Atovaquone/Proguanil Use and Zoster Vaccination Are Associated with Reduced Alzheimer’s Disease Risk in Two Cohorts: Implications for a LatentToxoplasma gondiiMechanism

INTRODUCTION Identifying modifiable risk factors for Alzheimer’s disease (AD) may shed light on novel mechanisms and inform prevention strategies. Increasing evidence suggests that latent pathogens may contribute to AD pathogenesis via chronic neuroinflammation.

We conducted a large-scale, dual-cohort study to identify exposures associated with reduced Alzheimer’s disease (AD) risk. In a national Israeli cohort (Leumit Health Services; 2004-2024), we analyzed 9,124 AD patients and 18,248 matched controls. We systematically screened medication exposures in the matched cohort for associations with significantly reduced AD risk (OR < 0.5, FDR < 0.05). To account for potential residual confounding, we applied conditional logistic regression models adjusted for age, sex, socioeconomic status, and relevant comorbidities. Findings were independently validated in the U.S.-based TriNetX network, which includes electronic health records from over 120 million patients across 69 healthcare organizations. Propensity score-matched Cox proportional hazards models were used to estimate hazard ratios (HRs) for dementia incidence across stratified age groups.

Atovaquone/proguanil (Ato/Pro), an antiprotozoal agent active against Toxoplasma gondii, was strongly associated with reduced AD risk in both cohorts (LHS: OR 0.36 [95% CI, 0.20-0.61]; TriNetX: HRs 0.34-0.51, p = 10-17 to 10-40 across age groups 50-59, 60-69, and 70-79). Both recombinant and live attenuated varicella-zoster virus (VZV) vaccines were also significantly protective (ORs 0.16-0.37), and T. gondii seropositivity was associated with a 2.43-fold increased risk of dementia (p = 0.0013). Notably, Ato/Pro’s protective effect was more pronounced in individuals without prior VZV vaccination (HR 0.51 [0.43-0.59]) compared to vaccinated individuals (HR 0.71 [0.59-0.85]).

This dual-cohort study - spanning over 120 million patients across two nations - demonstrates strong and reproducible associations linking Ato/Pro use and VZV vaccination to reduced AD risk. The findings support a mechanistic model in which latent T. gondii infection, potentially reactivated by herpesvirus co-infection may contribute to AD pathogenesis. Ato/Pro may protect by eliminating or suppressing T. gondii, while VZV vaccination may reduce viral triggers of parasite reactivation. These results point to novel preventive strategies and reinforce the infectious hypothesis of Alzheimer’s disease. Competing Interest Statement The authors have declared no competing interest. Funding Statement This study was funded internally by Leumit Health Services. Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Ethics approval was granted by the Leumit Health Services Institutional Review Board (LEU-0001-24), with a waiver of informed consent due to retrospective, anonymized data analysis. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Footnotes Editing. Minor additions to the discussion. Added subplots with confidence intervals to Figure 1 Data Availability Access to patients data is limited to researchers approved by the Institutional Review Board. Abbreviations - AD - Alzheimer’s Disease - Ato/Pro - Atovaquone/Proguanil - aOR - adjusted Odds Ratio - BBB - Blood-brain barrier - BMI - Body Mass Index - BP - Blood pressure - CNS - Central nervous system - COX-2 - Cyclooxygenase-2 - EHR - Electronic health records - FDR - False Discovery Rate - HCO - Healthcare organizations - HR - Hazards Ratio - ICD - International Classification of Diseases - IM - Intramuscular - LHS - Leumit Health Services - OR - Odds Ratio - PO - Per os - PSM - Propensity score matching - SES - Socioeconomic status - SMD - Standardized Mean Difference - T3 - Triiodothyronine