Does underreporting of measles cases affect the true incidence of Subacute Sclerosing Panencephalitis (SSPE)?

Understanding SSPE Risk with Measles Underreporting

Yes, the 11% reporting rate means SSPE is actually much MORE common than the published rates suggest, not more rare—the true risk of SSPE is approximately 9-10 times higher than previously estimated when accounting for underreporting. 1

The Mathematics of Underreporting

The CDC documented that before measles vaccine licensure in 1963, approximately 400,000 measles cases were reported annually in the United States, but the actual number approached 3.5 million per year—meaning only about 11% of true infections were officially reported. 1 This dramatic underreporting has critical implications:

• When calculating SSPE risk per measles case, the denominator (total measles cases) is artificially small because most cases go unreported. 1
• The numerator (SSPE cases) remains relatively stable because SSPE is severe enough to eventually come to medical attention, even if the original measles infection was never reported. 2
• This creates a mathematical situation where published SSPE rates (typically cited as 4-11 per 100,000 measles cases) substantially underestimate the true risk. 1

Evidence from the 1989-1991 Measles Resurgence

The most compelling data comes from careful follow-up of the 1989-1991 measles outbreak in the United States, where researchers could track both reported measles cases and subsequent SSPE development:

• Among children under 5 years who contracted measles during 1988-1991, the SSPE incidence was 1:1,367 cases. 2
• For infants under 12 months at time of measles infection, the risk was even higher at 1:609 cases. 2
• These rates are approximately 10-fold higher than the previous 1982 estimate for the United States. 3

This California study identified 17 SSPE cases through comprehensive surveillance including death certificate searches and investigations of undiagnosed neurologic disease—methods that captured cases regardless of whether the original measles infection was reported. 2

Why SSPE Cases Get Counted Despite Measles Underreporting

SSPE eventually forces medical attention due to its devastating progression through personality changes, intellectual decline, seizures, myoclonic jerks, and ultimately coma and death. 4, 5 The disease characteristics ensure case ascertainment:

• The distinctive EEG pattern showing periodic complexes with 1:1 relationship to myoclonic jerks is pathognomonic. 5
• Detection of intrathecal measles-specific antibodies in CSF confirms the diagnosis. 5
• The progressive, fatal nature means patients eventually reach tertiary care centers where diagnosis is made, even if the precipitating measles infection occurred years earlier and was never reported. 2

Clinical Implications

Clinicians should maintain high suspicion for SSPE in patients with compatible neurologic symptoms, even in older patients with no documented history of measles infection, because the original measles case was likely unreported. 2 Among the California SSPE cases:

• 71% had a history of measles-like illness, but this means 29% had no known measles history at all. 2
• The median latency period was 9.5 years (range 2.5-34 years) between measles infection and SSPE diagnosis. 2
• Males outnumbered females 2.4:1. 2

The Prevention Imperative

Widespread measles vaccination has essentially eliminated SSPE from the United States, and vaccination is the only effective prevention strategy. 4, 1 The evidence is unequivocal:

• MMR vaccine does not increase the risk for SSPE under any circumstances. 1, 6
• When rare SSPE cases occur in vaccinated children without known measles history, evidence indicates these children had unrecognized measles infection before vaccination—the SSPE resulted from that natural infection, not the vaccine. 5, 6
• Successful measles vaccination programs directly and indirectly protect populations against SSPE and have the potential to eliminate SSPE through measles elimination. 6

Common Pitfall to Avoid

Do not assume that low reported measles incidence means low SSPE risk—the opposite is true. When measles cases are underreported, the calculated SSPE risk per case appears artificially low, masking the true danger of measles infection. 1, 3 This underestimation has led to more cases of SSPE than were originally recognized as being prevented by measles immunization. 3