When COVID-19 infects a human (host) the virus enters host cells and hijacks the machinery of the cell to replicate. The new virus particles are then released from the infected cells and can infect more cells. The infection initially activates an innate immune response, generic to all infections, which does not result in immune memory. Over a number of days the host then develops an adaptive immune response that is longer lasting and has memory. The adaptive immune response has two distinct arms, one using T-Cells and the other B-Cells.
The T-Cell immune response is also known as Cellular Immunity. T-Cells (specifically Killer T-Cells) kill host cells infected with the virus. T-Cell immunity is particularly important for viruses and may result in lasting immunity to COVID-19 effective against mutated strains. It seems likely that Adenovirus vaccines (such AstraZeneca) induce more T-Cell immunity than mRNA vaccines (such as Pfizer/BioNTech). Unfortunately, tests for T-Cell immunity are specialized and not readily available commercially.
The B-Cell immune response, also known as humoral immunity, produces antibodies. Neutralizing antibodies coat the virus and prevent its entry into host cells. The antibodies produced by an infection (or vaccination) will reduce over time but the B-Cells that produce the antibodies are stored and subsequent exposure to the virus will produce a much faster antibody response. Tests for antibody levels are readily available.
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