Because persons have apparently contracted SSPE without ever knowingly having had measles, it could not be ruled out that the measles vaccine strain caused the infection. William Bellini, PhD, and colleagues at the Centers for Disease Control and Prevention (CDC), sought to evaluate that notion.
Brain tissue specimens from 11 patients suspected of having SSPE were examined. Five of the 11 patients with samples and 7 additional SSPE patients identified in case reports were related to infections during the resurgence of measles in the United States during a drop in measles vaccination rates between 1989 and 1991. The ages of the patients ranged from 5 to 36 years, with a mean of 14.
The researchers discovered wild-type measles virus in brain tissues from individuals with SSPE who had no previous diagnosis of measles and had been vaccinated. Case histories and demographics, when available, suggested that most of the individuals had very likely contracted measles prior to being vaccinated.
The fact that 12 SSPE patients identified in the study had measles between 1989 and 1991 raises the incidence of measles-induced SSPE to a level approximately 10 times higher than the statistic often cited. That figure, based on data available in 1982, estimated that 8.5 in 1 million persons contracted SSPE, a rate grossly underestimating the risk, now thought to be between 7 and 11 per 100,000.
The higher incidence could be due to an underreporting of diagnosed measles cases, despite clinical guidelines on reporting infections. Other possibilities include a higher incidence of measles during the 1989-1991 resurgence in the population with the highest risk for SSPE, namely children under the age of five, or because wild-type viruses of the genotype identified are more likely to cause SSPE.
The measles vaccine is known to be highly effective. Two doses provide immunity to 99 percent of those vaccinated. Since the risk of SSPE was much higher than originally estimated, and since the vaccine had not caused SSPE in the cases studied, the researchers concluded that measles vaccination programs prevent many more of the fatal brain infections than previously thought.
In an accompanying editorial, Samuel L Katz, MD, of Duke University, who was part of the team that developed the measles vaccine, strongly urged support for global efforts to reduce measles mortality through immunization programs. Despite recent reductions in the worldwide toll of measles, he noted, the disease still accounts for almost half of the 1.6 million annual childhood deaths due to vaccine-preventable diseases.