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

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. Author summary: In recent years, the reemergence of chikungunya has gained global attention and threatened to become a global outbreak. Although the epidemiology of chikungunya is known globally, the viral activity in Vietnam has not been thoroughly investigated. In this paper, we used information from a systematic review and serological survey to understand the past activity of chikungunya in Vietnam. In the serological survey, we tested age -stratified population serum samples in the central and southern parts of Vietnam from 2015 for chikungunya IgG, which is indicative of past exposure to chikungunya. Our results show almost no positive individuals below the age of 30 years old, indicating very low to no virus transmission for the last 30 years. In individuals over 30 years old, there were more positive individuals, and we estimate widespread transmission with an average of 2–4% of the population infected each year. Because of the lack of recent transmission in these locations, the susceptible population is increasing and is now high, suggesting Vietnam is vulnerable to future outbreaks if chikungunya is reintroduced. Hence, chikungunya surveillance along with surveillance for other arboviruses, such as dengue, should be conducted in Vietnam.