Endline visits

At the end of the intervention period, participants will be invited to the mobile biospecimen collection site for the two endline data collection visits (35 weeks and 38 weeks for WRA and children; 11 weeks and 12 weeks for lactating women). The second endline assessment is included to enhance statistical power. A 3-week period between endline visits was chosen based on the desired timing between the isotope dose administration and blood collection for assessment of total body vitamin A stores. Delivery of study bouillon cubes will continue through the last endline assessment for each household. Data collection procedures at the first endline visit will be similar to those described at the baseline visit. Briefly, data collection will include anthropometric measurements and morbidity symptoms (all participants); spot urine samples, blood pressure measurement and venous blood collection (WRA and children); breast milk collection (lactating women); and stool collection (children).

In addition, at the first endline visit (35 wk), WRA will receive an oral dose of ¹³C₁₀-retinyl acetate (2 mg retinol equivalents, sourced from Buchem BV, Apeldorn, Netherlands) to assess total body vitamin A stores. ¹³C₁₀-retinyl acetate will be used at endline to avoid any potential interference from the baseline dose of d6-retinyl acetate. To assess eligibility for the endline isotope dose, participants will undergo screening procedures identical to those at the pre-intervention screening visit, including assessment of morbidity symptoms, vitamin A intake, pregnancy (based on measurement of HCG in urine), anemia, malaria infection, and inflammation. Based on results of these assessments, WRA will either be 1) eligible to receive the endline isotope dose and continue with the study; 2), deferred and scheduled to be re-screened for isotope dosing at the 2^nd endline visit (recent morbidity, CRP > 5 g/L, positive malaria RDT); 3) excluded from isotope dosing and the second endline blood collection (participants with Hb < 80 g/L), or 4) excluded from the study (positive pregnancy test) (see S3 File for detailed exclusion and deferral criteria).

Data collection procedures at the second endline assessment will include a subset of the outcomes measured at the first endline assessment, measured using identical methods. These will include morbidity symptoms (all groups), venous blood collection (WRA and children) and breast milk collection (lactating women). WRA who are deferred at the first endline visit, and do not receive the vitamin A isotope dose, will be re-screened at the second endline visit using the same procedures for eligibility described above. WRA who are eligible to receive the isotope dose at the second endline will be asked to return for an additional endline visit to collect a follow-up venous blood sample for RID assessment 3 weeks later.

In between the two endline visits (35wk and 38wk), a single endline child development assessment will be completed for each task. After the second endline visit, a final home visit will be conducted to complete the study exit form and collect all remaining study bouillon cubes from the household. In each household, the baseline assessment of hypothetical willingness-to-pay for a fortified bouillon cube will be repeated at endline.