Moderna Vaccine and the Immune Response: How Vaccines Work

Influenza vaccines are ready to trigger an immune reaction by mimicking virus infection. They're usually manufactured using inactivated or killed virus particles.

Taken from various circulating influenza strains. Inactive fractioned viral components contain the sub-Varian particles hemagglutinin HA and neuraminidase NA.

Photo by Artem Podrez 

In Canada, influenza vaccines are administered via intramuscular injection. These fractioned particles containing the foreign antigens hemagglutinin and neuraminidase are released from the vaccine into the bloodstream.

There they are met with an immune response mediated by various immune cells, including macrophages T lymphocytes or T-cells and B lymphocytes or B-cells.

A major mechanism of action involves macrophage phagocytosis of hemagglutinin, breaking it into smaller components. After ingestion macrophages display the hemagglutinin antigen on their surface in combination with a specific receptor known as the major histocompatibility complex or MHC.

T-cells are now able to recognize and bind foreign antigens that are associated with the MHC. Upon binding to the MHC receptor, T-cells become activated and proliferate into either cytotoxic T-cells, regulatory suppressor T-cells, or helper T-cells.

Activated helper T-cells, Express hemagglutinin receptors specific to the vaccine strand on their surface and play a major role in antibody generation and memory B-cells activation.

Unlike T-cells, B-cells are able to ingest hemagglutinin independent of the MHC. Once internalized B-cells processed the hemagglutinin antigen and presented on their surface in combination with an MHC

When activated helper t-cells interact with activated B-cells expressing antigen MHC receptors. They begin secreting lymphokines which have several effects. Lymphokines trigger activated B-cell proliferation which leads to either their differentiation into memory B-cells or into plasma cells.

Plasma cells produce hemagglutinin antibodies specific to the strain of influenza contained in the vaccine. Memory B-cells aid in future immune response when exposed to an active influenza virus.

When an infected host sneezes towards an uninfected person the nasopharynx is exposed to aerosol droplets containing whole live influenza virus. Once inhaled the influenza virus attempts viral colonization of nasopharyngeal epithelial cells.

Assuming that the vaccine strain matches that of the live virus. Hemagglutinin antibodies block the viral attachment of live influenza virus to host epithelial cells and overall disease is either avoided or diminished in severity.