Update: Sao Paulo Measles Outbreak

During 1997 through 20 January 1998, a provisional total of 26,722 confirmed measles cases was reported from the countries of the Americas. Of these, 25,599 (96%) were reported from Brazil. Of the Brazil cases, 20,459 (80%) occurred in the state of Sao Paulo. The outbreak began during late 1996 with a total of 27 confirmed cases. During 1997, cases were reported from over 250 of the state's 645 municipalities. Of the total cases, 18,542 (91%) were reported from the Greater Sao Paulo metropolitan area.

The age-groups most affected by the Sao Paulo outbreak were infants under 1 year of age, (440 cases/100,000 population), followed by young adults 20-29 years, (164 cases/100,000), children 1-4 years (47 cases/100,000 ) and children 5-9 years (32 cases/100,000).

As of 20 January, a total of 20 measles deaths were reported (1 death per 1,022 reported cases, total case-fatality rate of 0.10%); 17 (85%) were residents of the Greater Sao Paulo metropolitan area.

The age distribution of persons dying of measles as follows: 11 (55%) were infants less than one year of age, 3 (15%) were children 1-4 years of age, 2 (10%) were children 5-9 years of age, and 4 (20%) were young adults 20-29 years of age. The following age-specific case-fatality rates were observed: in infants < 1 year of age (0.38%), children 1-4 years of age (0.25%), children 5-9 years of age (0.20%) and young adults 20-29 years of age (0.04%).

The following strategies were implemented with the goal of reducing measles virus circulation:

• Lowering the age of routine measles vaccination from 9 months to 6 months.
• Selective vaccination of unvaccinated children under 5 years of age in June 1997 (161,987 doses administered).
• Vaccination of health workers (182,562 doses administered).
• Extended contact vaccination of persons under 30 years of age to reach those possibly exposed cases of measles, including households, neighborhoods, workplace and schools (856,534 doses administered).
• Indiscriminate vaccination of children 6 months through 4 years 11 months of age in August 1997 (3,085,221 doses administered). Coverage estimated to be 100% using official population data.
• Selective vaccination of school-aged children between 5-15, years of age between September and November 1997 (298,039 doses administered).
• Intensification of routine vaccination against measles for children between the ages of 9 and 15 months of age.

The Center for Epidemiological Surveillance of the Sao Paulo State Health Secretariat, in collaboration with the National Health Foundation of the Ministry of Health and the State Promotion for Mass Immunization and Education (FESIMA), along with PAHO is conducting a detailed study to determine the risk factors for acquiring measles in this outbreak. This study seeks to track the dynamic of measles virus transmission and other factors that may explain the occurrence of this epidemic.

Source: Sao Paulo State Health Secretariat, Brazil.

Editorial Note: Although the outbreak investigation is continuing, the Sao Paulo experience clearly demonstrates both the infectiousness and lethality of measles virus. Following a prolonged period of low measles incidence, the virus returned with a vengeance in Sao Paulo State. Measles has demonstrated its ability to find susceptible persons, even in areas with high vaccination coverage.

Several factors appear to have combined to create conditions which facilitated measles transmission in Sao Paulo. First, the failure to conduct a follow-up vaccination campaign in 1995, combined with low routine vaccination coverage ("keep-up" vaccination) among infants allowed for the accumulation of susceptible children in Sao Paulo. Second, the presence of large numbers of susceptible young adults who, for a variety of reasons, escaped both natural measles infection and measles vaccination increased the risk of a measles outbreak. Third, measles virus was imported into Sao Paulo, most probably from Europe. Finally, the high population density of the city facilitated contact between persons infected with measles and susceptible persons.

Available surveillance data suggest that the major outbreak control activities implemented in Sao Paulo helped to reduce the number of susceptibles and slow the epidemic. However, these control measures were very expensive in terms of financial and human resources, not to mention the opportunity costs of the interventions. Over 4.5 million persons were vaccinated in these efforts. Combined with the direct costs associated with medical care and the indirect costs due to decreased productivity, both acutely and chronically, this outbreak was very costly.

The overriding objective of PAHO's measles eradication strategy is the prevention of measles outbreaks. It is far better (and cheaper) to prevent an outbreak than to be forced to attempt to control an outbreak. Measles outbreaks can be prevented by achieving and maintaining of high population immunity in susceptible populations, combined with the absence of imported measles virus.

Sao Paulo will now need to redouble its efforts to prevent future measles outbreaks. High coverage levels of measles vaccination must be achieved and maintained for infants at their first birthday. Follow-up campaigns must be conducted every four years to assure the highest possible level of measles population immunity. A two-dose schedule is clearly not an appropriate measles eradication strategy unless nearly universal and coverage can be assured in a timely manner for both doses of measles vaccine. Moreover, efforts need to be made to assure immunity in adolescents and young adults who are at highest risk for exposure to measles virus. These interventions, combined with the reduction of measles importations from other regions of the world, will greatly decrease the risk of another major measles outbreak in Sao Paulo.