The TIRS trial: Enrollment procedures and baseline characterization of a pediatric cohort to quantify the epidemiologic impact of targeted indoor residual spraying on Aedes-borne viruses in Merida, Mexico

Abstract

Aedes mosquito-borne viruses (ABVs) place a substantial strain on public health resources in the Americas. Vector control of Aedes mosquitoes is an important public health strategy to decrease or prevent spread of ABVs. The ongoing Targeted Indoor Residual Spraying (TIRS) trial is an NIH-sponsored clinical trial to study the efficacy of a novel, proactive vector control technique to prevent dengue virus (DENV), Zika virus (ZIKV), and chikungunya virus (CHIKV) infections in the endemic city of Merida, Yucatan, Mexico. The primary outcome of the trial is laboratory-confirmed ABV infections in neighborhood clusters. Despite the difficulties caused by the COVID-19 pandemic, by early 2021 the TIRS trial completed enrollment of 4,792 children aged 2-15 years in 50 neighborhood clusters which were allocated to control or intervention arms via a covariate-constrained randomization algorithm. Here, we describe the makeup and ABV seroprevalence of participants and mosquito population characteristics in both arms before TIRS administration. Baseline surveys showed similar distribution of age, sex, and socio-economic factors between the arms. Serum samples from 1,399 children were tested by commercially available ELISAs for presence of anti-ABV antibodies. We found that 45.1% of children were seropositive for one or more flaviviruses and 24.0% were seropositive for CHIKV. Of the flavivirus-positive participants, most were positive for ZIKV-neutralizing antibodies by focus reduction neutralization testing which indicated a higher proportion of participants with previous ZIKV than DENV infections within the cohort. Both study arms had statistically similar seroprevalence for all viruses tested, similar socio-demographic compositions, similar levels of Ae. aegypti infestation, and similar observed mosquito susceptibility to insecticides. These findings describe a population with a high rate of previous exposure to ZIKV and lower titers of neutralizing antibodies against DENV serotypes, suggesting susceptibility to future outbreaks of flaviviruses is possible, but proactive vector control may mitigate these risks.

Fig 1. Distribution of blinded control and intervention clusters.

A map of Merida, Yucatan, Mexico with blinded control and intervention clusters (Arm A clusters in purple and Arm B clusters in blue) The pink coloring shows the ABV hotspot area identified for Merida [52]. The source of the census tract boundaries was the Instituto de Estadistica y Geografia (INEGI), 2010.

Fig 2. Summary of prospective mobility patterns of study participants in the TIRS trial.

(A) The proportion of time participants enrolled in each study arm spent at their house (from a 24h day). (B) The time all participants spent at home, separated by day of the week. (C) The frequency of visits (in times per week) to places other than their home.

Fig 3. Overall seroprevalence of participants by age.

Seroprevalence (%) or participants with antibodies against flavivirus (black) and CHIKV (blue) at the indicated age at time of sampling.

Fig 4. Flavivirus exposure history of trial participants.

(A) Calculated FRNT₅₀ titers of 631 flavivirus-reactive participants were determined for ZIKV and DENV serotypes 1–4. (B) Participants with FRNT₅₀ titers ≥4-fold higher against any one serotype were considered to be exposed to a monotypic infection while those with titers to two or more serotypes with <4-fold difference were considered multitypic. Multitypic participants were split into ZIKV-exposed (Multi ZIKV⁺) or unexposed (Multi ZIKV^-) based on whether ZIKV titers were within 4-fold of any DENV serotype titer. (C-D) Heatmaps of FRNT₅₀ titers against ZIKV and DENV 1–4 for all participants.

Fig 5. Density of mosquitoes in the TIRS clusters.

Boxplots showing the Median (Q1-Q3, range) of the number of eggs per ovitrap per year (during the five years before the start of the study), stratified by the blinded control and intervention study arms (red for Arm A, blue for Arm B). Each box includes the z-score and P-value of the parameter ‘Arm’ in the GLMM evaluating the difference in the number of eggs per trap by study arm.

Fig 6. Susceptibility of mosquitoes to insecticides.

Mortality (%) of Ae. aegypti exposed to different insecticide active ingredients using the CDC bottle bioassay. Dots indicate the mortality for each of the 16 clusters analyzed, colors differentiate between treatment and control clusters (blinded as Arm A and Arm B). Large dots and bars show the mean and 95% CI, respectively.