Study settings
An outbreak district (or District A) in Surat Thani province has a total area of 835.1 km2. It is characterized by plain areas surrounded by hills, forests, and rubber plantations. Its population of 50,905 resided in 17,337 households. The other outbreak district (or District B) is located in Narathiwat province, one of the southernmost provinces of Thailand. It has a total area of 372.6 km2. Small rivers run from the mountains creating peat swamp forests in the area. The local population of 47,965 resided in 11,285 households.21
During the outbreaks, local health officers followed the national guideline22 to control the infection. All index patients underwent reverse transcriptase-polymerase chain reaction (RT-PCR) testing for disease confirmation. The guideline also included the screening of their household contacts, and all pregnant women in the outbreak districts for the infection by the same test. Furthermore, intensive space spraying of insecticide and mosquito surveillance were implemented in the whole affected village area. Our research team retrospectively reviewed the medical records at one month after the end of the outbreak and had the meetings with the health officers to plan our current study.
The review and the meetings revealed that ZIKV infection were confirmed by RT-PCR in 24 and 18 patients in District A and B, respectively. The outbreak covered 12 villages in 6 subdistricts of District A and 5 villages in 3 subdistricts of District B. Figure 1 displays maps of both districts. White areas denote affected subdistricts with the number of confirmed ZIKV cases. Grey areas are subdistricts adjacent to the outbreak districts and black areas denote non-adjacent and non-affected subdistricts.
Study design
A cross-sectional serological survey was conducted in the two affected districts approximately 18 months after each outbreak. In each study district, we recruited two study populations, non-pregnant adults and pregnant women. Details in sampling technique for each group are as follow.
1. Non-pregnant adults
Based on the preceding outbreak records, we recruited all index cases, their household contacts, and a random sample of residents who lived varying distances from the nearest index case's house. All study subjects were at least 18 years old and lived in the area for more than 18 months. Exclusion criteria included pregnancy, immunodeficiency disease, and current use of immunosuppressive drugs.
In order to test our hypothesis, we stratified the non-case population into five groups based on the distance from their household to the house of the nearest index case.
- Household members of the index cases.
- Other residents of the subdistrict where the number of the confirmed cases was highest, who lived within 100 meters from an index case
- Similar to #2 but the distance to the nearest index case was between 101 and 400 meters
- Similar to #2 but the distance to the nearest index case was between 401 and 1,000 meters
- Residents of a randomly selected village in a non-affected and non-adjacent subdistrict (black areas shown in
Figure 1)
Based on the limitation of a finite number of cases and the population, for each district, we planned to recruit 400 eligible non-pregnant including all of the index’s household members (i) and addition to a hundred of their neighbors who lived within 100 meters (ii), and 100 non-pregnant subjects in each other three distance stratum (iii, iv, and v). /..Having a significance level set to 0.05 and a power of 80%, this sample size would allow us to detect a 20% difference in the prevalence of ZIKV neutralizing antibodies among the distance strata.
2. Pregnant women
All pregnant women aged 18 years or above, attending an antenatal care clinic, and living in one of the same districts as a case for more than 12 months were recruited. Exclusion criteria included known cases of ZIKV identified during the outbreak period, major psychiatric or physical illness, cognitive impairment, inability to communicate in Thai, immunocompromised, rheumatologic disorders, and autoimmune diseases. Four hundred pregnant women per district were recruited regardless of their gestational age and distance from their house to the house of the nearest index case. This sample size was calculated based on initial expected seroprevalence of 20%, +/- 4% and alpha = 0.05. They were treated as a separate stratum in the descriptive analysis, but we did not analyze for risk factors among pregnant women because the risk behavior information was not available.
Data collection
In the non-pregnant adult group, the survey was conducted in District A during March-May 2018 and District B during July-September 2018. A team of local health volunteers were trained as research assistants and instructed to recruit participants and conduct the interviews. The recruitment process involved visiting potential participants at their home, explaining to them the objectives of the study, and requesting their informed consent to participate in the study. Consenting participants were interviewed using a structured questionnaire to collect individual and household information. The distance between the center of each participant’s household and the nearest index case’s household was estimated using Google Maps®. The participants were invited to the health centers for venipuncture at the end of the week where a 10-mL blood sample was taken by a local health officer.
Between July 2018 and May 2019 in District A and B, consecutive pregnant women who attended the antenatal clinics were invited by the research team. Informed consent was obtained. Then a blood sample was taken for serological test. At least 30 minutes after venipuncture, each blood specimen was centrifuged at 3,200 revolutions per minute, divided into 4 aliquots and stored at -20◦C in a freezer at the district hospital, and finally shipped in lots to the Center of Vaccine Development (CVD), Institute of Molecular Biosciences, Mahidol University in Bangkok as it is the WHO-approved reference laboratory on serology and virology for arboviruses.
In the non-pregnant adult group, the survey was conducted in District A during March-May 2018 and District B during July-September 2018. A team of local health volunteers were trained as research assistants and instructed to recruit participants and conduct the interviews. The recruitment process involved visiting potential participants at their home, explaining to them the objectives of the study, and requesting their informed consent to participate in the study. Consenting participants were interviewed using a structured questionnaire to collect individual and household information. The distance between the center of each participant’s household and the nearest index case’s household was estimated using Google Maps®. The participants were invited to the health centers for venipuncture at the end of the week where a 10-mL blood sample was taken by a local health officer.
Between July 2018 and May 2019 in District A and B, consecutive pregnant women who attended the antenatal clinics were invited by the research team. Informed consent was obtained. Then a blood sample was taken for serological test. At least 30 minutes after venipuncture, each blood specimen was centrifuged at 3,200 revolutions per minute, divided into 4 aliquots and stored at -20◦C in a freezer at the district hospital, and finally shipped in lots to the Center of Vaccine Development (CVD), Institute of Molecular Biosciences, Mahidol University in Bangkok as it is the WHO-approved reference laboratory on serology and virology for arboviruses.
Laboratory tests
Plaque reduction neutralization tests (PRNT) for ZIKV-neutralizing antibodies were performed using the following procedures. Rhesus monkey kidney epithelial cells (LLC-MK2) were first seeded in 6-well plates at 1 × 105 cells/well for 7 days. The serum samples were four-fold serially diluted by phosphate buffer solution pH 7.5 with 30% fetal bovine serum, and then mixed with Zika virus strain MR766 at 50 plaque-forming unit (pfu)/well (for a final starting dilution of 1:10) for 1 hour. Following infection, cells were overlaid with Dulbecco's Modified Eagle Medium containing fetal bovine serum, 3.0% carboxymethyl cellulose, and neutral red. Plaques were visualized and counted at 7 days after infection. Probit analysis was used to determine the titer and interpreted as a PRNT50 and later on PRNT90 titer per a reviewer’s suggestion which is the reciprocal of the dilution showing a 50% and 90% reduction, respectively, in plaque count. A neutralization titer ≥1:10 by PRNT90 was considered as a seropositive.23,24
Random samples of 10 positive (PRNT90 titers> 1:10) and 10 negative (PRNT90 titer < 1:10) serum samples were used to further test for neutralizing antibody against dengue virus (DENV) serotype 1-4 (strain 16007,16681 16562, and C036/06 respectively) and Japanese encephalitis virus (Beijing strain).
Statistical analysis
The main outcome variable was whether the subject had a positive neutralizing antibody defined by a PRNT90 titer above 1:10. The detailed titer was further analyzed against the titer of neutralizing antibodies against other types of flavivirus.
The main independent variable was the distance from the participant's household to the nearest index case's house. Other independent variables were personal characteristics of the subjects such as age, occupation, behavior related to protective measures against mosquito bites such as the use of mosquito repellents, domestic garbage management, and self-reported history of dengue and chikungunya infection. Prevalence estimation, statistical tests, and the regression in non-pregnant data were computed using the ‘survey’ package to adjust the standard errors based on the sampling weights.25Variations in seroprevalence among different geographical locations were observed in previous studies,26–28 thus the seroprevalence in the two districts were described separately. For non-pregnant participants, the estimated prevalence of seropositive cases was stratified by the distance band between the household of the participant and the household of the nearest index case. Chi-square test was used to initially determine whether there is a significant difference between seroprevalence of affected and non-affected subdistrict. Proportional trend test was used to determine whether there is a linear trend in the prevalence across gradient of distance from the affected subdistrict.
In order to inquire more power to examine associated factors of seropositivity, the data of the two districts were combined and ‘district’ was handled as one of the independent variables. Predictors for seropositivity from non-pregnant adults were tested using the Rao-Scott Chi-Square test. Independent variables that showed an initial association with ZIKV seropositivity (P-value<0.2) were included in the multivariate logistic regression model to adjust for potential confounding effects. Likelihood ratio test (LR test) is used to test whether the model with that predictive factor is fit the data significantly better than the more restrictive model. Wald test is used to test whether removing of that level is substantially harm the fit of the model.
For cross-neutralization, titers of neutralizing antibodies against ZIKV were plotted against those of the flaviviruses, one-by-one, on a logarithmic scale.
Statistical significance was set at 0.05. All statistical analysis was undertaken using R software.