Iron deficiency (ID) anemia (IDA) is a global public health problem, with the highest prevalence in Africa. Vaccines often underperform in low- and middle-income countries (LMIC), and undernutrition, including ID, likely plays a role. Recent studies have shown the importance of iron status in vaccine response. Intravenous iron given at time of vaccination improved response to yellow fever and COVID-19 vaccines in IDA Kenyan women. Whether oral iron treatment would have a similar beneficial effect on vaccine response is uncertain. Also, timing of oral iron treatment needs further investigation. The co-primary objectives of this study are to assess 1) whether IDA in Kenyan women impairs vaccine response, and whether oral iron treatment improves their response; 2) the timing of oral iron treatment to improve vaccine response (prior to vaccination vs at time of vaccination). We will conduct a double-blind randomized controlled trial in southern Kenya to assess the effects of iron supplementation on response to three single-shot vaccines: Johnson \& Johnson COVID- 19 (JJ COVID-19), the quadrivalent meningococcal vaccine (MenACWY) and the typhoid Vi polysaccharide vaccine (Typhim Vi). Women with IDA will be recruited and randomly assigned to three study groups: group 1 (pre- treatment) will receive 100 mg oral iron as ferrous sulfate (FeSO4) daily on days 1-56; group 2 (simultaneous treatment) will receive matching placebo daily on days 1-28, and 200 mg oral iron as FeSO4 daily on days 29-56; and group 3 (control) will receive matching placebo daily on days 1-56. Women in all groups will receive the JJ COVID-19 vaccine, the MenACWY and the Typhim Vi vaccine on day 28. Cellular immune response and serology will be measured at 28 days after vaccination in all groups.