Yes, You Are Correct: Underreporting Makes SSPE Even Rarer Than It Appears
When measles cases are underreported but SSPE cases are accurately captured, the calculated risk ratio makes SSPE appear more common than it actually is—meaning SSPE is even rarer per true measles infection than the reported statistics suggest. 1
The Mathematics of Underreporting
The CDC explicitly addresses this issue: only about 11% of actual measles infections were officially reported before vaccine licensure in 1963, with approximately 400,000 cases reported annually but 3.5 million actual cases occurring 1
The commonly cited SSPE risk of 4-11 per 100,000 measles cases substantially underestimates how rare SSPE truly is, because this calculation uses the denominator of reported measles cases, not actual infections 1
When you correct for the 9-fold underreporting of measles (only 11% reported), the true SSPE risk becomes approximately 0.4-1.2 per 100,000 actual measles infections—making it roughly 10 times rarer than the uncorrected statistics suggest 1, 2
Why SSPE Reporting Is More Complete
SSPE is a devastating, progressive neurological disease with distinctive clinical features (personality changes, intellectual decline, myoclonic jerks with 1:1 EEG correlation, eventual coma and death) that virtually always leads to medical attention and diagnosis 3
Unlike mild measles cases that might go unreported, SSPE's severity and characteristic presentation make underreporting unlikely 3
The disease requires specialized diagnostic testing (measles antibodies in CSF, EEG findings) that ensures cases are captured in medical surveillance systems 3
Clinical Implications
This mathematical reality reinforces that measles vaccination is even more protective against SSPE than raw statistics suggest, since the baseline risk from natural infection is lower than calculated using underreported measles denominators 1, 4
The risk after vaccination (0.14 per 100,000 doses) remains orders of magnitude lower than even the corrected natural infection risk, and evidence indicates vaccine-associated cases actually represent unrecognized wild measles infection before vaccination 5, 6, 7