Researchers studied whether a special mix of helpful bacteria could boost how well COVID vaccines work in older adults and people with diabetes. They gave 369 people either a supplement containing three types of beneficial bacteria or a placebo for three months after vaccination. The study found that people whose guts had room for these new bacteria showed stronger immune responses to the vaccine. This suggests that your existing gut bacteria play an important role in determining whether a probiotic supplement can help your body fight off COVID-19 more effectively.

The Quick Take

  • What they studied: Whether a supplement containing three types of helpful gut bacteria could improve how well COVID vaccines protect older adults and people with diabetes
  • Who participated: 369 people (mostly older adults and those with diabetes) who had recently received a COVID vaccine. Researchers analyzed detailed information from 280 people’s stool samples and 276 people’s metabolic markers
  • Key finding: People who took the bacterial supplement developed stronger immune responses to the COVID vaccine, but only if their gut bacteria had ‘room’ for the new bacteria to take hold. The strength of the immune response was linked to how well the new bacteria established themselves in the gut
  • What it means for you: If you’re older or have diabetes, a probiotic supplement might help your COVID vaccine work better—but whether it helps depends on your current gut bacteria. This is not a replacement for vaccination, but a potential way to boost its effectiveness. Talk to your doctor before starting any supplement

The Research Details

This was a controlled experiment where researchers randomly assigned 369 people to receive either a special supplement (called SIM01) containing three types of beneficial bacteria or a dummy pill (placebo) for three months. All participants had recently received a COVID vaccine—some got the Pfizer vaccine and others got the Sinovac vaccine. The researchers collected stool samples at the start and after three months, and measured immune markers at one month and six months after vaccination.

The scientists used advanced laboratory techniques to identify exactly which bacteria were present in each person’s gut and how they changed over time. They also measured specific chemicals produced by the bacteria and tracked how well the vaccine worked by measuring antibodies (immune proteins) in the blood.

This approach allowed researchers to understand not just whether the supplement worked, but why it worked for some people and not others—specifically by looking at whether each person’s existing gut bacteria could accommodate the new bacteria from the supplement.

Understanding how your existing gut bacteria affect whether a probiotic supplement works is crucial because it explains why the same supplement might help one person but not another. This research moves beyond just asking ‘does it work?’ to asking ‘when does it work and for whom?’ This personalized approach could help doctors recommend supplements more effectively

This study has several strengths: it included a large number of participants (369), used a control group for comparison, randomly assigned people to groups, and employed advanced genetic testing to identify bacteria at the species level. The researchers measured multiple markers of immune function and tracked people for six months. However, the study focused specifically on older adults and people with diabetes, so results may not apply equally to younger, healthier people. The study was published in a peer-reviewed scientific journal, meaning other experts reviewed the work before publication

What the Results Show

The main finding was that the supplement containing three beneficial bacteria (Bifidobacterium adolescentis, B. bididum, and B. longum) helped people develop stronger immune responses to the COVID vaccine—but only when their baseline gut bacteria had lower levels of these bacteria already present. In other words, people whose guts had ‘room’ for the new bacteria benefited most.

When the supplement worked, the three bacterial species successfully established themselves in the gut over the three-month period. This successful establishment was directly linked to stronger antibody responses six months after vaccination. For people who received the Pfizer vaccine specifically, the strength of the immune response was also connected to increased levels of a compound called benzoic acid, which is produced by these beneficial bacteria.

The researchers discovered they could predict ahead of time which people would benefit most from the supplement by looking at their baseline gut bacteria composition. People with lower initial levels of these beneficial bacteria and reduced capacity for breaking down carbohydrates were the best candidates for the supplement.

The study found that the supplement’s effectiveness was specifically tied to how well the three bacterial strains ’took root’ in each person’s gut. This engraftment (successful colonization) was measurable and predictable based on each person’s starting gut bacteria profile. The research also showed that metabolic changes in the gut—specifically increases in benzoic acid—were part of the mechanism by which the bacteria improved immune function. These findings suggest that the bacteria don’t just passively sit in the gut; they actively produce compounds that enhance immune responses

Previous research has suggested that probiotics might help immune function, but this study provides more detailed evidence about why some people benefit and others don’t. It moves beyond simple ‘probiotic works or doesn’t work’ conclusions to show that the existing gut bacteria ecosystem matters tremendously. This aligns with growing scientific understanding that personalized medicine—tailoring treatments to individual characteristics—is more effective than one-size-fits-all approaches

The study focused on older adults and people with diabetes, so we don’t know if the same results would apply to younger, healthier people. The study only tested one specific supplement formula (SIM01), so other probiotic products might work differently. The research was conducted in a controlled setting, and real-world results might differ. Additionally, while the study measured immune markers in the blood, it didn’t directly measure whether people who took the supplement were actually better protected against COVID-19 infection in daily life. The study also didn’t include information about participants’ diets, which can significantly affect gut bacteria

The Bottom Line

For older adults and people with diabetes: A probiotic supplement containing these three Bifidobacterium species may help COVID vaccines work better, but only if your gut bacteria profile suggests you have room for these bacteria to establish themselves. This is not a substitute for vaccination. Consider discussing with your doctor whether a probiotic supplement might be appropriate for you, especially if you have diabetes or are over 65. The supplement needs to be taken for at least three months to be effective. (Confidence level: Moderate—this is based on one study in a specific population)

This research is most relevant to older adults (65+) and people with diabetes who have recently been vaccinated against COVID-19 and want to maximize their immune protection. It may be less relevant to younger, healthier people, though more research is needed. People with severe immune system disorders should consult their doctor before starting any probiotic supplement. This is not a replacement for vaccination

The study showed that the supplement needed three months to establish the beneficial bacteria in the gut. Immune benefits were measurable at one month and continued to improve through six months of follow-up. You shouldn’t expect immediate results; give any probiotic supplement at least three months to work

Want to Apply This Research?

  • Track probiotic supplement adherence daily (yes/no taken) and measure COVID antibody levels or immune markers monthly if available through your healthcare provider. Also track any digestive changes (bloating, gas, bowel regularity) as these often change when introducing probiotics
  • Set a daily reminder to take the probiotic supplement at the same time each day (consistency matters for establishing gut bacteria). Pair it with a consistent meal to improve absorption. Keep a simple log of whether you took it each day to maintain accountability over the three-month period needed for effectiveness
  • Check in monthly with your healthcare provider to monitor immune markers if available. Track digestive comfort and any changes in symptoms. After three months, discuss with your doctor whether the supplement appears to be helping based on any available immune testing. Continue for at least six months to see full benefits, as shown in the research

This research describes a specific study in older adults and people with diabetes and should not be considered medical advice. Probiotic supplements are not regulated the same way as medications and may not be appropriate for everyone, particularly those with compromised immune systems or certain medical conditions. Always consult with your healthcare provider before starting any new supplement, especially if you take medications or have underlying health conditions. This supplement is not a substitute for COVID-19 vaccination. The findings are based on one study and should be confirmed by additional research before being widely recommended. Individual results may vary based on your unique gut bacteria and health status.