Workers were invited to the factory’s health clinic for venipuncture conducted by a licensed laboratory nurse. Approximately 20 ml of blood was collected from each worker via BD Vacutainer SST Plus Blood Collection Tubes (Becton, Dickenson and Company, Franklin Lakes, NJ). The blood was subsequently centrifuged at 400 x g and the serum fraction was aliquoted, and cryopreserved at -80°C