SSPE Incidence in Older Children with Measles
No, the true incidence of SSPE following measles infection is substantially higher than 1 in a million, particularly for children infected at younger ages, with the highest quality evidence showing rates of approximately 1 in 600-1,400 for children under 5 years old. 1
Actual SSPE Risk Following Measles Infection
The "1 in a million" figure dramatically underestimates the true risk of SSPE after measles infection:
- For children under 12 months: The incidence is approximately 1 in 609 measles cases 1
- For children under 5 years: The incidence is approximately 1 in 1,367 measles cases 1
- Overall measles cases: Studies report 4-11 per 100,000 (1 in 9,000 to 1 in 25,000) measles infections develop SSPE 2, 3
- Historical data: The risk ranges from 6.1 to 40.9 per million measles cases, with a mean of 16.1 per million (approximately 1 in 62,000) 4
Critical Context: Age at Infection Matters Most
The risk of SSPE is not uniform across age groups—younger children face dramatically higher risk:
- 71% of SSPE cases had measles illness before 15 months of age 1
- 46% of SSPE cases contracted measles before age 2 years 5
- Children infected under 5 years have the highest risk of developing SSPE 3
- The latency period averages 7-9.5 years between measles infection and SSPE onset 1, 5
Why the "1 in Million" Misconception Exists
The confusion stems from massive measles underreporting:
- Before 1963, approximately 400,000 measles cases were reported annually in the United States, but the actual number approached 3.5 million per year 6
- This means only 11% of actual measles infections were officially reported 6
- When SSPE incidence is calculated against reported (rather than actual) measles cases, the risk appears artificially low 6
Measles Vaccination Dramatically Reduces SSPE Risk
Measles vaccination has essentially eliminated SSPE in highly vaccinated populations:
- The risk of SSPE following measles vaccination is 0.5 to 1.1 cases per million doses, compared to 5.2 to 9.7 cases per million following natural measles infection 5
- Widespread measles vaccination has essentially eliminated SSPE from the United States 7
- MMR vaccine does not increase the risk for SSPE, even in persons who previously had measles 7, 6
Clinical Implications
For unvaccinated children, particularly infants:
- Protection requires avoidance of travel to endemic areas or early vaccination at 6-11 months prior to travel 1
- Immunocompromised children (HIV, leukemia, lymphoma) face increased risk of severe measles and potentially SSPE 2, 6
- Males develop SSPE 2.4 times more frequently than females 1
Common pitfall: Assuming older children who contract measles have negligible SSPE risk. While the highest risk is in children infected under 5 years, SSPE can occur after measles at any age, with latency periods extending up to 34 years 1.