Evidence of previous but not current transmission of chikungunya virus in southern and central Vietnam: Results from a systematic review and a seroprevalence study in four locations

Abstract

Background: Arbovirus infections are a serious concern in tropical countries due to their high levels of transmission and morbidity. With the outbreaks of chikungunya (CHIKV) in surrounding regions in recent years and the fact that the environment in Vietnam is suitable for the vectors of CHIKV, the possibility of transmission of CHIKV in Vietnam is of great interest. However, information about CHIKV activity in Vietnam remains limited.

Methodology: In order to address this question, we performed a systematic review of CHIKV in Vietnam and a CHIKV seroprevalence survey. The seroprevalence survey tested for CHIKV IgG in population serum samples from individuals of all ages in 2015 from four locations in Vietnam.

Principal findings: The four locations were An Giang province (n = 137), Ho Chi Minh City (n = 136), Dak Lak province (n = 137), and Hue City (n = 136). The findings give us evidence of some CHIKV activity: 73/546 of overall samples were seropositive (13.4%). The age-adjusted seroprevalences were 12.30% (6.58-18.02), 13.42% (7.16-19.68), 7.97% (3.56-12.38), and 3.72% (1.75-5.69) in An Giang province, Ho Chi Minh City, Dak Lak province, and Hue City respectively. However, the age-stratified seroprevalence suggests that the last transmission ended around 30 years ago, consistent with results from the systematic review. We see no evidence for on-going transmission in three of the locations, though with some evidence of recent exposure in Dak Lak, most likely due to transmission in neighbouring countries. Before the 1980s, when transmission was occurring, we estimate on average 2-4% of the population were infected each year in HCMC and An Giang and Hue (though transmision ended earlier in Hue). We estimate lower transmission in Dak Lak, with around 1% of the population infected each year.

Conclusion: In conclusion, we find evidence of past CHIKV transmission in central and southern Vietnam, but no evidence of recent sustained transmission. When transmission of CHIKV did occur, it appeared to be widespread and affect a geographically diverse population. The estimated susceptibility of the population to chikungunya is continually increasing, therefore the possibility of future CHIKV transmission in Vietnam remains.

Fig 1. Locations of serum sample collection and recent outbreaks locations: From north to south, Hue City, Dak Lak province, Ho Chi Minh City, and An Giang province are shown in red.

Locations with recent outbreaks are shown in orange; Kampong Speu province in Cambodia (2012) and Champasak province in Laos (2012). The map was generated in R software version 3.3.3 [37].

Fig 2. Flowchart describing the systematic review procedure searching for information of CHIKV activity in Vietnam.

A) Searching procedure in Google news and ProMED. B) Searching procedure in international journals and national Vietnamese journals. Abbreviations: WOS = Web of Science, VJPM = Vietnam Journal of Preventive Medicine, and NLVN = National Library of Vietnam.

Fig 3. The stacked barplots of the numbers of negative, positive, borderline ELISA results by age group in each location: An Giang province, Ho Chi Minh City, Dak Lak province, and Hue City.

The colored bars represent the negative, positive, and borderline respectively (as shown in the legend).

Fig 4. Percentage of positive individuals and the binomial proportion confidence intervals by age group in each location: The binomial 95% confidence intervals are corrected by known 90% sensitivity and 90% specificity of the diagnosis test [69].

The colors represent each location, with red for An Giang province, brown for Dak Lak province, orange for Ho Chi Minh City, and purple for Hue City.

Fig 5. FOI estimates in each period by location from the best fit model.

The blue boxplots show the credible intervals with means, 1^st quartile and 3^rd quartile of annual FOI estimation by time period in each location.

Fig 6. Model fit of the best fitting model to the age-specific seropositive proportion in each location.

The black dots are the seropositive proportion in each age group along with corresponding binomial 95% confidence intervals, corrected by reported 90% sensitivity and 90% specificity of the test [69]. The black lines are the model output generated from the model simulated with the mean parameters estimates. The grey areas represent the 95% credible intervals.

Fig 7. The predicted susceptible proportions over time from the best model in each location.

In all locations, the boxplots represent the generated proportion with its 95% credible intervals (also shown 1^st quartile, 3^rd quartile, and the medians) with the solid lines showing the mean value of each interval. The two scenarios, with or without endemic transmission before in 1930 are shown in the plot by yellow and blue respectively. The black dots are the age-adjusted seropositive proportions from our serosurvey in 2015.