrastructure that kept it contained.
Geography dominates individual risk. An unvaccinated child in a school district with 98% MMR coverage faces a near-zero annual exposure probability. The same child in a community where coverage has dropped below 90% faces outbreak-level risk, with household secondary attack rates approaching 90%. The 50% lifetime estimate is a national average that obscures this variance — the real number for any given child is either much higher or much lower, depending almost entirely on where they live and who they spend time with.
Related tidbits
Parents often put an unvaccinated child's odds of catching measles at 5-20%. With declining herd immunity, the figure is closer to ~50% by age 18. Before the vaccine, more than 90% of unvaccinated children caught it by age 15. This is one where the intuition runs low.
Claim ledger
Every number below is what each source reported, with the verbatim quote we relied on and how we arrived at our figure. Click any link to verify directly.
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[1] CDC Epidemiology and Prevention of Vaccine-Preventable Diseases (Pink Book) — Chapter 13: Measles
Chapter 13: Measles- Statistic
Before 1963, >90% of persons had measles by age 15; 3-4 million cases/year; ~500 deaths/year in the US- Excerpt
“"Before 1963, approximately 500,000 cases and 500 measles deaths were reported annually, with epidemic cycles every 2-3 years. However, the actual number of cases was estimated at 3-4 million annually. More than 50% of persons had measles by age 6, and more than 90% by age 15." ”
- Source data from
- 2024-05-08
- Accessed
- 2026-04-18 · archived copy
- Calculation
- CDC Pink Book establishes the pre-vaccine baseline: >90% cumulative infection by age 15 in unvaccinated populations. With 3-4 million annual cases in a US child population of ~50 million, the annual attack rate was roughly 6-8%. Cumulative over 15 years: 1 - (1 - 0.07)^15 ≈ 0.67 to 0.90, consistent with the >90% figure (accounting for epidemic clustering and household transmission).
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[2] World Health Organization — Measles Fact Sheet
Measles Fact Sheet- Statistic
R0 of 12-18; 90% of susceptible contacts become infected; caused 136,000 deaths globally in 2022- Excerpt
“"Measles is one of the most contagious diseases in the world. Any person who is not immune (who has not been vaccinated or was vaccinated but did not develop immunity) can get measles." ”
- Source data from
- 2024-05-10
- Accessed
- 2026-04-18 · archived copy
- Calculation
- WHO confirms the R0 range of 12-18, making measles the most contagious common infectious disease. The 90% secondary attack rate among susceptible contacts means that in any exposure event, the probability of transmission is near-certain. This underpins the pre-vaccine >90% cumulative infection rate and explains why even small declines in vaccination coverage produce outbreaks.
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[3] The Lancet Infectious Diseases (Guerra et al.) — The basic reproduction number (R0) of measles: a systematic review
The basic reproduction number (R0) of measles: a systematic review- Statistic
Median R0 of 12.5 (range 3.7-203.3) across studies; most estimates cluster between 12-18- Excerpt
“"We identified 58 studies... The median R0 value across studies was 12.5." ”
- Source data from
- 2017-12-01
- Accessed
- 2026-04-18 · archived copy
- Calculation
- Guerra et al. systematic review of R0 estimates confirms measles as exceptionally transmissible. The R0 range means that in a fully susceptible population, each case generates 12-18 secondary cases on average. This makes herd immunity thresholds very high (1 - 1/R0 = 92-94%), explaining why vaccination coverage drops from 95% to 92.5% can trigger outbreaks.
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[4] CDC — Measles Cases and Outbreaks
Measles Cases and Outbreaks- Statistic
2,144 confirmed measles cases in 2025; 93% among unvaccinated or unknown vaccination status- Excerpt
“"As of May 30, 2025, 2,144 confirmed measles cases and 3 deaths have been reported in the United States." ”
- Source data from
- 2025-05-30
- Accessed
- 2026-04-18 · archived copy
- Calculation
- The 2025 outbreak data demonstrates the real-world consequence of declining vaccination. With 2,144 cases and 93% among unvaccinated individuals, this represents the highest case count since measles was declared eliminated in 2000. Three deaths confirm the ongoing lethality of the disease even in a modern healthcare setting.
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[5] CDC — Measles Cases and Outbreaks (full-year 2025)
Measles Cases and Outbreaks (full-year 2025)- Statistic
2,288 confirmed measles cases across 48 outbreaks in the US for the full year 2025; worst US measles year since 1991-1992- Excerpt
“"For the full year of 2025, a total of 2,288 confirmed* measles cases were reported in the United States. There were 48 outbreaks** reported in 2025, and 90% of confirmed cases (2,065 of 2,288) were outbreak-associated." ”
- Source data from
- 2026-06-11
- Accessed
- 2026-06-14 · archived copy
- Calculation
- The full-year 2025 total of 2,288 confirmed cases (updated from the May-2025 mid-year snapshot of 2,144) is the highest annual US count since 1991-1992 and far above the post-elimination norm of a few hundred cases. With 90% of cases outbreak-associated and 93% among unvaccinated individuals, the data corroborates -- but does not raise -- the current-era ~50% cumulative point estimate, which already spans a 0.10-0.90 uncertainty band reflecting local coverage variance. The number is a national case count, not a per-child incidence rate, so it cannot be converted to an individual lifetime probability without estimation; it is recorded here as a present-day exposure caveat rather than a headline revision.
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[6] CDC — Vaccination Coverage and Exemptions among Kindergartners (SchoolVaxView)
Vaccination Coverage and Exemptions among Kindergartners (SchoolVaxView)- Statistic
Kindergarten MMR coverage was 92.5% in 2024-2025, down from 95.2% in 2019-2020; below the ~95% herd-immunity threshold- Excerpt
“"MMR vaccination coverage was 92.5% during the 2024-25 school year." ”
- Source data from
- 2025-08-01
- Accessed
- 2026-06-14 · archived copy
- Calculation
- CDC SchoolVaxView documents kindergarten MMR coverage of 92.5% for 2024-2025, down from 95.2% in 2019-2020 -- a decline of 2.7 percentage points that pushes the US below the ~95% threshold (1 - 1/R0 for R0 of 12-18) needed to interrupt measles transmission. This declining herd immunity is the mechanism behind the elevated current-era exposure risk for unvaccinated children captured in the normalized estimate.