SSPE Risk by Age at Measles Infection
The risk of SSPE following measles infection at age 5 years or older is approximately 1 in 100,000 measles cases, which is 16 times lower than the risk when measles occurs before age 1 year. 1
Age-Stratified SSPE Risk Data
The evidence clearly demonstrates that age at measles infection is the most critical determinant of SSPE risk, with a dramatic inverse relationship between age and subsequent SSPE development:
Specific Risk by Age Group
- Under 1 year of age: 18 per 100,000 measles cases (highest risk) 2
- Under 2 years of age: Nearly half of all SSPE cases had measles before age 2 1
- Under 5 years of age: 6.5-11 per 100,000 measles cases 3; California data shows 1 in 1,367 cases 4
- Age 5 years or older: Approximately 1 per 100,000 measles cases (16-fold lower risk than infection under age 1) 1
Overall Population Risk
The baseline SSPE risk across all ages is 4-11 per 100,000 measles cases 5, 6, 3, but this aggregate figure masks the profound age-dependent variation. Children who contract measles at age 5 or older represent the lowest-risk group for SSPE development. 1
Critical Clinical Context
Why Age Matters
- 71% of SSPE cases had documented measles-like illness before 15 months of age 4
- The median latency period between measles infection and SSPE onset is 8-9.5 years 1, 4
- Male predominance exists (2.4-2.8:1 ratio) across all age groups 1, 4
Underreporting Caveat
These risk estimates are based on reported measles cases, which represent only approximately 11% of actual infections. 6 The true denominator of measles cases is much larger, meaning the actual per-infection SSPE risk may be lower than these figures suggest, though the age-dependent pattern remains consistent.
Prevention Implications
Measles vaccination has essentially eliminated SSPE in highly vaccinated populations and remains the only effective prevention strategy. 7, 5, 6 The vaccine carries an SSPE risk of only 0.14 per 100,000 doses—dramatically lower than natural infection at any age—and does not increase SSPE risk even in previously infected individuals. 7, 2