Scientists are discovering that the trillions of bacteria living in your stomach and intestines might play a bigger role in how well vaccines protect you than anyone realized. A new review of research shows that your gut bacteria can influence how your body responds to vaccines like the flu shot and COVID-19 vaccine. Different types of bacteria produce special chemicals that help your immune system create stronger defenses. This means that what you eat, medicines you take, and even probiotic supplements could potentially affect how well vaccines work for you. Understanding this connection might help doctors improve vaccines for people whose immune systems don’t work as well.

The Quick Take

  • What they studied: How the bacteria living in your gut influence whether vaccines work well in your body
  • Who participated: This is a review that looked at many different human studies and animal experiments about gut bacteria and vaccine responses
  • Key finding: Research suggests that specific types of gut bacteria, especially one called Bifidobacterium adolescentis, are connected to how strong your immune response is after getting vaccinated
  • What it means for you: Your gut health might matter for vaccine effectiveness, but it’s too early to make major changes to your diet or take new supplements based on this alone. Talk to your doctor before making changes, especially if you’re immunocompromised

The Research Details

This is a review article, which means scientists looked at all the existing research on gut bacteria and vaccines and summarized what they found. They examined studies on vaccines for COVID-19, hepatitis B, and the flu. The researchers looked at both human studies (where they measured people’s immune responses) and animal experiments (where scientists studied bacteria’s effects in controlled settings). By combining information from many different studies, they could see patterns in how gut bacteria affects vaccine responses across different types of vaccines.

Understanding how gut bacteria influences vaccines is important because not everyone responds equally well to vaccines, and scientists don’t fully understand why. Age, sex, and genetics explain some differences, but not all. By identifying gut bacteria as a key player, researchers might be able to predict who will have weaker vaccine responses and find ways to improve those responses. This could be especially helpful for older adults and people with weak immune systems.

This is a review of existing research rather than a new experiment, so it summarizes what other scientists have found. The strength of the conclusions depends on the quality of the studies reviewed. The research is recent (2025) and published in a peer-reviewed journal, which means other experts checked the work. However, many of the individual studies are still relatively new, and more research is needed to confirm these connections in humans

What the Results Show

The review found that your gut bacteria composition—the specific mix of bacterial species you have—appears to be connected to how well your body responds to vaccines. Certain bacteria, particularly Bifidobacterium adolescentis, showed associations with stronger immune responses after vaccination. The bacteria don’t directly fight the virus; instead, they produce special chemicals called metabolites (like short-chain fatty acids and bile acids) that help train your immune cells to respond better to vaccines. These chemicals influence how your T-cells (immune fighters) develop and how many antibodies (protective proteins) your body makes. The research suggests this relationship works for multiple vaccines, including COVID-19, flu, and hepatitis B vaccines.

The review also found that things affecting your gut bacteria can change how well vaccines work. Antibiotics kill both harmful and helpful bacteria, and studies show that people taking antibiotics sometimes have weaker vaccine responses. Diet matters too—what you eat influences which bacteria thrive in your gut. Probiotic supplements (beneficial bacteria you consume) and prebiotic foods (that feed good bacteria) showed promise in some studies for improving vaccine responses. Animal studies showed that mice without any gut bacteria or treated with antibiotics had much weaker vaccine responses, while mice given beneficial bacteria had stronger responses.

This research builds on earlier discoveries that gut bacteria influence many aspects of health beyond digestion, including mood, weight, and disease risk. The connection to vaccines is relatively new and represents an exciting frontier in understanding immunity. Previous research focused mainly on age, genetics, and overall health status as factors affecting vaccine response. This review suggests that gut bacteria should be added to that list of important factors, though it’s not yet clear how much bacteria contributes compared to these other factors.

This is a review of existing studies, not new research, so the conclusions are only as strong as the studies reviewed. Many studies were done in animals, which don’t always translate directly to humans. The human studies are often small and sometimes measure different things, making it hard to compare results. Most research focused on specific vaccines and specific bacteria, so it’s unclear if findings apply to all vaccines or all people. The review doesn’t prove that changing your gut bacteria will definitely improve vaccine responses—it shows associations and possibilities that need more testing

The Bottom Line

Based on current evidence (moderate confidence): Maintain a healthy diet rich in fiber and diverse foods to support beneficial gut bacteria. Avoid unnecessary antibiotics when possible. If you’re considering probiotics or prebiotics specifically to improve vaccine response, discuss this with your doctor first, especially if you have a weakened immune system. Don’t make major dietary changes solely based on this research yet—more evidence is needed

This research is most relevant for: older adults who sometimes have weaker vaccine responses, people with compromised immune systems (like those with HIV or undergoing cancer treatment), and people taking long-term antibiotics. Everyone should know about this connection, but it’s especially important for these groups. People with healthy immune systems who eat reasonably well probably don’t need to make changes based on this research alone

If you made changes to support gut health (like improving diet or taking probiotics), you wouldn’t see effects on vaccine response immediately. Gut bacteria changes take weeks to months. Vaccine responses are typically measured 2-4 weeks after vaccination. So realistically, any benefits from microbiota changes would only be relevant if made well before vaccination—ideally several weeks in advance

Want to Apply This Research?

  • Track your fiber intake (aim for 25-30 grams daily) and note any probiotic or prebiotic supplements taken. Record this for at least 4 weeks before and after vaccination to monitor patterns
  • Users can set a goal to increase dietary fiber through whole grains, vegetables, and fruits. The app could send reminders to eat diverse plant-based foods and track consistency. Users could also log when they take probiotics or antibiotics to see correlations with how they feel
  • For users interested in optimizing vaccine response: track gut health indicators (digestion quality, energy levels) and dietary diversity for 8-12 weeks. If vaccination is planned, maintain these habits for at least 4 weeks before the appointment. After vaccination, note any side effects or illness patterns over the following months

This review summarizes current research on gut bacteria and vaccine responses, but the field is still developing. These findings do not replace medical advice from your healthcare provider. Do not start, stop, or change any medications or supplements based on this information without consulting your doctor first. This is especially important if you have a weakened immune system, are pregnant, nursing, or taking other medications. While maintaining good gut health through diet is generally beneficial, there is not yet enough evidence to recommend specific probiotic or prebiotic supplements solely for improving vaccine response. Always follow your healthcare provider’s vaccination recommendations.