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
Lifetime probability · lifetime, subgroup
1 in 2.0
50% lifetime chance
range 1 in 10 to 1 in 1.1
● your factors — click this risk ▾ to reveal
≈ 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. National MMR coverage has fallen from 95.2% in 2019-2020 to 92.5% in 2024-2025, below the ~95% threshold needed for reliable herd immunity. The 2025 US outbreak saw 2,144 cases with 93% among unvaccinated individuals. 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.
Risks at similar odds
Other risks with roughly the same likelihood — useful for calibration.
Painkiller before infant vaccination
What are the odds that giving an infant paracetamol prophylactically at vaccination blunts the immune response to the vaccine?
Unprotected sex
What are the odds of catching an STI from a single unprotected sexual encounter with a new partner?
Raw meat cross-contamination
What are the odds of getting sick from not washing hands or surfaces after handling raw meat or eggs?
Toy injury requiring ER (child)
What are the odds of a child needing an ER visit for a toy-related injury?
Recently viewed on this device
Stored locally — clear anytime.
Pick challenger
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 the 2025 outbreak produced 2,144 confirmed cases, the highest count since elimination was declared in 2000.
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.
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.
-
[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).
-
[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.
-
[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.
-
[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.