Recent studies indicate that declining levels of measles-specific IgG antibodies among vaccinated individuals without exposure to wild-type virus may reflect changes in population-level serological profiles, rather than direct measures of protective immunity. Such findings may have implications for understanding population susceptibility following viral introduction in measles-eliminated settings. This study aimed to analyze changes in the age distribution of protective measles antibodies in the population of Salvador, Brazil, between 2000 and 2024. A population-based seroprevalence study was conducted using a household-based cluster sampling design across 30 sentinel areas in Salvador, Brazil. A total of 1,508 individuals were included, and a comparable survey had been conducted in 2000. Data on sociodemographic characteristics, vaccination history, and prior measles infection were collected through household interviews. Venous blood samples were obtained for measurement of measles-specific IgG antibodies using ELISA. Seroprevalence estimates were calculated by age group, and logistic regression analysis was performed to identify factors associated with measles seropositivity. ELISA-based IgG detection does not directly measure functional neutralizing antibodies or protective immunity. Overall measles IgG seroprevalence was 82.4%, with antibody concentrations predominantly ranging from 275 to 1,000 IU/L. The highest seroprevalence was observed among individuals aged over 50 years (92.1%), whereas the highest proportion of seronegative individuals occurred in the 20–29-year age group (39.6%). This age group also showed the most pronounced decline in seropositivity between 2000 and 2024, from 95.7% to 60.4%. In multivariable analysis, only previous measles infection was positively associated with seropositivity (OR = 1.67; 95% CI: 1.04–2.66; p = 0.032). Although seroprevalence was higher than seronegativity in the study population, the observed levels remain below those generally considered sufficient to interrupt measles transmission (≈ 92–95%). These findings indicate important immunity gaps among young adults and children that may facilitate outbreak occurrence following virus introduction in large urban centers.
Vilar et al. (Wed,) studied this question.