Can Vaccines Be Used for Bacteria? A Look at How They Work

Vaccines work against bacteria, and they have for well over a century. Some of the earliest vaccines ever developed targeted bacterial diseases like typhoid, cholera, and plague, and today several of the most widely used childhood immunizations protect against bacterial infections such as tetanus, diphtheria, whooping cough, and pneumococcal disease. The public conversation around vaccines has been dominated by viral targets in recent years, but bacterial vaccines remain a cornerstone of public health, and a wave of new technology is expanding what they can do.

A Longer History Than Most People Realize

The popular image of vaccination starts with smallpox, a virus. But bacterial vaccines followed remarkably quickly. Louis Pasteur’s early work on anthrax and cholera in animals laid the groundwork, and when he turned to rabies, he discovered that killed microbes could also serve as vaccines. One of his students used that insight almost immediately to develop a vaccine against typhoid, a bacterial disease. By 1921, vaccines against diphtheria and tetanus had opened yet another path: immunization using specific molecules from the pathogenic bacteria rather than the whole organism.¹ Those early breakthroughs established the basic toolkit that bacterial vaccine development has built on ever since.

Today, bacterial vaccines protect against a wide range of diseases. Tetanus, diphtheria, and pertussis vaccines are given to children worldwide. Conjugate vaccines against pneumococcal bacteria and Haemophilus influenzae type b (Hib) have dramatically reduced childhood meningitis and pneumonia. Meningococcal vaccines cover several serogroups of the bacteria responsible for bacterial meningitis. Typhoid vaccines are still in use. So the short answer to whether vaccines can target bacteria is not just “yes” but “they already do, routinely, and have for over a hundred years.”

How Bacterial Vaccines Differ From Viral Ones

Viruses are structurally simple compared to bacteria. A virus is essentially a packet of genetic material wrapped in a protein coat, sometimes with an outer envelope. Bacteria are far more complex: they have cell walls, outer membranes, polysaccharide capsules, protein appendages, and they secrete toxins. That complexity means there are more potential targets for a vaccine, but also more ways for the bacteria to dodge immune recognition. It also means no single vaccine approach works for all bacterial diseases. Instead, there is a whole menu of strategies, each tailored to the particular way a given bacterium causes harm.

For bacteria that do their damage primarily through toxins, like the ones responsible for tetanus and diphtheria, the vaccine does not need to target the bacterium itself. Instead, it uses an inactivated form of the toxin, called a toxoid, to train the immune system to neutralize the poison before it can do harm. Researchers continue to refine this approach, including recent work on nanoparticle-based systems that use cell membrane coatings to intercept and neutralize bacterial toxins, allowing rapid loading of different toxin types for use as vaccine candidates.²

For bacteria that evade the immune system by hiding behind a polysaccharide capsule, conjugate vaccines are the breakthrough technology. Polysaccharides on their own are poor at triggering a lasting immune response, especially in young children. But linking (conjugating) those sugar chains to a carrier protein transforms them into something the immune system responds to much more vigorously and remembers for the long term.³ This is the technology behind the pneumococcal conjugate vaccine (PCV), the Hib vaccine, and several meningococcal vaccines. It was a genuine turning point in vaccinology.

Mucosal Immunity and Live Bacterial Vectors

Many bacterial infections begin at mucosal surfaces: the lining of the gut, the respiratory tract, or the urogenital tract. An injected vaccine can generate antibodies in the blood, but that does not always translate into strong protection right at the surface where infection starts. This has driven interest in live bacterial vectors, weakened bacteria that can be administered orally or nasally to stimulate local immune defenses at the mucosal surface itself. Research has shown that oral vaccines made from killed bacteria or protein subunits alone tend to produce weak responses and poor immune memory, while live bacterial carriers presenting antigens can induce mucosal immunity and provide local protection.⁴

The oral typhoid vaccine (Ty21a) is a real-world example: a live, weakened strain of Salmonella Typhi that you swallow as a capsule. It colonizes the gut briefly, provoking a local immune response without causing disease. This approach also has potential beyond the bacterium itself; researchers have experimented with using live attenuated bacteria as vehicles to deliver antigens from other pathogens, essentially turning a harmless bacterium into a custom delivery system for multiple vaccines at once.

The Stubborn Problem of Intracellular Bacteria

Some bacteria are exceptionally difficult to vaccinate against because of where they live inside the body. Pathogens like Mycobacterium tuberculosis, Chlamydia, and Rickettsia species invade host cells and hide inside them, which puts them out of reach of antibodies circulating in the blood. Defense against these bacteria depends heavily on cell-mediated immunity, particularly certain types of T cells that can recognize and kill infected cells.⁵

Building a vaccine that reliably triggers this kind of cellular response is harder than generating antibodies. Successful vaccines against obligate intracellular bacteria need to provoke strong T cell memory, including cytotoxic T cells capable of hunting down and destroying infected cells.⁶ Tuberculosis is the poster child for this challenge. BCG, the existing TB vaccine, was developed a century ago. It provides some protection in children but is unreliable in adults, and no replacement has yet made it through to widespread use despite decades of effort. The difficulty is not a lack of trying but a genuinely hard immunological problem.

When Bacteria Fight Back With Diversity

Bacteria are not a single fixed target. Many species exist as dozens or even hundreds of distinct serotypes, each wearing a slightly different surface coat. A vaccine targeting a handful of serotypes can be spectacularly effective at eliminating those specific strains, but that success creates an ecological opening for strains the vaccine does not cover. This phenomenon, called serotype replacement, is one of the most closely watched complications in bacterial vaccinology.

The pneumococcal conjugate vaccine is the best-studied example. The original seven-valent vaccine (PCV7) targeted the seven serotypes responsible for most disease at the time. Among people carrying the bacteria asymptomatically, the vaccine-targeted strains declined but non-vaccine serotypes moved in to fill the gap, so the overall rate of bacterial carriage barely changed.⁷ Fortunately, the increase in disease caused by replacement serotypes has generally been smaller than the reduction in disease from vaccine serotypes, so the net effect has still been positive. But the replacement trend is real. A study across ten European countries found that disease from non-vaccine serotypes rose by more than 80% compared to the pre-vaccination era after newer 10- and 13-valent vaccines were introduced.⁸

The practical consequence is an ongoing arms race between vaccine designers and bacterial diversity. Newer vaccines cover more serotypes, and higher-valent formulations are in development. But expanding the valency of a vaccine has limits, and the increasing diversity of disease-causing serotypes in high-income settings suggests that the benefits of simply adding more serotypes may diminish over time.⁹ Protein-based vaccines that target antigens shared across all serotypes are one potential way out of this cycle.

Vaccines as a Weapon Against Antibiotic Resistance

One of the strongest arguments for expanding the bacterial vaccine portfolio has nothing to do with the vaccines’ direct protective effects. It is about reducing the need for antibiotics in the first place. Every infection prevented by a vaccine is an infection that does not require antibiotic treatment, which means fewer opportunities for resistant bacteria to emerge and spread. The Hib and pneumococcal conjugate vaccines already have impressive track records in not only preventing life-threatening disease but also reducing antibiotic use and the prevalence of antibiotic-resistant strains.¹⁰

The economic stakes are enormous. A modeling study estimated that vaccines against key drug-resistant pathogens could have averted roughly $207 billion in hospital costs globally in 2019 alone, along with about $76 billion in lost productivity.¹¹ The World Health Organization has explicitly called for new bacterial vaccines as part of the global strategy to combat antimicrobial resistance, and several priority pathogens on the WHO’s antibiotic-resistance threat list, including Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa, are active targets for vaccine development.

Why Some Bacteria Still Have No Vaccine

Given that bacterial vaccines have been around for over a century, it is reasonable to wonder why some of the most dangerous bacteria still lack one. Staphylococcus aureus is a striking example. S. aureus infections are common, can be life-threatening, and are increasingly drug-resistant (MRSA), yet every vaccine candidate tested in large clinical trials has failed. Research suggests part of the problem is that the antibodies humans naturally generate against S. aureus surface proteins are surprisingly ineffective at killing the bacteria. In laboratory tests, human antibodies against several major S. aureus surface antigens showed poor killing activity, while antibodies targeting the bacterium’s toxins performed much better.¹² This hints that past vaccine attempts may have been aiming at the wrong targets, and that a successful S. aureus vaccine might need to focus on neutralizing toxins rather than opsonizing the bacterium itself.

Group A Streptococcus is another notoriously difficult target, but for a different reason. Certain strep proteins share structural similarities with human heart tissue, raising the specter that a vaccine could trigger autoimmune damage resembling rheumatic heart disease. This fear delayed vaccine development for decades. Recent preclinical work, however, has been encouraging: newer vaccine candidates designed to avoid the cross-reactive regions of strep proteins showed no signs of cardiac or neurological damage in animal models of rheumatic heart disease, while a control protein that included the problematic region did cause pathology.¹³ This is clearing a path for candidates to move into human trials.

Regulatory hurdles also slow progress. For some bacterial infections, the disease is rare enough that running a traditional large-scale efficacy trial is impractical. Group B Streptococcus (GBS), a leading cause of newborn infections, is one such case. WHO consultations have explored whether immunological evidence from smaller studies, combined with laboratory assays and animal data, could be used as the basis for licensure instead of requiring a massive placebo-controlled trial.¹⁴ Finding shortcuts through the regulatory pathway without sacrificing safety standards is one of the field’s active challenges.

Reverse Vaccinology and Genomic Approaches

For much of vaccination history, researchers identified targets the old-fashioned way: growing bacteria in culture, isolating components, and testing them one by one. The genomic era changed that. Reverse vaccinology starts with the bacterium’s genome sequence rather than the organism itself, using computational tools to scan for proteins likely to sit on the bacterial surface and provoke an immune response. This approach was first used successfully to develop a vaccine against meningococcal serogroup B, a strain that had resisted traditional methods because its polysaccharide capsule mimics human tissue and cannot safely be used as a vaccine antigen. The resulting vaccines, Bexsero and Trumenba, were developed through reverse vaccinology and molecular biology techniques.¹⁵

The success against meningococcal B demonstrated that genome-based approaches could crack problems that decades of conventional vaccine science had not solved. The same philosophy is now being applied to other hard targets, including S. aureus, Group A and Group B Streptococcus, and extra-intestinal pathogenic E. coli.

mRNA Technology Meets Bacterial Targets

The COVID-19 pandemic put mRNA vaccine technology in the spotlight, but its applications are not limited to viruses. Adapting mRNA platforms to bacterial pathogens is now an active area of research, though it comes with its own set of challenges. Bacterial antigens are structurally more complex than viral spike proteins, and effective bacterial vaccines often need to trigger a different balance of immune responses, emphasizing T-cell activation and mucosal protection rather than just circulating antibodies.¹⁶

Early preclinical results are promising. Self-amplifying mRNA constructs encoding antigens from Group B Streptococcus and Group A Streptococcus have been shown to produce the target proteins in mammalian cells, generate antigen-specific antibody responses, and provide consistent protection in animal models of bacterial infection.¹⁷ Progress in antigen design, mRNA engineering, and lipid nanoparticle delivery has also enabled early preclinical success against tuberculosis, Pseudomonas aeruginosa, and Streptococcus pneumoniae, though challenges in complex antigen expression, mucosal targeting, and immune durability remain.¹⁸ The speed and flexibility of mRNA manufacturing could be a major advantage if any of these candidates prove effective, since mRNA platforms can be updated quickly as new antigens are identified or bacterial strains shift.

Therapeutic Vaccines and Treating Existing Infections

Most vaccines are preventive: you get them before you encounter the pathogen. But there is growing interest in therapeutic bacterial vaccines, designed to boost the immune system of someone who already has an infection, particularly a chronic or drug-resistant one. This concept is being explored alongside other immunotherapeutic strategies, including monoclonal antibodies and immunomodulators, as a way to treat bacterial infections that no longer respond well to antibiotics.¹⁹ The idea is still largely in the preclinical and early clinical stages, but it represents a fundamentally different way of thinking about what vaccines are for.

Manufacturing Advances That Could Change the Landscape

One underappreciated bottleneck in bacterial vaccine development is manufacturing. Conjugate vaccines, for instance, require attaching sugars to proteins through a chemical process that is expensive, hard to standardize, and difficult to scale. A newer approach called bioconjugation sidesteps much of this complexity by engineering bacterial cells to produce the finished sugar-protein conjugate in a single biological step. This technology promises lower-cost, customizable glycoconjugate vaccines with a vast range of possible protein-glycan combinations.²⁰

Advances in chemoenzymatic synthesis are also making it possible to produce precisely defined bacterial sugar structures in the lab, which is relevant for developing vaccines against WHO-prioritized antibiotic-resistant pathogens like Neisseria meningitidis and others.²¹ If these manufacturing methods mature, they could make it economically feasible to produce vaccines against a broader set of bacterial pathogens, including those that primarily affect low-income countries where the cost of conventional conjugate vaccines has been a barrier.

Bacterial Vaccines in Animals and the Food Chain

The conversation around bacterial vaccines tends to focus on human health, but a huge number of bacterial vaccines are used in veterinary medicine. Livestock are routinely vaccinated against bacterial diseases like leptospirosis, clostridial infections, and certain strains of E. coli. These vaccines serve multiple purposes at once: improving animal welfare, reducing the use of antibiotics in agriculture (which contributes to resistance in human pathogens), protecting humans from zoonotic diseases that jump from animals to people, and keeping food products free of chemical residues.²²

Vaccinating poultry against Salmonella, for example, has been one of the most effective interventions for reducing Salmonella contamination in eggs and meat destined for human consumption. In an era of mounting concern about antibiotic resistance in the food supply, veterinary bacterial vaccines are quietly doing some of the most consequential work in public health.

The Childhood Mortality Gap That Bacterial Vaccines Could Close

Despite the successes already achieved, there is still a large gap between what bacterial vaccines could do and what they are currently doing. A significant share of childhood deaths worldwide are caused by infections like pneumonia and diarrhea that are, in principle, vaccine-preventable. Wider deployment of pneumococcal conjugate and rotavirus vaccines alone could produce a meaningful further decline in childhood mortality.²³ The barriers are not primarily scientific at this point but logistical and economic: cold chain infrastructure, supply, cost, and the political will to prioritize routine immunization programs. For bacterial vaccines that already exist and already work, the biggest remaining challenge is getting them into the arms of people who need them most.