What is the risk of developing Subacute Sclerosing Panencephalitis (SSPE) if measles is contracted at age 5?

Risk of SSPE After Measles Infection at Age 5

The risk of developing SSPE after contracting measles at age 5 is substantially lower than infection during infancy, estimated at approximately 4 per 100,000 measles cases overall, but the risk is 16 times lower when measles occurs after age 5 compared to infection under 1 year of age. 1

Age-Stratified Risk Profile

The risk of SSPE is heavily age-dependent at the time of initial measles infection:

• Children infected with measles under age 5 have the highest risk of developing SSPE, with the greatest danger occurring in those infected before 2 years of age 2, 3
• Measles infection under 1 year of age carries a risk of approximately 18 per 100,000 cases (or 1 in 5,556 cases), which is 16 times higher than measles occurring after age 5 4, 1
• The overall risk across all ages is approximately 4 per 100,000 measles cases (or 1 in 25,000 cases) 4, 1
• For children infected at age 5 specifically, the risk would be considerably lower than the overall average, falling into the lower-risk category since infection occurred after the high-risk period of infancy and early childhood 1

Recent California Data Shows Higher Risk Than Previously Thought

More recent surveillance data suggests the historical estimates may have been too conservative:

• A California study from 1988-1991 measles cases found SSPE incidence of 1 in 1,367 for children under 5 years at the time of measles infection 5
• For infants under 12 months, the California data showed 1 in 609 developed SSPE, confirming the dramatically elevated risk in the youngest children 5
• These rates are substantially higher than older estimates, suggesting that 4-11 per 100,000 may underestimate the true burden 6, 5

Clinical Latency Period

The timing between measles infection and SSPE onset is critical to understand:

• SSPE typically develops 7-10 years after the initial measles infection, with a median latency of 9.5 years (range: 2.5-34 years) 5
• A child infected at age 5 would most likely develop SSPE symptoms in early adolescence (around ages 12-15) if they are among the unfortunate minority who develop this complication 5
• The incubation period has been increasing over time due to declining measles incidence from vaccination programs, creating a transient epidemiologic effect 4

Key Risk Factors Beyond Age

Several additional factors modify SSPE risk:

• Male children have 2.4-2.8 times higher risk than females 1, 5
• Children who are third or subsequent siblings show excess risk 1
• Children with HIV infection or immunocompromise may face increased SSPE risk 2
• Geographic variation exists, with higher incidence in certain regions 1

Critical Prevention Context

• Measles vaccination is the only effective prevention strategy for SSPE, and the MMR vaccine does not cause SSPE 7, 8, 6
• The risk of SSPE after measles vaccination is no greater than 0.14 per 100,000 doses (essentially negligible), compared to 4-18 per 100,000 natural measles cases depending on age 4
• When SSPE cases occur in vaccinated children, evidence indicates they had unrecognized measles infection before vaccination, and the SSPE resulted from natural infection, not the vaccine 7, 8

Clinical Implications

For a child who contracted measles at age 5:

• The absolute risk remains low but not negligible, likely in the range of 1-2 per 100,000 cases based on the age-stratified data showing 16-fold lower risk compared to infant infection 1
• Vigilance for neurologic symptoms should continue through adolescence, as the median latency period would place symptom onset around ages 12-15 5
• SSPE presents with insidious personality changes, declining intellectual performance, seizures, myoclonic jerks, and progressive neurologic deterioration leading to death 8
• The condition is universally fatal in most cases, with no effective cure despite various immunomodulation attempts 2