Background Since the first azithromycin mass drug administration (MDA) in 1999, Tanzania has made remarkable progress towards trachoma elimination. Yet several districts have seen prevalence of trachomatous inflammation—follicular (TF) in children aged 1–9 years remain or rebound ≥5%. From 2022, Tanzania modified MDA implementation in these districts to either more frequent than annual (MFTA) or two additional annual MDA. Conducted in four districts receiving MFTA MDA (Longido and Ngorongoro) or annual MDA (Monduli and Simanjiro), our sentinel site monitoring study aimed to: 1) estimate TF prevalence, anti-Pgp3 serology measures, and ocular Chlamydia trachomatis (Ct) infection prevalence among children aged 1–9 years; 2) characterize ocular Ct infection prevalence over time; and, 3) identify factors associated with ocular Ct infection and increasing community Ct infection prevalence. Methods Ten sentinel sites, with the highest prevalence of active trachoma, were selected per district. At each site, during three monitoring rounds just before MDA in June 2022, January 2023, and October 2023, 50 children aged 1–9 years were examined for trachoma clinical signs and had conjunctival swab and dried blood spot samples taken. Results Though ocular Ct infection prevalence declined, there was no change in TF prevalence, Pgp3 seroprevalence, or Pgp3 seroconversion during follow-up. Younger age, female gender, and more time required to collect water were associated with higher infection prevalence, and greater distance to a health facility was associated with increasing community infection prevalence. Discussion Findings support shift from annual to MFTA MDA in Monduli, continued MFTA MDA in Longido and Ngorongoro, and ″Wait and Watch″ in Simanjiro. Annual monitoring and environmental improvement will be valuable alongside further investigation of relationships between water and sanitation conditions and ocular Ct infection in this setting. Prevalence surveys with additional molecular and serological testing are recommended in the four districts at the end of the MDA cycle.

AB and SB are employees of the International Trachoma Initiative (ITI), a program of The Task Force for Global Health, which receives an operating budget and research funds from Pfizer Inc., the manufacturers of Zithromax (azithromycin). EMHE receives salary support from ITI. The other authors declare no competing interests.

This study did not receive any funding.

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:

Tanzania National Health Research Ethics Committee (NIMR/HQ/R.8a/Vol. IX/3014) gave ethical approval for this work.

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

All data produced in the present study are available upon reasonable request to the authors.