Does the estimated risk of 4-11 Subacute Sclerosing Panencephalitis (SSPE) cases per 100,000 measles infections account for measles underreporting?

Does the 4-11 per 100,000 SSPE Risk Account for Measles Underreporting?

No, the 4-11 per 100,000 SSPE risk estimate does NOT account for measles underreporting and therefore substantially underestimates the true risk of developing SSPE following measles infection. 1

The Underreporting Problem

The critical issue is that only approximately 11% of actual measles infections are officially reported, according to the CDC. 1 This means:

• Before the measles vaccine was licensed in 1963, approximately 400,000 measles cases were reported annually in the United States 1
• However, the actual number of cases approached 3.5 million per year 1
• This represents a nearly 9-fold undercount of true measles infections 1

What This Means for SSPE Risk Calculation

When you see the 4-11 per 100,000 figure cited in the literature, this is calculated based on reported measles cases, not actual infections. 1, 2 The true denominator should be much larger, which means:

• The actual risk per true measles infection is lower than 4-11 per 100,000 when properly accounting for all infections
• However, the absolute number of SSPE cases remains the same regardless of reporting completeness
• This creates a paradox where the risk appears higher when using reported cases as the denominator

Real-World Evidence from Georgia

A 2020 study from Georgia provides concrete evidence of how underreporting affects risk calculations. 3 Following the 2004-2005 measles outbreak:

• The crude SSPE risk estimate was 1:1,396 (approximately 72 per 100,000) based on reported cases 3
• When adjusted for 50% reporting completeness, the risk became 1:2,792 3
• When adjusted for 25% reporting completeness, the risk became 1:5,584 3
• When adjusted for 10% reporting completeness (similar to US historical data), the risk became 1:13,960 (approximately 7 per 100,000) 3

This demonstrates that the commonly cited 4-11 per 100,000 figure likely represents calculations based on incomplete case reporting, making it appear as though SSPE risk is higher than it actually is per true infection. 3

Critical Clinical Caveat

While the per-infection risk may be lower when properly accounting for all cases, this does NOT diminish the importance of measles vaccination. The key points remain:

• SSPE is uniformly fatal in most cases 4, 5
• Measles vaccination has essentially eliminated SSPE in highly vaccinated populations 1, 6
• The MMR vaccine does not increase SSPE risk, even in persons who previously had measles 1
• Measles vaccination remains the only effective prevention strategy for SSPE 1, 6, 4

Highest Risk Groups

Regardless of the exact denominator used, certain populations face dramatically elevated SSPE risk:

• Children who contract measles before age 2 years face the highest risk 2, 7
• Measles contracted under 1 year carries a risk 16 times greater than measles over 5 years 7
• Immunocompromised individuals, particularly those with HIV infection, face increased risk 6, 2