Specimen collection
BLOOD
Stool samples were collected every two weeks from week −4 through week 14. All stool samples were kept in participants’ home freezers (−20°C) wrapped in ice packs, until they were transferred on ice to the research laboratory and stored at −80°C. Blood samples were collected at seven time points: weeks −3, 0, 4, 6, 8, 10, and 14. Blood for serum was collected into an SST-tiger top tube, spun at 1,200xg for 10 minutes, aliquoted, and stored at −80°C. Blood for plasma was collected into an EDTA tube, spun at 1,200×