Arboviral diseases, including dengue and chikungunya, are a certain public health concern in many tropical and sub-tropical countries [
1,
2].The main vector of dengue and chikungunya is
Aedesaegypti. However,
Aedesalbopictus can harbor and transmit viruses [
3,
4]. Also, the viruses may be directly transmitted through blood donation by asymptomatic donors [
5]. Dengue and chikungunya viruses have similar transmission modes, same vector and pathological mechanisms, and clinical presentations [
4]. The two febrile diseases are characterized by acute fever, high body temperature above 40
°C, muscle pain and headache, backache, and skin rashes [
6]. Infection by any of the four dengue serotypes may be occurring with different clinical presentations and often with unpredictable clinical evaluation and outcome. Therefore, dengue case is classified as dengue fever with or without warning signs and severe dengue characterized by severe plasma leakage, severe hemorrhagic, and severe organ impairment [
7]. In most cases, chikungunya is a self-limiting disease, though its complication, mainly joint pain, can persist for months or years post-infection, especially for older age [
8,
9].
Arboviral disease transmission is often heterogeneous due to the vectors and host distribution and underlying social and ecological determinants [
10]. The recent epidemics caused by these arboviruses have been associated with many factors, including urban expansions, population growth, and international travel and trade, which facilitate the spread of vectors and arboviruses into new niches amplification through the human-vector-human cycle [
4]. Also, the areas with high temperatures and heavy rainfall followed by flooding are most favorable for mosquitoes' growth and survival [
11]. Evidence of abundances of Aedes mosquitos has been documented in Zanzibar, where out of 200 samples, 124 (62%) were positive for immature stages of mosquitoes, of which 114 (94%) were positive for
Aedesaegypti larvae and pupae [
12]. In Tanzania mainland, a study conducted in the Morogoro region reported that immature
Aedes mosquitoes were present in breeding sites during the rainy season (18.87%) and dry season (4.64%) [
13]. Recent reports on dengue and chikungunya outbreaks show that diseases have spread in many parts globally, including Asia, the Pacific, Europe, and Africa [
14‐
16]. Dengue outbreaks were reported in 2010, 2014, and more recently in 2018 in Tanzania [
17]. The outbreak of 2019 was the worst documented dengue outbreak. Dar es Salaam was the epicenter, followed by Tanga with 6873 cases and 13 death reported [
18]. The outbreak of chikungunya in Zanzibar was reported on 4th May 2018, with around 50 cases per day seen in MnaziMmoja referral Hospital [
18]. Some studies have also documented chikungunya seroprevalence in different parts of Tanzania Mainland [
20,
21].
Dengue cases associated with transfusions and transplantations have been reported [
22,
23]. While these cases may not by themselves cause substantial public health alarm, but they may indicate possible future outbreak which may have huge public health and economic consequences. This study aimed to determine dengue and chikungunya’s seroprevalence in blood donors using the stored blood samples from Temeke referral hospital in Dar es Salaam and Zanzibar National Blood Bank in Zanzibar.