Analysis of data from a measles epidemic that occurred in California between 1988 and 1990 has revealed the greater-than-anticipated occurrence of subacute sclerosing panencephalitis (SSPE). Analysis of the 17 cases of SSPE suggests the rate could be as high as one in 600, depending on the age of the patients at the time of infection.
The finding, which once again emphasizes the importance of herd immunity, was presented by Kristen Wendorf, MD, MS, Immunization Branch, California Department of Public Health, Richmond, California at the annual conference of the Infectious Diseases Society of America (IDSA) in New Orleans, Louisiana, and was discussed in a press briefing by James Cherry, MD, University of California Los Angeles, one of the other study researchers.
SSPE develops years after measles has abated. Evident initially as behavioral changes that are not recognized as serious, the symptoms become more serious, and include seizures that become increasingly severe and more frequent with time. This complication of measles is always fatal; death occurs 1 to 3 years after diagnosis. Only after SSPE-associated death is the diagnosis definitive, with detection of characteristic brain changes and measles antibodies in the cerebrospinal fluid.
"We used to think it was quite rare," said Dr. Cherry. A prior study indicated a rate of one for every 1700 measles infections among children less than 5 years of age. Not only is the presently indicated rate much higher, but it could, in fact, be an underestimate, according to Dr. Cherry.
Many patients in the California outbreak had been vaccinated according to the inoculation schedule. However, they had been infected by the measles virus before the first injection of the live attenuated vaccine. If coverage of the vaccinated population had been near 100%, herd immunity would have been protective. The problem was the refusal by some parents to have their children immunized. If enough children go uninoculated, and particularly if they are concentrated in the same community or school, herd immunity can be lost.
The 17 cases of SSPE that occurred from 1998 through 2015 involved children born in the US in about half of the cases, with three-quarters being ethnic minorities. Measles infection that was confirmed or suspected based on febrile illness with rash occurred in 12 children who were younger than 15 months of age, with 11 being less than 1 year old.
Because the data is retrospective, the reported rate is not an absolute number. Still, the indication that SSPE is likely more prevalent than expected and the demonstration of the vulnerability of herd immunity are sobering.