Impact of a one-year interruption to vector control on Bioko Island, Equatorial Guinea

Over the last 20 years, malaria transmission on Bioko Island, Equatorial Guinea has declined dramatically thanks to the implementation of a comprehensive set of malaria control activities, centered around island-wide indoor residual spraying (IRS). In 2024 Bioko Island experienced a lapse in malaria control funding, and as a result vector control activities (including IRS) were interrupted. However, agreements with funders allowed for both a previously planned malaria indicator survey (MIS) and the subsequent implementation of indoor residual spraying (IRS) in late 2024.

This study analyses routine case data from public health facilities from 2019-2024 and annual cross-sectional MIS data from 2019-2024 using interrupted time series methods to quantify the impact of the interruption and reestablishment of malaria control activities on Bioko Island. Modeling included interaction terms to allow both pre-intervention trends and interruption effect sizes to vary by district.

In 2024, the number of confirmed cases reported was 41% higher than the 2021-2023 average, and the Plasmodium falciparum prevalence rate (PfPR) rose by three percentage points. Statistical modeling estimated that 23.1% (95% CI 9.2-33.7%) of 2024 cases were associated with the timing of vector control interruptions and potentially avertable if control activities had been maintained, and that the interruption was associated with an increased PfPR in 2024, above previous trends (adjusted OR 1.20, 95% CI 1.07-1.35). Moreover, the reintroduction in IRS in late 2024 was found to have potentially averted an additional 13.3% (95% CI 4.4-28.4%) increase in confirmed cases.

In just 1 year with interruptions to control, malaria transmission and burden quickly resurged on Bioko Island. However, Bioko's experience demonstrates that the reestablishment of control activities can equally rapidly contain, and reverse resurgence associated with control interruptions.