What will the antibody test tell you about?
Immune protection, of course. We are protected from infections by nonspecific innate immunity (natural killers, phagocytes, interferons, complement system), acquired humoral immunity (antibodies) and acquired T-cell immunity (T-killers).
The main role in protection is played by the acquired immunity, which is formed after meeting with an infection or antigen in a vaccine. It is at the formation of acquired immunity that vaccination is aimed. You can read about humoral and T-cell immunity in the articles https://habr.com/ru/post/569316/ https://habr.com/ru/post/569292/
The most protective antibodies that neutralize SARS-CoV-2 are antibodies to the spike protein (S-protein) and its RBD domain. Quantitative, commercially available IgG to RBD Abbott II and IgG to S protein Diasorin correlate well with the neutralizing ability of the sera measured in the live SARS-CoV-2 assays.
Unfortunately, it is impossible to take into account all these factors in order to predict the individual risks of the likelihood of infection and the severity of the course of COVID-19. However, some factors are especially important and can serve as a guideline for assessing individual risk. There is more and more evidence that humoral immunity is such a factor.
What research do we have?
1) Mathematical models.
Epidemiological efficacy of vaccines (according to CI data) correlates with the level of neutralizing antibodies. The dependence of the effectiveness of protection on the level of neutralizing antibodies is well described by the mathematical model [3]
2). Population studies…
A study of 4,372 AstraZeneca vaccinated in the UK (144 cases) confirmed a correlation between protection against SARS-CoV-2 infection and S protein IgG levels. 80% efficacy against alpha challenge was achieved at 264 BAU / ml [4]
A study of 39 breakthrough infections in 1,497 medical workers in Israel (85% of infections caused by the alpha strain) showed a correlation between the level of neutralizing antibodies and viral load [5]
In a population study of humoral and T-cell immunity of the Moscow Department of Health, 5,340 people took part. The presence of antibodies alone or antibodies and T-cell immunity provided a higher level of protection than T-cell immunity alone (the study began before the spread of the delta strain) [6]
A population study to determine the protective level of antibodies for the delta strain was carried out by the V1V2 project https://t.me/delta_self_research… In July-September 2021, the telegram group collected the latest results of analyzes for IgG antibodies to RBD or S protein after illness (in 1-2 waves) and vaccination among 967 residents of Moscow and St. Petersburg. Within 2 months after passing the test, 49 people fell ill. In 91.7% of the patients, the IgG level was less than 370 BAU / ml according to Abbott II (P <0.05) and less than 252 EU / ml according to Diasorin (p <0.001). Taking into account the dynamics of the decrease in antibodies, the real protective values at the time of the encounter with the virus could correspond to 320 BAU / ml according to Abbott II QUANT or 200 OU / ml according to Diasorin S1 / S2. More details [7]
The protective value of IgG of 300 BAU / ml against infection with the delta strain was announced on November 8 by A. Gunzburg. https://tass.ru/obschestvo/12859041 The study involved 4,000 Muscovites, but the details were not disclosed.
3) Laboratory research and cases from medical practice.
Studies of animals and humans with a deficiency of humoral or T-cell immunity have shown that humoral immunity protects against COVID-19 more effectively than T-cell immunity.
Depletion (removal) of T-helpers and T-killers in primates only slightly increased the recovery time. All animals with T-cell deficiency quickly recovered due to the synthesis of neutralizing antibodies and were protected from reinfection. [8]
In contrast, a deficiency in humoral immunity has been associated with a severe form of COVID-19 and long-term persistence of the virus in the body. Such patients were effectively treated with monoclonal antibodies and plasma of patients who had been ill, containing antibodies [9-13]
It is important to note that protective values of antibodies, at which infection becomes an unlikely event, have been described for other viral infectious diseases. For example, for hepatitis B, the protective IgG titer, which provides resistance to infection in vaccinated, is determined at 0.01 IU / ml (“Assessment of the quality and effectiveness of immunoprophylaxis”. N. I. Briko) [14]… For measles, the IgG protective value is 120 U / ml [15]
