An adjuvant is an ingredient added to some vaccines to enhance the immune response. The term “alum” refers to aluminum-containing adjuvants, specifically aluminum salts like aluminum hydroxide, aluminum phosphate, and potassium aluminum sulfate. These have been incorporated into vaccine formulations since the 1930s. Alum is one of the most widely used adjuvants globally, found in numerous common vaccines administered to both children and adults.
The Role of Alum in Vaccines
Many modern vaccines are not made from whole viruses or bacteria but from small, purified pieces of them, such as proteins. While this approach improves safety by eliminating any risk of infection, these purified components are often not potent enough on their own to provoke a sufficient immune response.
By amplifying the immune response, alum adjuvants provide several practical benefits. They can help reduce the amount of antigen required in each vaccine dose, which is an important consideration for manufacturing and supply. Furthermore, this enhanced response can decrease the total number of shots needed to achieve full immunity, ensuring the body builds a strong immunological memory to defend against the actual pathogen in the future.
How Alum Adjuvants Stimulate the Immune System
The mechanism by which alum stimulates the immune system is a topic of ongoing research, with current understanding having evolved significantly over time. For many years, the prevailing hypothesis was the “depot effect,” which suggested that alum particles formed a deposit at the injection site. This depot was thought to trap the vaccine’s antigens, releasing them slowly to provide a prolonged exposure to the immune system.
While the depot effect may play a minor role, contemporary research points to a more direct activation of the innate immune system. When injected, alum particles are recognized by specialized immune cells called antigen-presenting cells (APCs). This interaction triggers an internal cellular sensor known as the NLRP3 inflammasome.
Once activated, the inflammasome directs the cell to release pro-inflammatory signals, including cytokines like IL-1β and IL-18. These signaling molecules act as an alarm, recruiting a variety of other immune cells to the injection site. This localized inflammation and cellular recruitment create a heightened state of immune readiness, leading to a more efficient uptake of the vaccine antigen by APCs and a stronger subsequent adaptive immune response, which is responsible for long-term protection.
Safety and Regulation of Alum Adjuvants
Aluminum-containing adjuvants have been used in vaccines for over 90 years, administered in billions of doses worldwide, establishing a long-term safety profile. Regulatory bodies and global health organizations, including the U.S. Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO), continuously review safety data and affirm their use. The amount of aluminum in a single vaccine dose is extremely small, with quantities ranging from approximately 0.25 mg to less than 1.5 mg per dose depending on the specific vaccine.
To put this into perspective, humans are regularly exposed to aluminum through their diet. It is naturally present in water, fruits, and vegetables. An average adult in the U.S. consumes an estimated 7 to 9 milligrams of aluminum per day through food. The amount of aluminum in a vaccine is comparable to that found in about one liter of infant formula.
The types of aluminum salts used in vaccines are also poorly absorbed into the bloodstream and are efficiently filtered by the kidneys and cleared from the body. The most common side effects associated with alum-adjuvanted vaccines are local reactions at the injection site, such as redness, swelling, or soreness. These effects are generally mild and temporary, and they are an expected outcome of the adjuvant’s function, indicating that the innate immune system has been successfully activated to begin building protection.
Vaccines Containing Alum Adjuvants
Alum adjuvants are a component of many routine vaccines recommended for children and adults. They are not used in live-attenuated viral vaccines, such as those for measles, mumps, and rubella, because the weakened viruses in those vaccines are potent enough to stimulate the immune system without assistance. Instead, alum is found in vaccines made from inactivated (killed) pathogens or from purified components of a pathogen.
Common examples of vaccines that contain alum include the DTaP vaccine, which protects against diphtheria, tetanus, and pertussis (whooping cough). Alum is also used in vaccines for Hepatitis A, Hepatitis B, and Human Papillomavirus (HPV). Other vaccines that rely on alum’s immune-enhancing properties include certain pneumococcal and meningococcal vaccines, which protect against serious bacterial infections.