Global Vaccine Safety

Subacute sclerosing panencephalitis and measles vaccination

Extract from report of GACVS meeting of 1-2 December 2005, published in the WHO Weekly epidemiological Record on 13 January 2006

The Committee reviewed the epidemiology of subacute sclerosing panencephalitis (SSPE) and the purported relationship between measles immunization and the occurrence of SSPE. The deliberations were considerably helped by a commissioned report presented by experts from the Health Protection Agency (HPA) of the United Kingdom. The meeting was joined by experts from the Division of Viral and Rickettsial Diseases, CDC National Center for Infectious Diseases (NCID), who agreed with the general conclusions and recommendations from the HPA experts. Evidence was provided that the true incidence of SSPE is approximately 4–11 cases per 100 000 cases of measles, although with measles infection acquired very early in life the risk may be higher (18 per 100 000 cases). A risk as high as 27.9 SSPE cases per 100 000 cases of measles has been cited. In many countries with good measles control, an increasing age at onset of SSPE has been observed attributable to cases that acquired measles infection at a time when the disease was more prevalent.

Available epidemiological data are consistent with a directly protective effect of vaccine against SSPE mediated by preventing measles. In countries with good measles control through vaccination, a decline in new SSPE cases is seen a few years after the decline in measles incidence. However, given the latency of SSPE following natural measles infection, it would take at least 5 years before an impact on SSPE incidence is seen, and more than 10 years before a large decrease is seen. Even with the elimination of measles, cases of SSPE may still occur 20 to 30 years after the last measles cases because of the skew of the latency distribution. Re-emergence of SSPE cases has been seen after outbreaks of measles following a period of good measles control. Available epidemiological data, in line with virus genotyping data, do not suggest that measles vaccine virus can cause SSPE. Furthermore, epidemiological data do not suggest that the administration of measles vaccine can accelerate the course of SSPE or trigger SSPE in an individual who would have developed the disease at a later time without immunization. Neither can the vaccine lead to the development of SSPE where it would not otherwise have occurred in a person who has already a benign persistent wild measles infection at the time of vaccination.

For situations where cases of SSPE occur in vaccinated individuals who have no previous history of natural measles infection, the available evidence points to natural measles infection as the cause of SSPE, not vaccine.