Detection of Immunity Gap before Measles Outbreak, Ho Chi Minh City, Vietnam, 2024

Abstract

In 2022, we established a residual sample serosurveillance program in Ho Chi Minh City, Vietnam. During September 2022-April 2024, we found low measles antibody seroprevalence in children in the city's western region, where a measles outbreak began in May 2024. Serosurveillance could be a useful tool for outbreak prediction and prevention.

Keywords: Measles; Vietnam; disease outbreaks; epidemiology; population surveillance; serology; viruses.

Figure 1

Hospital locations, catchment areas, and number of cases per age in detection of immunity gap before a measles outbreak, Ho Chi Minh City, Vietnam, 2024. A) Children’s Hospital 1; B) Children’s Hospital 2; C) City Children’s Hospital. The first column shows seroprevalence as a function of time of collection for 2 age bands from each hospital; blue rectangles indicate the latest collections; dashed red lines represent the 95% critical vaccination threshold for measles; tick marks at top and bottom of graphs denote seropositive and negative samples; points represent monthly aggregated seroprevalence; error bars indicate 95% CI of the aggregated seroprevalence; shaded areas indicate 95% CI of the seroprevalence estimates by time of collection. Before the measles outbreak, the levels of seroprevalences were 70.7% (95% CI 57.6–81.1) for Children’s Hospital 1 in April 2024 (A), 58.1% (95% CI 44.1–70.9) for Children’s Hospital 2 in December 2023 (B), and 41.5% (95% CI 29.4–54.1) for City Children’s Hospital in December 2023 (C). The middle column shows age-stratified seroprevalence at the most recent timepoint (represented in blue rectangles from the first column). The third column maps the catchment areas based on sample addresses.

Figure 2

Comparison of seroprevalence and vaccination coverage in a study of immunity gap before a measles outbreak, Ho Chi Minh City, Vietnam, 2024. A) Seroprevalence among children receiving first measles vaccine dose only; B) seroprevalence among children receiving first and second measles vaccine doses. Bars show the 95% CI of seroprevalence (vertical) and vaccine coverage (horizontal). Seroprevalence remained below the administrative vaccination coverage estimates for children who received both first and second measles doses but corresponded more closely with second-dose coverage. The blue dashed line indicates 1:1 relationship of vaccination coverage to seroprevalence. A wider view, administrative coverage data for each district and neighboring provinces, and antibody concentration of 17 children <9 months of age is available in the Appendix.