We found no evidence of a difference in the risk of infection (RR 0.99, 95% CI 0.55 to 1.80, I2 = 9%) or in the Hb concentration at ‘short-term’ (≤ 7 days) follow-up (mean difference − 0.32 g/L, 95% CI − 3.28 to 2.64, I2 = 37%). This evidence concerns the gene GSTM1 and infection.