Background Probiotics have been shown to be immunomodulatory and can affect antibody responses following vaccination. Several immunisations are associated with suboptimal seroconversion rates leaving a substantial part of the population exposed to infection.

Objectives To evaluate the influence of probiotic supplementation on the immune response of infants following mumps, measles, rubella and varicella vaccination.

Methods A randomised, placebo-controlled, double blinded prospective trial was performed in a cohort of healthy infants. Study subjects were randomly assigned to receive probiotics or placebo for a total of 5 months, starting 2 months prior to vaccination. Antibody levels against vaccine components were measured 3 months after immunisation. Treatment-related and vaccine-related adverse events were recorded.

Findings 47 infants completed the study, 25 in the probiotic group and 22 in the placebo group. There was no statistically significant difference in the number of infants failing to reach protective antibody titres against the different vaccine components (three infants in the placebo group against one in the treatment group for rubella, two each for mumps, four children vs two for measles). When combining all results in both groups, a larger percentage of failures to seroconvert occurred in the placebo group (17% vs 8%, p=0.052), a result of borderline significance. The number of infants needed to treat in order to prevent one failed vaccine component was 12. There was no difference in the rate of treatment related adverse effects between the two groups. There was a significant trend toward fewer vaccine related adverse effects in the treatment group.

Conclusions Oral probiotics given to infants during the period of immunisation do not interfere with the immune response to mumps, measles, rubella and varicella vaccine, and may improve seroconversion rates.