The SARS-CoV-2 virus, responsible for COVID-19, is characterized by spike proteins on its surface. These structures are the machinery the virus uses to initiate an infection. The spike protein is the primary target for COVID-19 vaccines, which are designed to teach the body’s immune system to recognize and fight this specific part of the virus. This focus allows vaccines to prepare the immune system without exposing the body to the full virus.
The Role of the Spike Protein in Viral Infection
In a natural infection, the spike protein of the SARS-CoV-2 virus functions like a key seeking a lock on the surface of human cells. This “lock” is a receptor called angiotensin-converting enzyme 2, or ACE2. The spike protein binds to ACE2 receptors, which are found on various cells, particularly in the lungs. This binding action triggers a process that allows the viral membrane to fuse with the host cell membrane.
Once fusion occurs, the virus releases its genetic material into the human cell. Inside, the virus hijacks the cell’s machinery, forcing it to produce countless copies of itself. Each new virus particle is complete with its own set of spike proteins, ready to spread and infect more cells.
Vaccine-Instructed Spike Protein Synthesis
COVID-19 vaccines provide the body with instructions to build a harmless version of the spike protein. Messenger RNA (mRNA) vaccines, from Pfizer-BioNTech and Moderna, deliver these instructions via an mRNA molecule. This mRNA is encased in a protective lipid nanoparticle that helps it enter muscle cells at the injection site. Once inside, the cell’s ribosomes read the mRNA blueprint and begin assembling spike proteins.
Viral vector vaccines, like the one from Johnson & Johnson, use a different approach. This type of vaccine uses a modified, harmless adenovirus as a delivery vehicle. The adenovirus is engineered to carry a segment of DNA that codes for the SARS-CoV-2 spike protein. After injection, the vector virus enters human cells and releases this DNA into the nucleus, where it is transcribed into mRNA, which is then used to manufacture the spike protein.
For both vaccine types, the goal is for the body’s own cells to produce only the spike protein. These newly synthesized proteins are then displayed on the surface of the cells, where they can be detected by the immune system.
The Body’s Immune Response and Clearance
Once cells at the injection site and nearby lymph nodes begin displaying the vaccine-instructed spike protein, the immune system recognizes it as foreign. This recognition activates a coordinated response. Specialized immune cells, known as antigen-presenting cells, engulf the spike protein and show it to other immune cells, including T-cells and B-cells.
The B-cells are stimulated to produce antibodies, which are proteins that can specifically bind to the spike protein. These antibodies circulate in the bloodstream and can neutralize the actual virus if it enters the body later by blocking the spike protein from attaching to human cells. The body also generates memory B-cells and T-cells, which remain long-term to mount a faster response upon future exposure.
The components from the vaccine do not remain in the body indefinitely. The mRNA from Pfizer and Moderna vaccines is broken down by natural cellular processes within a few days. The spike proteins produced by the cells last for up to a few weeks before they are also degraded and cleared away. This is a normal process that ensures the vaccine’s components are disposed of after training the immune system.
Addressing Common Concerns about Vaccine Spike Proteins
A primary concern involves the safety of the vaccine-produced spike protein. The quantity generated from vaccination is small and localized to the injection site and nearby lymph nodes. In contrast, a natural infection produces large amounts of spike protein as the virus replicates, which contributes to the inflammation and tissue damage seen in severe COVID-19.
Another concern is the idea of “shedding” spike proteins. This is not supported by how these vaccines function, as they instruct cells to produce only the spike protein, not the complete, infectious virus. Without the full virus, there is no replication and no way for a vaccinated person to transmit viral components. Shedding is a phenomenon associated with different vaccine technologies, such as certain live-attenuated viruses.
The protein generated by vaccines is also engineered to be less adept at entering cells than the version on the live virus. This modification helps focus its role on stimulating the immune system with less potential for harm.