Overview of the workshop

The PAC-SIT workshop aimed to advance the implementation of SIT against Aedes species mosquitoes. The objectives of the meeting were to review principles and components of SIT, share experiences and case studies, learn through on-site observation; discuss challenges and officially launch the PAC-SIT project. The workshop began with welcoming addresses given by senior officials from the hosting institution, government of French Polynesia, CDC, IAEA, TDR and WHO, highlighting the importance of controlling arboviral diseases and potential of SIT as a tool for durable control. Workshop sessions dealt with SIT implementation, country and regional experiences, and working groups on community engagement, epidemiology, and entomological considerations for SIT trials (Fig. 1). Central to discussion were the PAC-SIT Phase 3 trials in French Polynesia and the Cook Islands to evaluate the impact of SIT on both mosquito populations and human disease. Participants were given the opportunity to visit the Medical Entomology Laboratory at Institut Louis Malardé, which specializes in operational research on innovative mosquito surveillance and control, and its production facilities for SIT- and Wolbachia- mosquitoes. Overall, the workshop provided a collaborative platform for researchers and stakeholders to share knowledge and insights on SIT implementation, contributing to sustainable health outcomes.

Fig. 1

Logic diagram and overview of the PAC-SIT workshop structure. The workshop comprised topical sessions on various aspects of the sterile insect technique (SIT) implementation. The sessions provided contextual information, delved into specifics of SIT trials, discussed challenges and solutions, offered opportunities to visit field sites and production facilities, and concluded with recommendations for SIT development and the PAC-SIT project. PAC-SIT Pacific Islands Consortium for the Evaluation of Aedes SIT

SIT implementation

Members of the SIT Scientific Advisory Committee presented key SIT implementation topics, particularly relating to SIT evaluation and program development (Fig. 2). A major challenge in SIT and more generally of mosquito “Rear and Release” technologies is mass production of field competitive sterile male mosquitoes. Experts addressed building and running a mass production facility, irradiation processes, and quality control. Presentations highlighted best practices for trials to collect data on entomological impact, epidemiological trial principles and study designs, factors to consider for cost effectiveness analyses, and key approaches for community engagement, such as stakeholder mapping.

Fig. 2

Sterile insect technique development and evaluation phases, adapted from references [11, 12]. SIT sterile insect technique; WHO World Health Organization

SIT trials

Countries invited to the workshop presented SIT trials conducted in their regions of the world. The presentations were not exhaustive of all SIT field trials ongoing, instead presenting specific country experiences across various phases of SIT testing [11]. In addition to the trials described below, the PAC-SIT project plan and monitoring strategy was presented.

Despite a broad range of suppression trials conducted worldwide, robust evidence on the impact of SIT on disease epidemiology remains to be demonstrated. The PAC-SIT plan to implement SIT in two Pacific Islands to assess its effects on vector mosquito populations and human disease represents a significant step forward. This Phase 3 trial targets Ae. aegypti in Paea, Tahiti (population 13,000), French Polynesia and Aitutaki, Cook Islands (population 2200), both of which are experiencing dengue transmission this year. The study will use an integrated vector management (IVM) approach that combines community participation to remove larval containers around households with the release of sterile males. Mass production and irradiation of mosquitoes will be conducted by Institut Louis Malardé in Tahiti. ILM has been producing and releasing Ae. polynesiensis incompatible male mosquitoes since 2015. With advanced mosquito production and sterilization technology, their facility aims to produce up to 1,000,000 sterile Ae. aegypti males per week for the study. Weekly ground releases of chilled and compacted sterile adult males will be conducted, adjusting the releases based on field sterility levels inferred from ovitraps.

In both Paea and Aitutaki, study areas will span approximately 170 hectares per treatment, including buffer zones. The releases will occur over a 12-month implementation period with a release ratio of 10∶1 for sterile∶wild type males. Epidemiological metrics will include dengue seroprevalence and anti-mosquito saliva antibodies as a proxy for exposure to Ae. aegypti bites [13]. Entomological metrics will include egg and adult abundance, and bi-annual mark release and recapture (MRR) studies. Community engagement and acceptance was secured through stakeholder analysis and the development of a communication plan with culturally relevant communication materials. (Dr. Hervé Bossin and Dr. Françoise Mathieu-Daudé, Institut Louis Malardé, French Polynesia, unpublished).

African region (AFR) SIT Programs

Historically, SIT was successful against tsetse fly vectors in the African region. SIT elimination of tsetse from the island of Unguja, Zanzibar allowed farmers to increase production in the absence of animal trypanosomiasis [14, 15]. More recently, tsetse were eradicated from a 1000 km2 area around Dakar in Senegal [16], completely interrupting transmission of trypanosomiasis in cattle [17].

For mosquitoes, SIT against Ae. albopictus has been tested in La Réunion Island in the Indian Ocean [18], currently expanding to Phase 3 trials (pre-operational). The Phase 2 trials (small scale field trials) under controlled conditions carried out 2021–2022 were preceded by MRR studies in 2019 and 2021 and studies measuring egg counts, hatch rates, and adult abundance in response to SIT. The Phase 2 study included an immuno-epidemiological evaluation of Ae. albopictus-specific antibody responses as a measure of human-vector contact, and a knowledge, attitudes, and practices (KAP) survey to gauge community acceptability and support. The trial covered an area of 20 hectares, with weekly releases of 120,000–300,000 males, averaging 6000–15,000 males released per hectare. A Phase 3 study under natural conditions is planned and a new “boosted SIT” strategy that pairs SIT with Insect growth regulator (e.g. pyriproxyfen) autodissemination [19] will be tested (Dr. Frédéric Simard, Research Institute for Development, France, unpublished).

American region (AMR) SIT programs

All countries performing SIT trials in the American region are at Phase 2 and some will move to Phase 3 in the near future.

In Cuba, the Instituto Pedro Kouri and the national integrated mosquito management program conducted a Phase 2 trial against Ae. aegypti in Havana from February to September 2020 [20]. 40,000–80,000 sterile males were released twice a week, achieving up to 80% suppression. The entomological metrics were egg counts, hatch rates, induced sterility, and sterile male dispersion, survival, and competitiveness measured by MRR. SIT releases were consequently expanded across Havana in 2022 to evaluate the effectiveness of SIT to reduce mosquito abundance and dengue transmission (Dr. Rene Gato, Institute of Tropical Medicine Pedro Kourí [21]).

In Brazil (Recife and Juazeiro), a Phase 2 Ae. aegypti SIT trial was performed from October 2021 to January 2022. Efforts to optimize production, transport, and release procedures were presented. Before mosquito release, wild populations were suppressed with an integrated vector management (IVM) approach utilizing larvicides and mosquito-disseminated pyriproxyfen [22, 23]. Sterile males were released one to two times a week, with 0.5–0.7 million mosquitoes released per week, 5000–9000 males per hectare. Data collected included egg and adult abundance and hatch rates, and a MRR study in March 2021. Engagement of community leaders and local authorities, and public education is ongoing. The importance of applying SIT in an IVM framework was highlighted. The next steps for SIT implementation will be to expand to more areas in Brazil (Dr. Virginio Jair, Moscamed, Brazil, unpublished).

In the United States of America (USA), a Phase 2 Ae. aegypti trial on Captiva Island, Florida was implemented between 2020 and 2022. Sterile males were released across 300 hectares (3 km2) for both treatment and control zones, with releases exceeding 9 million mosquitoes in 2022. Entomological metrics included egg and adult abundance, and quarterly MRR studies [24]. Community outreach was conducted via town hall and local stakeholder meetings, and media outlets. Due to damage from Hurricane Ian in September 2022, the trial was relocated to Fort Myers, Florida (presented by Ms. Nicole Foley, CDC on behalf of Mrs. Rachel Morreale and Dr. David Hoel, Lee County Mosquito Control District, FL, unpublished).

European region (EUR) SIT programs

In Europe, where the establishment of invasive Aedes species in some areas is a concern, SIT could be an important tool for integrated mosquito management, advancing EU policy objectives to reduce biocide usage [Regulation (EU) No 528/2012] and proactively mitigating risk of outbreaks from introduced dengue, Zika, yellow fever and chikungunya. Several countries have initiated SIT programs, and most are in Phases 1 and 2 of development and testing. Notably, Italy and Switzerland have advanced to Phase 3, focusing on vector control to reduce nuisance biting rather than emphasizing epidemiological outcomes. This focus underscores regional priorities in these locales, where invasive Aedes species have established yet Aedes-borne diseases pose minimal public health threats compared to endemic regions like Asia or Latin America. In Ticino, Switzerland, a Phase 3 Ae. albopictus SIT trial occurred April through October 2023. Sterile males were produced in and transported from Italy (Centre Agricoltura Ambiente). 3000 males were released per hectare, with a total of 15,000 released per week. Entomological outcomes included fertility, dispersion, and survival. The importance of IVM in controlling local Ae. albopictus [25] was emphasized. (Dr. Elenora Flacio, University of Applied Sciences and Arts of Southern Switzerland, unpublished).

Southeast Asia region (SEAR) SIT programs

In Sri Lanka, a Phase 2 Ae. albopictus trial in Gampaha District has been completed [26]. The country utilizes IVM to control Ae. albopictus, however, insecticide resistance has hindered effective control, leading to the implementation of a SIT pilot during 2020–2021. Community engagement was conducted via door-to-door visits and focus groups. The trial area spanned 30 hectares with a target of 100,000 sterile males released per week. Entomological indicators included egg counts and hatch rates, adult abundance, and MRR [27] for sterile male dispersion and survival. Sri Lanka is developing production and irradiation capabilities to support a Phase 3 trial with expanded release areas (300 ha). (Dr. Anoja F. Dheerasinghe, Ministry of Health, Sri Lanka, unpublished).

Western Pacific (WPR) SIT programs

Indonesia started Ae. aegypti Phase 1 trials with laboratory activities in 2021 and moved to Phase 2 in 2022. The trial area covers 6.48 hectares for Phase 1 and 3.25 hectares for Phase 2, with 5881 males per hectare released in Phase 1 and 8970 males per hectare released in Phase 2. The trials monitored egg counts and hatch rates, and adult abundance. Community engagement with key stakeholders included a KAP survey to gauge community understanding of dengue and SIT. Plans for an epidemiological trial are underway (Mr. Hadian Iman Sasmita, National Research and Innovation Agency, Indonesia, unpublished).

In Malaysia, sterile males were released during 2019–2021 [28] and entomological data collected on larval counts and sterile male competitiveness, survival, and dispersal. The trial areas spanned from 4.55 to 8.41 hectares and between 1300–8000 males/hectare were released, totaling up to 2.2 million sterile males per study site. The country is now testing SIT under controlled field conditions (Phase 2), and presented staffing and training needs for the trials against Ae. aegypti in Malaka, Malaysia (Dr. Cheong Yoon Ling, Institute for Medical Research, Malaysia, unpublished).

Singapore is one of the countries most advanced in SIT testing, currently in Phase 3 for the SIT-incompatible insect technique (IIT) against Ae. aegypti. The SIT-IIT strategy involves releasing males that are both X-ray-sterilized and Wolbachia-infected to reduce mosquito populations [29]. For mosquito population surveillance, Singapore validated a Gravitrap Aegypti Index (GAI) [30]. National Environment Agency (Singapore) tested different mosquito release strategies ("rolling-carpet” and “targeted”) before starting this field study (NEA | Multi-site Field Study). The current Phase 3 study is a two-arm, cluster randomized trial that compares dengue case incidence between sites [31, 32]. Communities were surveyed for awareness, attitudes, and knowledge about SIT-IIT [33]. Engaging the community has been essential to the success of Singapore’s SIT-IIT program (Mr. Youming Ng, National Environment Agency, Singapore, unpublished).

Challenges and solutions

Following the country SIT trials status updates, working groups were convened to discuss community engagement, epidemiology, and entomological considerations for SIT trials. An overview of priority challenges and solutions is given in Table 1.

Table 1 Summary outcomes of working group discussions on community engagement, entomology and epidemiology

The community engagement working group reviewed effective community engagement strategies applicable to SIT, stressing the importance of understanding the socio-cultural, historical, political, and economic context. They recommended involving a dedicated social scientist and communication focal point to support projects, along with the need for guidelines on developing, implementing, and evaluating SIT communication strategies and development of standardized tools like questionnaires and surveys. The group highlighted effective practices such as direct engagement methods (e.g., face-to-face interactions, door-to-door visits), participation in local events, and festivals to promote community involvement. They emphasized the value of participatory approaches that integrate community perspectives and transparent communication to build trust. Stakeholder mapping and collaboration with diverse stakeholders, including good will ambassadors and media partnerships, were also identified as crucial strategies to enhance community engagement and project success.

The epidemiology working group focused on addressing knowledge gaps and challenges in vector-borne disease epidemiology. They highlighted the importance and difficulties of implementing robust study designs like RCTs, which while providing important data, are costly and difficult to implement. The group emphasized serological markers and other surrogates for epidemiological impact, demonstrating impact in low disease incidence areas, and demonstrating epidemic prevention. They also discussed strategies to account for disease transmission outside of trial areas and debated the need for RCTs and alternative study designs to demonstrate efficacy, stressing the integration of scientific rigor with practical feasibility and standardized data collection.

The entomology working group discussed challenges, needs, and solutions across topics including SIT production, release strategies, and field evaluations. They noted high costs of production facility start-up, due in part to cost of irradiators, and the need for stable funding mechanisms and staffing. Distributed production (e.g., release centers that supply multiple programs) and increased translational research with industry towards more efficient equipment could reduce costs and allow more programs to access SIT. For field evaluations, egg density, sterility rate, and adult density are commonly measured, and adult indices should be prioritized. Challenges include connecting entomologic to epidemiologic outcomes, estimation of human-vector contact, and measures of mosquito population longevity/age structure. Other priority areas discussed included the need for more open knowledge and data sharing, and more advocacy for novel control tools like SIT.