Dengue and CHIKV infections are often mild, and may be undiagnosed or misdiagnosed. Hence, we have only considered those who self-reported that they were treated for dengue or CHIKV. Dengue is the dominant vector-borne viral disease in India; population level serosurvey carried out in 2017-2018 (5-45 years) showed 48.7% seropositivity for dengue  vs. 18.1% for CHIKV . Analysis of LASI data shows dengue and CHIKV prevalence to be 0.87% and 2.3%, respectively. Dengue is endemic in most States of India , and population level serosurvey carried out in 2017-2018 in the age group of 5-45 years has reported a seropositivity of 60.3%, 5%, 18.3%, 62.3% and 76.9% in the Northern, North-Eastern, Eastern, Western and Southern regions respectively, with an overall 48.7% seropositivity for India . The low self-reported prevalence could be due to the high seropositivity across India, except for the North-Eastern and Eastern regions. The North Indian States of Delhi, Uttar Pradesh, Punjab and Haryana are the only ones to report ≥2% prevalence. Delhi is highly endemic for dengue, and multiple serotypes co-circulate . Secondary infections resulting in severe dengue illness are known to occur due to the circulation of numerous serotypes , and may explain the highest self-reported prevalence (5.6%) in Delhi.
The high prevalence of CHIKV could be explained by the study period of the LASI survey. Even though, the LASI survey was carried out in 2017-18, the respondents were asked to self-report if they had the disease in the preceding two years. In 2016, there was a massive outbreak of CHIKV in North India [26, 27]. The highest prevalence (>4%) of the self-reported CHIKV cases were in the northern States of Delhi, Uttar Pradesh, Haryana and Rajasthan. Even though, the population level serosurvey shows South India to have the highest seropositivity (43.1%) , the self-reported cases in the LASI survey are lower. The Southern States were the most affected in the CHIKV outbreak in 2005-06 [28–30]. A multicentric hospital-based study carried out in 2008-2009 to detect CHIKV cases by RT-PCR and/ or IgM-ELISA reported highest positive cases in South India (49.36%), followed by West (16.28%), and the lowest was in North (0.56%) . Prior exposure to CHIKV could explain the low self-reported prevalence rates in the South when compared to North India. The eastern States of Odisha and West Bengal, and the adjacent States of Bihar, Jharkhand and Chhattisgarh have <1% prevalence, and this overlaps well with the 4.4% seropositivity in the East . Similarly, the prevalence was 0% in the North-East, and is in line with the 0.3% seropositivity in this region . In line with the population level serosurvey data , LASI survey shows the Eastern and the North-Eastern region of India to have low prevalence of CHIKV, and are susceptible to future outbreaks.
Analysis of LASI data indicates urban residence, wealth, education and location of water-source to be the common risk factors for dengue and CHIKV in India. In addition, adults (45-54 years) are also at more risk for dengue, while for CHIKV, caste (SC and forward), pucca/semi-pucca house type are additional risk factors. Among the various factors of dengue transmission, urbanization, globalization and lack of effective vector control are considered to be the three major drivers . Ae. aegypti, the primary driver of dengue and CHIKV lives in urban and peri-urban human habitation. In urban tropics, large swathes of human and Ae. aegypti population live in intimate association, and provide the perfect setting for the maintenance and generation of epidemic strains of vector-borne viruses [32, 33]. In this analysis, urban residence increases the odds for both dengue and CHIKV. Positive association has been reported with dengue and CHIKV prevalence, and population density [34–38]. Even though dengue is present both in rural and urban India, incidence in urban areas are much higher; a nation-wide dengue serosurvey has recorded 70.9% (64·3–76·6) seropositivity in urban compared to 42.3% (36·0–48·9) in rural districts . The urban incidence of CHIKV is even higher; 40·2% (31·7–49·3) in urban vs. 11·5% (8·8–15·0) in rural . Ae. aegypti’s breeding preferences coupled with population density makes urban areas a significant risk factor for vector-borne viral diseases in India. Among all the States and UT of India, the National Capital Territory of Delhi and the Union Territory of Chandigarh are most urbanized with 97.5% and 97.25% urban population respectively, followed by Daman and Diu at 75.2% . Delhi shares borders with Haryana and Uttar Pradesh, and the urban expansion has accelerated in the border regions of these States . Thus, this region has emerged as hot spot of dengue and CHIKV prevalence in the country. Even though Himachal Pradesh is bordering this hot spotregion, the level of urbanization in Himachal Pradesh is least (10%) in the country, and this could explain the low period prevalence of dengue and CHIKV. Overall, urbanization appears to be a major driver of dengue and CHIKV.
Population based national serosurveys show that the incidence of dengue and CHIKV increases with age; compared to 5-8 (Dengue: 28.3%; CHIKV: 9.2%) and 9-17 (Dengue: 41.0%; CHIKV: 14%), seropositivity is high in the 18-45 age group (Dengue: 56.2%; CHIKV: 21.6%) [7, 12]. The age group more susceptible to clinical dengue infection varies among different geographical regions, and is influenced by host immunity and the circulating viral genotypes. Epidemiology of the 2017 dengue outbreak in Sri Lanka show adults ≥ 50 years are least affected . In Taiwan, dengue prevalence from 2010-2015 show significantly higher prevalence rates in adults ≥ 60 years . Cyclical pattern of dengue epidemics driven by DENV-1 and DENV-2 serotypes have been observed in Singapore from 2004-2016; in DENV-2 predominant years (2007-12 and 2016), the incidence rate of dengue in 55+ age group is almost equal to the 15-24 years age group, while in DENV-1 predominant years (2004-2006 and 2013-2015), the incidence rate in 55+ years is about half . In the 2007 epidemic in Brazil, there was a shift in the age pattern, with dengue hemorrhagic fever affecting predominantly children<15 (>53%), compared to 22.6% in 2001 . For pan-India, reliable estimates of age-stratified dengue caseloads are not available in the public domain. A nine year (2007-2015) dengue trend in Mumbai, western India shows dengue morbidity to be highest in young adults aged 21-40 years . Analysis of the LASI data among the three age groups (45-54, 55-69 and ≥70) shows adults in the 45-54 years age group to have higher odds for dengue. One possible reason for the higher likelihood in this group could be their active life style related to employment, which would also make them travel frequently. A case-control study in Odisha, India shows the odds of dengue are three times higher in individuals whose work requires long travel .
Location of water source outside the house was found to increase the odds of both dengue and CHIKV. An individual-level cohort study carried out in Vietnam shows households that do not have access to tap water close to their dwelling have increased risk of dengue fever . Lack of access to piped water supply will lead to households resorting to using containers for water-storage; these storage containers will provide the ideal breeding sites for mosquitoes resulting in increased dengue risk for the household . A retrospective study carried out in Delhi has identified lack of access to tap water to be a key factor in dengue IgG seropositivity . Lack of proper toilet facility in the household also increases the likelihood of CHIKV. Ae. aegypti’s peak biting periods are early in the Morning, and in the period before dusk ; the need to use outside toilet facilities increases the likelihood of mosquito bites and vector-borne diseases.
Individuals with less than 6 years of schooling have higher odds of dengue and CHIKV. Several studies have shown the association between low education levels and dengue [20, 48, 49]. Education helps in understanding the etiology of the disease, mode of transmission, symptoms, treatment, prevention and control measures . Wealthy households have higher odds of dengue and CHIKV. Also, residents in pucca houses have higher likelihood of getting infected with CHIKV. Possible reasons include: 1) wealth is likely to be positively associated with urban residence; both dengue and CHIKV have higher prevalence in densely populated urban settings in India, and 2) health seeking behaviour may be better in wealthy households. In Delhi, dengue burden was higher in wealthier districts despite lower mosquito load . In contrast, low SES is shown to be a key risk factor of dengue in Brazil [21, 22, 48, 50] and Cuba . Unlike dengue hemorrhagic fever and dengue shock syndrome, dengue fever is self-limiting characterized by fever, myalgia, headache and constitutional systems . The well-educated individuals from wealthy urban background are more likely to get diagnosed promptly compared to the lower socio-economic class, and this may have increased the odds of dengue and CHIKV in the former. Future studies in different SES settings of India should be carried out to better understand the association between SES and dengue/CHIKV incidence.
Among the different social groups, ST have lower odds of CHIKV. The forest dominated Northeast (except Assam) and Central India States (Chhattisgarh, Jharkhand, Odisha, and Madhya Pradesh), Odisha and the have a high percentage (>20%) of ST , and malaria [54–57]. Except Madhya Pradesh (2.9%), the CHIKV prevalence is very low in all the other ST dominated States. Furthermore, the share of the ST population in urban areas is a meager (2.4%) and could be a key reason behind the lower odds in the ST .