What is the incidence of Subacute Sclerosing Panencephalitis (SSPE) following measles vaccination?

Incidence of SSPE Following Measles Vaccination

The incidence of SSPE following measles vaccination is extraordinarily low at 0.14 to 1.1 cases per million vaccine doses, and importantly, the vaccine does not cause SSPE—it prevents it. 1, 2, 3, 4

Key Epidemiologic Data

The risk stratification clearly demonstrates vaccination's protective effect:

• Post-vaccination SSPE risk: 0.14-1.1 cases per 1 million doses 2, 3, 4
• Post-natural measles SSPE risk: 40-97 cases per 1 million measles infections (approximately 4-11 per 100,000 cases) 2, 3, 4
• This represents a 28-fold to 69-fold lower risk with vaccination compared to natural infection 2, 3

Critical Clinical Context: Vaccination Does Not Cause SSPE

The Advisory Committee on Immunization Practices definitively states that measles vaccine administration does not increase the risk for SSPE, even among persons who have previously had measles disease or received prior measles vaccine. 1, 5

Why Rare Cases Occur in Vaccinated Children

When SSPE has been reported in vaccinated children without documented measles history, evidence indicates these children likely had unrecognized measles infection before vaccination, and the SSPE resulted from that natural infection, not the vaccine 1, 5:

• The mean interval from vaccination to SSPE onset was only 3.3 years in vaccine-associated cases 4
• The mean interval from natural measles to SSPE onset was 7.0 years 4
• This shorter interval in "vaccine-associated" cases suggests prior unrecognized measles infection that was already incubating when vaccination occurred 1, 4
• No relationship exists between age at vaccination and SSPE risk, unlike natural measles where younger age dramatically increases risk 4

Impact of Vaccination Programs

Measles vaccination has substantially reduced SSPE occurrence, with near elimination of cases after widespread vaccination implementation. 1, 5

• SSPE incidence in Israel dropped sharply in 1977, exactly 10 years (the median latency period) after mass vaccination introduction 6
• England and Wales showed a downward trend in SSPE incidence following measles reduction from vaccination 2, 3
• The United States experienced declining SSPE rates since 1969, corresponding with vaccine uptake 4

Common Pitfalls to Avoid

Do not confuse SSPE with acute post-vaccination adverse events, which if they occur (extremely rare at approximately 1 per 2 million doses), present around 10 days after vaccination, not years later 7:

• Febrile seizures occur 5-12 days post-MMR at approximately 1 per 3,000 doses and cause no residual neurologic disorders 7
• Vaccine-related adverse events cluster in the first 2-3 weeks post-vaccination 7
• SSPE develops years after measles infection (median 7-8 years), not after vaccination 2, 3, 4

Risk Factors for SSPE (All Related to Natural Measles)

The following increase SSPE risk after natural measles infection 2, 4:

• Measles before age 2 years: 46% of SSPE cases had measles under age 2 4
• Measles before age 1 year: 16-fold higher risk compared to measles after age 5 years 2
• Male sex: Male to female ratio of 2.8:1 2
• Birth order: Excess cases in third and subsequent children 2

The Bottom Line for Clinical Practice

Measles vaccination is the only effective prevention strategy for SSPE. 5, 7, 8 The extraordinarily rare cases of SSPE in vaccinated children represent either incomplete vaccine efficacy allowing breakthrough measles infection, or more commonly, unrecognized measles infection that occurred before vaccination 1, 6. The vaccine itself does not cause or accelerate SSPE development 1, 5, 6.