Declining Vaccine Efficacy

Declining Vaccine Efficacy

Subject – Science and Tech

Context – Recent data suggest that the effectiveness of Covid-19 vaccines has been declining.

Concept –

• The data indicate that while the protection against infection (as shown by effectiveness against symptomatic Covid) has declined (especially in Israel and USA), the protection against hospitalisation from Covid remains high.
• Ideally, a public vaccination programme should protect the population from infection, transmission, and hospitalisation (and subsequent mortality).
• Initial vaccine efficacy data showed high efficacy versus infection and hospitalisation. (The correlation between infection and transmission is not automatic and has to be proven. Some post-clinical trial data showed this for the current vaccines.)

Working of Immune System –

• The virus, as it ‘infects’ the body, is primarily found in two locations. One is the circulation system that it uses to travel around the body. The second is the cells of various tissues that the virus invades and uses to multiply.
• Logically, therefore, the immune system has two main ‘arms’ to confront the virus in these two locations.
• One is the antibody arm. Antibodies ‘lock in’ on certain surface proteins of the circulating virus, thereby preventing it from invading our cells. Further, they ‘tag’ the virus for destruction. Thus, antibodies can be thought as a first line of defence, but they become ineffective once the virus enters the cells.
• At this point, the second arm of the immune response becomes relevant.
• This arm is aptly named the Killer T cell arm. These cells target our own bodies’ cells that harbour the virus and within which the virus replicates.
  • The T cells kill such cells, thereby eliminating the virus within them. Proceeding with a simplistic view that disease is caused once the virus takes hold in our bodies’ cells, a strong T-cell immune function can protect from severe disease even if the antibody response is weak.
• Vaccination establishes the two arms of the immune response; these two arms can mature differently with time and in response to variants. Circulating antibody levels decline with time.
• Even though there is “memory” in the system to produce antibodies on-demand, jumpstarting this memory at a subsequent encounter can take time.
• A weak and delayed antibody response would result in an infection (symptomatic Covid), but if the T-cell response is intact, the individual would be protected from severe disease.
• Additionally, vaccine effectiveness can reduce because the immune system that has been primed by one variant has to counter a new variant.
• Even here, the differences in response of the two arms to a variant are crucial. The antibody arm reacts to the viral surface proteins (primarily the shape, or the 3D configuration), and thus changes in this surface protein can reduce the effectiveness of the antibody response.
• The T cells, however, react to smaller fragments of the surface and other viral proteins. Since the T cells respond to a broader set of targets — more proteins (surface and non-surface) and more sites on the proteins (multiple fragments) as compared to antibodies that respond to a specific site (or sites) on the surface protein controlled by the local “shape” at the site, the T-cell response can be more resistant to variants.
• Recent studies tracking a few individuals have reported that the T-cell response to Covid vaccines is durable and effective versus the variants.
• The simplified picture also indicates that circulating antibodies are not the entirety of the available protective resources. Antibody tests are easier to implement at large scale as compared to T-cell measurements and are therefore widely available.