Probiotics are everywhere these days. They are supposed to prevent all kinds of diseases, from infectious to immunological to allergic. Some of the claims have strong evidence, some not. A pilot study by Youngster I, et al, in which the role of probiotics before immunisations is studied, is yet another positive discovery, but there are several limitations that force us to be cautions and not overly excited about it.

The study is a small prospective double blind randomised trial from an urban medical center in Israel. Healthy infants aged 8-10 months were recruited. Fifty six infants were randomised to receive either a probiotic combination of four microorganisms or cornflour, matched in appearance and texture. A sachet a day was given orally for 5 months (from 10 to 15 months of age). At 12 months, infants received the MMRV vaccine. At 15 months, the authors measured if protective antibody levels were achieved. During all the duration of the study, a coordinator contacted the parents every two weeks to ask for compliance and side effects.

In short, there was no significant difference between groups in either the proportion of infants achieving protective levels of antibodies, or the number of side effects. When the authors used a composite outcome they observed a trend in favour of probiotics. Considering the proportion of samples tested where the titre of specific IgG did not achieve protective levels for each of the four diseases of the MMRV vaccine, the p value almost reached statistical significance: 15 infants in the placebo group versus 8 in the probiotic group (p = 0.052). The same happened when assessing adverse events: 11 infants in the placebo group experienced “any adverse effect” versus 4 in the probiotic group (p = 0.051).

Composite outcomes can be misleading. Other limitations, such as the small sample size and the unclear allocation concealment also bring uncertainty the results of this pilot trial. So, there may be a role for probiotics when vaccinating, but right now we need a lot more evidence.