Notably, we observed a strongly positive correlation between the signature cytokine of type 2 polarisation, IL-4, in T cells and hepatitis B vaccination when administered early, i.e., within the first few hours of life (e.g., odds ratio (OR) of 10.8 if hepatitis B vaccine was given within the first 24 h compared with no vaccination (95% confidence interval (CI) 2.9–4.0, p = 0.0004)) [8]. Here, IL4 is linked to hepatitis A virus infection.