A bivalent vaccine is a formulation designed to elicit an immune response against two different antigens. The prefix “bi-” signifies two, while “valent” pertains to the specific component the vaccine targets. This means the vaccine can protect against two separate viruses or two distinct strains of the same virus, providing broader protection than a vaccine targeting only a single entity.
How Bivalent Vaccines Work
To understand how a bivalent vaccine functions, it is useful to compare it with a monovalent vaccine, which protects against a single target. A bivalent vaccine contains components, such as messenger RNA (mRNA) instructions or pieces of antigens, for two different strains of a pathogen. This combination is administered in a single dose.
When introduced into the body, these components instruct cells to produce antigens specific to each of the two targeted viral strains. The immune system recognizes these antigens as foreign and begins to mount a defense. This process involves generating specialized antibodies and memory cells for both antigens simultaneously.
This dual-pronged approach trains the immune system to recognize and neutralize two different threats. It is analogous to upgrading a security system to identify two distinct types of intruders instead of just one. The result is a more comprehensive immune memory, preparing the body to combat either of the targeted strains upon future exposure.
The Bivalent COVID-19 Vaccine
The bivalent COVID-19 vaccine is a prominent example of this technology, developed to address the evolution of the SARS-CoV-2 virus. These vaccines were formulated to target two threats: the original ancestral strain of SARS-CoV-2 and a strain from the Omicron variant family. For instance, different bivalent formulations targeted the BA.1 or the BA.4 and BA.5 Omicron subvariants alongside the original virus.
The rationale for this combination was strategic. Including the ancestral strain aimed to maintain the protection established by the original monovalent vaccines. At the same time, incorporating an Omicron component was a direct response to variants circulating widely that could evade prior immunity.
This approach was designed to broaden the immune response. By exposing the immune system to both the original and a newer variant, the vaccine prompts the production of a wider range of antibodies. This enhances the body’s ability to neutralize the targeted strains and potentially future variants, strengthening overall protection against severe disease.
Bivalent Technology in Other Vaccines
Bivalent technology extends beyond the response to COVID-19 and is an established strategy in vaccine development. A significant example is the bivalent human papillomavirus (HPV) vaccine, which protects against HPV types 16 and 18. These two high-risk types are responsible for approximately 70% of all cervical cancers.
The concept of targeting multiple strains is also fundamental to seasonal influenza vaccines, which are often developed as trivalent or quadrivalent formulations. A trivalent vaccine protects against three flu viruses—two influenza A strains and one influenza B strain. A quadrivalent vaccine adds a second influenza B strain for broader coverage.
This principle allows for adapting vaccines to target the most relevant circulating strains of a pathogen. For many years, quadrivalent flu vaccines were preferred, but recommendations have shifted toward trivalent vaccines for the 2024-2025 season. This change was due to the disappearance of the B/Yamagata lineage virus.
Adapting Vaccines for New Variants
The development of bivalent and multivalent vaccines is a direct response to viral evolution. Viruses, especially RNA viruses like coronaviruses and influenza, are prone to mutations as they replicate. These genetic changes can lead to new variants that differ from their predecessors.
New variants can possess alterations in their surface proteins that allow them to partially evade immunity from past infections or previous vaccines. The immune system’s antibodies may not recognize these altered proteins as effectively, reducing the level of protection. This is a primary driver for updating vaccine formulations.
Bivalent technology is a proactive strategy that allows vaccine science to keep pace with this viral evolution. By incorporating components of newer, circulating variants, these updated vaccines help broaden and refresh the immune system’s defensive capabilities. This ensures that protection remains robust against a constantly changing viral landscape.