As Covid-19 ravages through the population, the latest buzz word around town is immunity and the bounties that it promises. There is talk of "immunity passports" which, based on antibody tests, will enable the positive ones to re-join society, keeping the negative ones sequestered for the safety of all. Attractive as this may sound, there are several scientific and ethical reasons why this concept is problematic for society.
Let us first examine the science behind antibody testing.
Immunity: Execute, control, and conclude
The first line of defence against infection are the cells that are on surveillance duty in the body, the White Blood Cells. They recognise common patterns on invading pathogens, the viruses or bacteria, and mop them up as best they can. However, if the invaders start to infiltrate cells and multiply, or the initial assault is overwhelming, the body triggers a specific immune response and this is where the much acclaimed antibodies start being produced.
These antibodies help by preventing the virus from entering and infecting new cells, or coating the virus so that it is then easily targeted by surveillance cells and destroyed; the final victory resulting in a sea of antibodies. In addition to orchestrating the initial victory, they also create a reservoir of memory cells, so that a subsequent attack by the same offender will be dealt with promptly and efficiently. Therein lies the promise of the antibody response.
Antibody response to Covid-19, as we know it
From what we know so far, following infection by the virus Sars-Cov2 that causes Covid-19, detectable IgM and IgG antibodies develop within days to weeks from the onset of symptoms in infected individuals, but are detectable in only about 20% of patients during the early part of the disease. However, by the 3rd or 4th week since the symptoms' onset, almost all patients develop IgM and IgG, with a simultaneous increase in both.
Interpreting diagnostic tests. - Courtesy: https://jamanetwork.com/journals/jama/fullarticle/2765837
Five weeks after the onset of symptoms, the IgM begins to decline, almost disappearing by week 7. IgG however remains persistently elevated beyond the seven week period. Why some patients do not seem to develop detectable antibodies and an immune response is still uncertain.
With regards to this particular virus, there is still uncertainty regarding the relationship between antibody response and clinical improvement. The findings from a small study of nine patients with Covid-19...