In many official maps, malaria in Indonesia appears to be receding. Case numbers have dropped drastically in the past decade, and Indonesia aims for malaria elimination by 2030. But behind these promising figures lies an important question: has malaria transmission truly stopped, or has it simply gone undetected by conventional diagnostic methods?

This is the question explored by dr. Inke Nadia Diniyanti Lubis, Sp.A(K), Ph.D, a lecturer at the Faculty of Medicine, Universitas Sumatera Utara, together with a cross-country research team in the scientific article titled:
“Serology reveals comparable patterns in the transmission intensities of Plasmodium falciparum and Plasmodium vivax in Langkat district, North Sumatera Province, Indonesia,”
published in Frontiers in Cellular and Infection Microbiology in 2025.

With collaborators Irbah Rea Alvieda Nainggolan, Meliani, Beby Syofiani Hasibuan, Kumuthamalar Sangaran, Luqman Samsudin, Sriwipa Chuangchaiya, Paul Cliff Simon Divis, Ranti Permatasari, and Zulkarnain Md Idris, Inke proposed a different approach: instead of counting parasites in the blood, they read the community’s “immune memory” through antibodies.

The research site was Langkat District, now categorized as near-elimination but previously known as a malaria hotspot with multiple parasite species—including the zoonotic Plasmodium knowlesi.

The team conducted a cross-sectional survey in June 2019. A total of 339 residents from various age groups participated. They did not need to be sick; in fact, the study aimed to capture past exposure to malaria rather than acute infection.

At the survey post, participants completed a short questionnaire about age, occupation, sleeping habits, bed net use, and whether they had recently spent nights in the forest. Afterward, the team collected finger-prick blood samples on specialized filter paper. These dried blood spots were later taken to the laboratory for antibody analysis.

This approach appears simple, but behind it lies complex serological and statistical analysis.

In the laboratory, Inke’s team measured IgG antibodies to four blood-stage malaria antigens: two belonging to Plasmodium falciparum (PfAMA-1 and PfMSP-119) and two belonging to Plasmodium vivax (PvAMA-1 and PvMSP-119).

These four antigens were selected because they represent the erythrocytic stage (the symptomatic stage), have long been used as exposure markers in various studies, and are relatively stable—making them suitable for capturing medium- to long-term exposure rather than infections from only the past few days.

Methodologically, the team used ELISA (enzyme-linked immunosorbent assay) and strict analytical standards. Optical density (OD) values were normalized against positive controls and processed using a finite mixture model to determine seropositivity thresholds—a procedure ensuring that individuals classified as “previously exposed” truly had meaningful antibody responses.

At first glance, malaria numbers in Langkat appear low. Previous studies recorded only 0.3% microscopic infections and 0.9% submicroscopic infections in the community.

However, when Inke and her team examined antibodies, the picture changed:
• Seroprevalensi PfAMA-1: 10.6%
• PfMSP-119: 13%
• PvAMA-1: 18.6%
• PvMSP-119: 7.4%

Combined, 20.7% of residents had been exposed to P. falciparum, and 20.7% had been exposed to P. vivax—almost identical.

This means that although few clinical cases are recorded, around one in five people in Langkat carries immunological traces of malaria exposure. This is an important signal for elimination programs: transmission has not disappeared; it has merely become quieter.

The team then used a reversible catalytic model to estimate the seroconversion rate (SCR)—the probability of becoming seropositive per year. The results indicated slightly higher P. falciparum transmission compared to P. vivax, though their confidence intervals overlapped.

For policymakers, these figures are far more informative than simple annual case counts, as they represent cumulative exposure across ages.

One practical finding was the risk factor of “sleeping overnight in the forest.” In multivariate logistic regression analysis, this was the only variable significantly associated with P. vivax seropositivity—with an adjusted odds ratio of about 3.93. This means individuals who had spent a night in the forest within the previous two weeks were nearly four times more likely to have been exposed to P. vivax.

This finding aligns with vector ecology: several P. vivax-transmitting mosquitoes in Sumatra (e.g., Anopheles kochi) tend to bite outdoors, near forested areas, at night. This makes forest-goers, farmers, and communities living near forest edges key targets in elimination strategies.

Meanwhile, no demographic or behavioral variables were significantly associated with P. falciparum seropositivity in adjusted models. This suggests that P. falciparum transmission is more diffusely distributed in the population or that its intensity is now so low that specific risk patterns are hard to detect.

In the author contribution section, Inke is listed as holding a central role: conceptualization, data collection, formal analysis, project administration, funding acquisition, supervision, drafting the manuscript, and final editing.

This positions her as a key figure in designing how serology can be used to understand malaria transmission patterns in near-elimination settings, strengthening collaborative networks between USU’s Faculty of Medicine and institutions in Malaysia and Thailand, and ensuring the statistical analysis is not only technically correct but also relevant to public health policy.

For the USU Faculty of Medicine, researchers like Inke demonstrate that local research has progressed to the level of “advanced epidemiological analysis,” no longer limited to reporting case numbers or microscopy results. The serological approach used opens opportunities for more sensitive surveillance in low-transmission areas, identification of risk pockets (such as communities frequently sleeping in the forest), and more nuanced long-term evaluation of interventions (such as bed net distribution or vector control).

Nationally, Indonesia is in a critical phase toward its malaria elimination target for 2030. At this stage, the challenge is no longer merely “reducing cases,” but “not missing hidden transmission.”

Inke and her collaborators highlight key points such as:
Serology should be considered as a routine support tool for elimination efforts, especially in areas with very low clinical cases.
P. falciparum and P. vivax are both still circulating in the community, meaning elimination strategies cannot focus on only one species.
Forest-goers must become special targets for prevention and education, as they have a higher risk of P. vivax exposure.
For USU, this article strengthens the university’s position as a strategic scientific partner in global health issues, particularly malaria in Southeast Asia.