A minimum of 500 µl will be collected at each bleed preferably via vein using a 23-25 gauge butterfly into a vacutainer tube (heel stick is acceptable if infant is noted to have poor venous access). Specimens will be labeled with a unique identifier and be kept cold (2-8° C), not frozen, until specimen is spun down and sera separated which should be done at end of each day (specimens should not sit out for > 12 hours).