Background
For very preterm infants, human milk is often fortified with formula products based on processed bovine milk. Intact bovine colostrum (BC), rich in anti-inflammatory milk factors, is considered an alternative. We investigated if BC affects anti-inflammatory/TH2 immunity and infection risk in very preterm infants.

Methods
For a secondary analysis of a multicenter, randomized controlled trial (NCT03537365), very preterm infants (26–31 weeks gestation, 23% small for gestational age, SGA) were randomized to receive BC (ColoDan, Biofiber, Denmark, n = 113) or conventional fortifier (PreNAN, Nestlé, Switzerland, n = 116). Infection was defined as antibiotic treatment for five or more consecutive days and 29 cytokines/chemokines were measured in plasma before and after start of fortification.

Results
In general, infection risk after start of fortification was associated with low gestational age, SGA status and antibiotics use prior to fortification. Adjusted for confounders, infants fortified with BC showed more infection episodes (20 vs 12%, P < 0.05) and higher cumulative infection risk (hazard ratio, HR 1.9, P = 0.06), particularly for SGA infants (HR 3.6, P < 0.05). Additionally, BC-fortified infants had higher levels of TH2-related cytokines/chemokines (IL-10, MDC, MCP4) and reduced levels of cytokines related to TH1/TH17-responses (IL-15, IL-17, GM-CSF). The differences were most pronounced in SGA infants, displaying higher levels of TH2-related IL-4, IL-6, and IL-13, and lower interferon-γ and IL-1α levels in the BC group.

Conclusion
Infants fortified with BC displayed a delayed shift from TH2- to TH1-biased systemic immunity, notably in SGA infants, possibly influenced by multiple confounding factors, alongside elevated antibiotic use, suggesting increased susceptibility to infection.