Strep throat is a common bacterial infection, leading many to question if immunity is possible. While the human body can build some protection, it differs from the lifelong immunity seen with certain viral infections like chickenpox. This article explores the complexities of strep immunity, explaining why recurrent infections are possible and what ongoing research aims to achieve for broader protection.
The Many Varieties of Strep Bacteria
Strep throat is caused by Group A Streptococcus (GAS), also known as Streptococcus pyogenes. This bacterium exists in numerous distinct types or “strains,” with over 200 different strains characterized worldwide.
These strains are distinguished by variations in a surface protein called the M protein. The M protein extends from the bacterial cell wall and is a target for the human immune system. Each distinct M protein type, or M-type, presents a unique antigenic signature. Common M-types include M1, M12, M28, and M3.
How Your Body Learns to Fight Strep (Sometimes)
When your body encounters a specific M-type of Group A Streptococcus, your immune system produces specific antibodies to recognize and neutralize that particular strain. These antibodies target the unique surface proteins, like the M protein, of the invading bacteria.
This immune response provides “strain-specific” immunity. If you are exposed to the same M-type again, your body’s existing antibodies can quickly act, often preventing a new infection or reducing its severity.
Why Strep Can Keep Coming Back
Despite developing strain-specific immunity, individuals can experience recurrent strep throat infections. This occurs because many different M-types of Group A Streptococcus circulate. Immunity to one M-type does not protect against infection from another.
Children often encounter strep throat multiple times due to frequent exposure in close-contact environments like schools and daycares, increasing their chances of encountering new M-types. Adults tend to experience fewer strep infections, likely because they have developed immunity against many common M-types over the years. Other factors contributing to recurrence include incomplete antibiotic treatment or being a “carrier” of the bacteria without symptoms.
The Quest for Broad Strep Immunity
The challenge of recurrent strep infections and the potential for serious complications, such as acute rheumatic fever and rheumatic heart disease, drives ongoing efforts to develop a vaccine. A hurdle in vaccine development is creating a vaccine that offers broad protection against the numerous diverse M-types of Group A Streptococcus. Researchers aim to identify a common characteristic among all strep strains that the immune system can target, rather than relying on strain-specific responses.
Current vaccine candidates often focus on parts of the M protein shared across multiple strains or on other universally present bacterial components. Such a vaccine would reduce the global burden of strep-related illnesses, particularly where complications like rheumatic heart disease are prevalent. While progress has been made, with some candidates reaching early human trials, a widely available, universal strep vaccine has not yet been achieved.