The measles vaccine’s effectiveness during an epidemic: a test negative case control study (Tunisia, 2019)

BACKGROUND: Despite the availability of a safe and effective vaccine globally, measles remains a highly contagious viral disease with considerable mortality rates. Measles outbreaks occurred in many countries in 2019. This study aimed to describe the 2019 measles outbreak in Tunisia and to evaluate the effectiveness of measles immunization within the framework of an outbreak. METHODS: We performed a test negative case control study from January to December 2019. All suspected cases during 2019 epidemic in four governorates in Tunisia (Monastir, Mahdia, Kairouan, and Kasserine) and declared to the regional directorate of primary health care (RDPH) were included. The vaccine effectiveness (VE) was calculated using the standard case-control protocol, with the formula VE = (1- odds vaccinated/odds unvaccinated) × 100. In this context, the odds vaccinated/odds unvaccinated represents the adjusted odds ratio for receiving at least one dose of the measles vaccine in comparison to no doses. Data analysis was performed using SPSS version 21.0 software. RESULTS: During our study period, 2688 suspected cases were reported, of which 396 (14.7%) were laboratory confirmed through serological testing. Infants under one year old comprised 42.2% of confirmed cases. The hospitalization rate for suspected cases was 61.5% and 27 deaths were recorded. Among suspected cases with available vaccination records, the majority (70.1%) were unvaccinated. For confirmed measles cases, IgM + seropositivity rates were 81.8% for unvaccinated cohort and 38% in the vaccinated cohort. In outbreak context, the Vaccine Effectiveness (VE) on measles infection was 86.4% (95% CI: 79.2–91.1). The measles outbreak response immunization (ORI) effectiveness on the risk of infection was 97.8% (95% CI: 90.7–99.5). Global VE on the risk of hospitalization was 81% (95%CI: 64.6–89.8) with an ORI of 91.4% (95%CI: 69.0–98.0). CONCLUSION: The measles outbreak in Tunisia in 2019 led to significant hospitalizations and fatalities, notably affecting infants under 1 year old. The effectiveness of measles vaccination during this outbreak was influenced by vaccination coverage. Implementation of outbreak response immunization (ORI) effectively curbed disease transmission and bolstered herd immunity in the affected areas.