GSTM1 and pertussis: Associations between hemoglobin (Hb) and moderate to severe anemia (Hb <9.5 g/dl), and anti-diphtheria, anti-pertussis, anti-pneumococcus serotype 19, and anti-measles serum IgG concentrations and/or seroconversion, in the birth cohort of Kenyan infants.