Recent seroepidemiological studies support dengue in Taiwan has not reached endemic yet

[[abstract]]Background: Dengue epidemics in southern Taiwan have been documented for decades and most outbreaks have been triggered by imported cases from Southeast Asia. Since 1998, most local dengue outbreaks have happened in different magnitudes and scales in southern Taiwan, with the two greatest numbers of cases occurring in Kaohsiung City and Tainan City. From 2014 to 2015, there had been two largest outbreaks caused by DENV-1 and DENV-2 in two consecutive years in Kaohsiung City, while Tainan City had encountered the most severe outbreak of DENV-2 in 2015. Methods and materials:: To understand the post-outbreak herd immunity against dengue virus (DENV) infection, we investigated the seroprevalence rates of DENV infection from 5253 serum samples of older residents in Kaohsiung and Tainan Cities. Seropositive samples were verified to exclude Japanese encephalitis virus (JEV) cross-reaction and the DENV-reconfirmed samples were used to identify viral serotypes and secondary infection. Results: The overall seroprevalence rates of DENV infection was 19.27%, from 5.88% among 80 years. Both DENV seroprevalence rates in Kaohsiung and Tainan increased with age and were significantly lower below 50 years of age than those 70 to 79 and > 80 age groups. Additionally, DENV-1, DENV-2, and DENV-3 covered wider age groups in Kaohsiung, whereas DENV-2 was dominant in Tainan. Conclusion: Both the overall and age-specific seroprevalence rates in southern Taiwan were much lower than those in many Southeast Asian countries. Besides, the predominant DENV serotype in each of the large-scale past epidemics of dengue in Taiwan is also different from multiple DENV serotypes have been co-circulating in many dengue-endemic countries in Southeast Asia. Therefore, our seroepidemiological results plus the predominant DENV serotype causing major outbreaks verify that dengue in Taiwan has not been endemic yet