Probiotics may give over-60s a stronger flu jab response

Vaccines are only as good as the immune response they provoke, and older adults are precisely the group in whom that response can be least reliable. Anything cheap and widely available that could raise the odds of a protective response to the seasonal flu jab would therefore be worth knowing about. Probiotics have long been proposed as immune modulators, but individual trials in older people have been small and scattered across different products and outcomes. This review pulled together the randomised evidence on whether oral probiotics change how people aged 60 and over respond to vaccination.

The authors searched three electronic databases up to January 2023 for randomised controlled trials of oral probiotics with vaccine response outcomes in people aged 60 and over. Ten trials involving 1,560 participants, reported across nine papers, met the criteria. Nine trials involved the seasonal influenza vaccine and one a COVID-19 vaccine; all used lactobacilli, some in combination with bifidobacteria. Outcomes included anti-vaccine antibody titres or concentrations, seroconversion and seroprotection, and study quality was assessed with the Cochrane risk of bias tool for randomised trials.

Comparing probiotic and placebo groups, the weighted mean response ratios were 1.29 for antibody titres, 1.16 for seroprotection and 2.00 for seroconversion. Meta-analysis showed probiotics increased seroconversion rates to all three strains of the seasonal influenza vaccine, with odds ratios of 2.74 for the H1N1 strain, 1.90 for H3N2 and 1.72 for the B strain. The findings showed a low level of disagreement between studies, which strengthens the pattern. The important qualifier is that several studies carried a high risk of bias because of missing outcome data.

The consistency across strains and outcomes supports an idea that has circulated for years: that the gut and its bacteria influence how well the immune system responds to vaccination. Almost all of the evidence, though, concerns one vaccine and one family of bacteria, since nine of the ten trials used the flu jab and every trial used lactobacilli. The single COVID-19 vaccine study means little can yet be said beyond influenza. The authors' own conclusion is carefully hedged: lactobacilli may improve the vaccine response, but further research is needed before anyone can be certain.

For an older adult, or anyone thinking ahead to later vaccinations, this is worth filing as promising rather than proven. The pooled trials point the same way, towards better odds of mounting an antibody response after the flu jab, but the missing data problems in several trials mean the picture could still shift. Nothing here identifies which product, strain or duration matters, so it does not support buying anything specific. It does support watching this space, because a supplement that measurably improved vaccine response in older adults would be a genuinely useful tool.