No abstract

Elimination of an infectious disease is often understood to mean the total absence of cases in a population. This situation can occur only if the entire population is immune as a result of either natural disease or vaccination. However, this costly and unrealistic scenario is not necessary to ensure elimination, more appropriately defined as a situation in which sustained transmission cannot occur and secondary spread from importations of disease will end naturally, without intervention. The authors describe the size and duration of outbreaks caused by imported infections after indigenous transmission has been eliminated. They show that the status of the elimination process can be monitored by assessing the proportion of cases imported and the distribution of outbreak sizes. Measles in Canada, the United States, and the United Kingdom provides a good example of the relevance of these criteria. Surveillance of the size and duration of these outbreaks enables maintenance of elimination to be monitored.

A modified cohort method has been proposed for estimating the relative incidence of rare adverse reactions after vaccination. The method requires only a sample of the cases, thus avoiding the need for following large population cohorts or selecting controls. This case series method has statistical power equivalent to that of the full cohort method when the risk periods after vaccination are short and vaccine coverage is high. The method also eliminates confounding by variables associated with both the outcome and avoidance of vaccination. In this paper, the cohort, case-control, and case series methods are reviewed, and their underlying assumptions and performances are compared. Theoretical results are illustrated using data on febrile convulsions after measles-mumps-rubella vaccination in the United Kingdom.

Laboratory reports and data on hospital admissions were used to estimate the number of hospitalizations due to group A rotavirus infection in England and Wales. Between January 1990 and December 1994, there were 75,059 laboratory reports of rotavirus infection, and 66,062 of these were in children <5 years old; rotavirus represented 39% of all pathogens identified in fecal specimens from this age group. Between April 1993 and March 1994, 1904 hospital admissions coded as "infectious intestinal disease" and 2354 coded as "noninfective gastroenteritis" occurred in children<5 in the North Thames region (a health authority representing 13% of the population in England and Wales). By modeling admission and laboratory reporting data, it was estimated that 54% of hospitalizations for intestinal infectious disease and 34% for noninfective gastroenteritis were attributable to rotavirus. By extrapolation of the North Thames data, it was estimated that 17,810 rotavirus-related hospitalizations (5/1000 children <5 years old) occurred in England and Wales during the same period. Effective vaccines have the potential to substantially reduce the number of hospital admissions due to group A rotavirus infection.

A mass immunization campaign with a Urabe-containing measles-mumps-rubella vaccine was carried out in 1997 in the city of Salvador, northeastern Brazil, with a target population of children aged 1-11 years. There was an outbreak of aseptic meningitis following the mass campaign. Cases of aseptic meningitis were ascertained through data collected from the records of children admitted to the local referral hospital for infectious diseases between March and October of 1997, using previously defined eligibility criteria. Vaccination histories were obtained through home visits or telephone calls. Eighty-seven cases fulfilled the study criteria. Of those, 58 cases were diagnosed after the vaccination campaign. An elevated risk of aseptic meningitis was observed 3 weeks after Brazil's national vaccination day compared with the risk in the prevaccination period (relative risk = 14.3; 95% confidence interval: 7.9, 25.7). This result was confirmed by a case series analysis (relative risk = 30.4; 95% confidence interval: 11.5, 80.8). The estimated risk of aseptic meningitis was 1 in 14,000 doses. This study confirms a link between measles-mumps-rubella vaccination and aseptic meningitis. The authors discuss the implications of this for the organization and planning of mass immunization campaigns.