Sero-Prevalence Surveillance to Predict Vaccine-Preventable Disease Outbreaks; A Lesson from the 2014 Measles Epidemic in Northern Vietnam

Abstract

Background: During the first half of 2014, a severe outbreak of measles occurred in northern Vietnam, causing 15 033 confirmed cases and 146 deaths.

Methods: To evaluate the population-level seroprevalence of protection against measles in the period before the outbreak, we made use of an existing age-stratified serum bank, collected over the year before the outbreak, between November 2012 and December 2013, from 4 sites across the country (Hanoi, Hue, Dak Lak, and Ho Chi Minh City). Data from the UNICEF's Multiple Indicator Clustered Surveys (MICS), carried out in Vietnam during the first quarter of 2014, were used to assess the vaccine coverage in 6 ecological regions of Vietnam.

Results: Results revealed a large discrepancy between levels of protection, as estimated from the serology and vaccine coverage estimated by UNICEF's MICS. Variation in seroprevalence across locations and age groups corresponded with reported numbers of measles cases, most of which were among the 0-2-year-old age group and in the northern part of the country.

Conclusions: Our study presents a strong case in favor of a serosurveillance sentinel network that could be used to proactively tune vaccination policies and other public health interventions.

Keywords: Vietnam; measles; seroprevalence; vaccination.

Figure 1.

A, Map showing locations of the 4 sampled hospitals in Vietnam: Hanoi, Hue, Dak Lak, and Ho Chi Minh City. B, Weekly measles notifications for the 2014 epidemic in Vietnam. Gray dots show the timing of serum sampling in the 4 sampling sites (see Supplementary Figure 1 for more details), the two vertical lines show the timing of the UNICEF Multiple Indicator Clustered Surveys.

Figure 2.

A, B, Population size (A) and density (B) for Vietnam as from the 2009 population and housing census [34]. C, D, Incidence (C) and incidence rate (incidence divided by population size) (D) for the 2014 epidemic, as reported by the World Health Organization’s Expanded Programme on Immunization. Yellow triangles show the 4 serum sampling sites of 2013 (Figure 1A).

Figure 3.

Estimated levels of seroprevalence (circles) together with 95% confidence intervals (error bars) by age class in years (x-axes) for the 200- and 275-IU/L thresholds (first and second rows, respectively) for the 4 locations (by columns). Vertical dashed gray lines show the age classes.

Figure 4.

Population protection and vaccine coverage against measles in 2013–2014. The map on the left shows the 6 ecological regions of Vietnam, color-coded, together with the serum sampling sites (red triangles) (Figure 1A). The chart on the right shows the estimates (circles) with 95% confidence intervals (horizontal bars) of the seroprevalence (white background), taken from Figure 3, and the vaccine coverage (colored background), increasing from 0% to 100% from left to right, and ordered by age class from bottom to top (see the legend on the right). For the seroprevalence estimates, both the 275-IU/L (bottom) and 200-IU/L (top) thresholds are shown for each age class. For the vaccine coverage estimates, both the rural (bottom) and urban (top) strata are shown for each age class.