SSPE Risk After Measles at Age 5 or Older
The claim that SSPE risk is ~1 in 300,000 to >1 in 1,000 for measles contracted at age 5 or older is not supported by the available evidence, which consistently shows that while SSPE risk is dramatically lower in older children compared to infants, the overall risk across all ages remains approximately 4-11 per 100,000 measles cases, with the highest risk concentrated in children infected before age 5. 1, 2, 3
Age-Stratified Risk Data
The most robust epidemiological data comes from population-level surveillance studies that demonstrate clear age-dependent risk patterns:
Children infected with measles before age 5 years carry the highest SSPE risk, with particularly elevated rates in those infected before 12 months of age (approximately 1 in 609 cases) and before 5 years (approximately 1 in 1,367 cases). 4
The overall SSPE incidence across all age groups is reported as 4-11 per 100,000 measles cases (equivalent to approximately 1 in 9,000 to 1 in 25,000), not the 1 in 300,000 to 1 in 1,000 range suggested in the question. 1, 2
England and Wales surveillance data (1970-1989) estimated the overall SSPE risk after measles at 4.0 × 10⁻⁵ (1 in 25,000), with the risk after measles under one year of age at 18 × 10⁻⁵ (1 in 5,556). 3
Why Age-Specific Data for ≥5 Years Is Limited
The vast majority of SSPE cases result from measles infection acquired before age 5, which explains why published surveillance studies focus on this younger age group and do not provide separate risk estimates for infections at age 5 or older. 2, 5, 4
All 12 SSPE cases with documented measles history in the California surveillance study (1998-2015) had measles illness prior to 15 months of age, demonstrating the concentration of risk in early childhood infections. 4
The latency period between measles infection and SSPE onset averages 9.5 years (range 2.5-34 years), meaning that even infections in older children could theoretically progress to SSPE, though this appears exceedingly rare in documented cases. 4
Critical Interpretation Caveats
"Effectively zero observed cases" in modern datasets for infections at age ≥5 years likely reflects two factors: (1) the genuine biological phenomenon that SSPE risk decreases substantially with age at infection, and (2) the dramatic reduction in measles cases overall due to vaccination, which reduces the denominator of exposed individuals in all age groups. 1, 3
The absence of observed cases does not equate to a quantifiable risk of 1 in 300,000 or lower—it may simply reflect insufficient sample sizes in modern surveillance to detect rare events in this specific age stratum. 4, 3
Historical data showing increased intervals between measles and SSPE onset in later years represents a transient effect from declining measles incidence post-vaccination, not a change in biological risk. 3
Clinical Bottom Line
While SSPE risk is unquestionably lower for measles infections occurring at age 5 or older compared to infant infections, the available evidence does not support the specific risk estimates of 1 in 300,000 to >1 in 1,000 cited in the question. The concentration of documented SSPE cases in children infected before age 5 reflects genuine age-dependent susceptibility, but the lack of age-stratified data for older children prevents precise risk quantification in that population. 2, 4, 3
The only evidence-based prevention strategy remains measles vaccination, which has essentially eliminated SSPE in highly vaccinated populations regardless of the age at which infection might otherwise occur. 1, 6