Since December 2019, SARS-CoV-2 (Severe Acute Respiratory Syndrome-related Coronavirus-2) has been responsible for a worldwide pandemic.
The immunity developed from contact with the virus (post-disease) comes in three forms:
- Localised (particularly with action of dendritic cells and IgA in the mucous membranes).
- Cellular (CD4 and CD8 T lymphocytes targeted against all proteins of the virus).
- Humoral (production by B lymphocytes of antibodies targeted against the spike (S) and nucleocapsid (N) antigens of the virus: with the appearance of anti-N, anti-S and, more specifically, anti-S1 and anti-RBD type antibodies).
In the bitter struggle against this virus, the marketing of vaccines with different mechanisms of action began in December 2020. For each vaccine, a vaccine protocol is defined for the first injections as well as for the booster injections according to the 2020 Vaccination Plan.
Initial publications on the study of post-vaccine immunity show a similar response to the disease with the appearance of a humoral response and a cellular response(*).
Since the vaccines currently on the market only provide one precursor or vector of protein S, the anti-N antibodies are not developed and only anti-S antibodies appear (and targeting its sub-domains, S1 and RBD).
Thus, screening for anti-S or anti-RBD antibodies can be used to search for the presence of humoral immune markers after a Covid infection (or to signal a previous contact if diagnosis could not be made), as they seem to persist for longer than anti-N antibodies.
Furthermore, anti-S or RBD antibodies can also be used post-vaccination.
According to current medical knowledge, the time that these markers are present (post-disease or post-vaccination) is not yet known, nor is their formal efficacy (however, reinfection rates appear to be anecdotal with respect to the number of infections reported worldwide).
(*) It should be noted that whether post-disease or post-vaccination, the humoral response and the cellular response are not necessarily correlated.
Brief overview of the mechanism of action of the Virus (French Health Authority Report “Immunity and SARS-CoV-2” from 25 Nov. 2020)
Virus structure
Genome structure
The spike protein has 2 regions: S1 and S2
The RBD or Receptor Binding Domain is located in region S1.
The RBD interacts with the ACE2 (or Angiotensin-Converting Enzyme 2) receptor, which is an analogue of the angiotensin-converting enzyme 1 involved in regulating blood pressure.
The ACE2 receptors are present in the epithelial cells of the bronchi and the pulmonary alveoli (but also in the arteries, heart, kidneys and the brain).
This binding of the virus by means of its S-RBD part to the ACE 2 receptors has a very high affinity and will lead to penetration of the virus into the cells. It will then begin its replication cycle.
Replication cycle of SARS-CoV-2
What is an anti-S or anti-RBD antibody?
Anti S-RBD antibodies come from a humoral-type immune response following contact with SARS-CoV-2.
Their role will be to prevent the virus, on second contact, from penetrating the cells by inhibiting its binding to the ACE2 receptor. Replication will no longer be possible and the infection will be stopped.
How can we determine that these antibodies have a neutralising action on the virus?
The gold standard used to determine the neutralising action of an antibody on a virus is achieved by a “true” serum neutralisation technique using a live virus that must be handled in a P3 safety laboratory. According to several publications, it has been demonstrated that the presence of anti-S1, anti-RBD and anti-S antibodies is correlated (for each sub-type of antibody) with serum neutralisation. Anti-S-RBD are therefore considered to be neutralising, while conversely anti-N antibodies do not have any neutralising activity.
The rate of anti-RBD or anti-S antibodies will allow evaluation of the presence of neutralising antibodies targeted at the virus not only during a natural infection but also at the time of vaccination.
What are the conditions for performance of the assay of anti-RBD antibodies?
- Analysis code: SRBD
- Minimum sample of 500 microlitres of serum
- Transport temperature: refrigerated
- Price: Contact the International Division
- No special Clinical Data Sheet
- For the instruction, state: “Assay of anti-S or anti-RBD antibodies”
For non-specific requests such as:
- “Screening for post-disease or post-vaccination immunity or antibodies”
Screening for anti-S/RBD will be carried out
For vaccination, the optimal response is between D10-D14 after the booster (2nd injection) - “Covid serology” without specification
The current screening for anti-N antibodies will be carried out
Important reminder: The existence of anti-RBD antibodies or anti-S antibodies will make it possible to reach a conclusion regarding the existence of an anti-CoVID19 humoral immunisation process and proof of previous contact with the virus or of vaccination.
However, pending better experience with the virus, this will not allow any conclusion regarding formal, long-term protection against the disease.
