Aim: To characterize the immune responses to routine vaccination (Diphtheria, Polio, Pertussis, Tetanus, Haemophilus influenzae b, Hepatitis B) in very preterm neonates of the PRIMAL Clinical Study, which are randomized to the administration of probiotics or placebo.

Design: We will characterize

• the acute non-specific inflammatory response 1-3 days after vaccination
• the primary adaptive immune response 5-7 days after vaccination
• And the long-term adaptive immune response (12 months) after vaccination in participants of the PRIMAL study cohort.

Expected outcome: We expect probiotics to better understand if and how the gut immaturity and the gut microbiota influence the immune response to routine vaccination.