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How do I know if we have achieved full immunity against the disease after receiving the COVID-19 vaccine? Tests determining the level of IgG are helpful in this. It is best to perform them 7-14 days after receiving the second dose, experts from the Science Against Pandemic initiative say.
The COVID-19 vaccination program is ongoing, and at the same time more and more people are interested in serological tests that allow to partially assess the immune system’s response to the administered preparation, medical experts note from the Science Against Pandemic initiative. In order to obtain such information, it is worth knowing what type of test to choose and when to conduct it.
- An effective tool that allows you to check whether our body has developed resistance to the coronavirus is the serological test – argue scientists associated with the Science against Pandemic initiative
- However, it is worth knowing that the tests differ in terms of reliability. So which one should you choose?
- Experts also explain when exactly we should perform such a test so that the answer is reliable
- You can find more such stories on the TvoiLokony home page
Vaccine: when does antibody production start?
Dr hab. Piotr Rzymski from the Medical University of Karola Marcinkowski in Poznań emphasizes that after the administration of the COVID-19 vaccine, several different processes of the immune system are stimulated. Only one of them involves the production of specific antibodies against the SARS-CoV-2 virus.
So, is it possible to verify whether we have developed the correct answer? «Yes, although there are some limitations to bear in mind. First, that the tests differ in their reliability; the type of test also matters. When it comes to checking long-term protection, IgG antibodies will be important, as they appear quite late after the disease, but also remain in the body for the longest time »- reminds Prof. Roman.
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The immunological tests available on the market allow to assess the presence of various classes of antibodies against SARS-CoV-2 in the serum: IgA, IgM and IgG. The approved vaccines stimulate the production of antibodies of all three classes. However, in practice – as Prof. Krzysztof Pyrć from the Jagiellonian University – to verify the immune system response after vaccination, tests determining the level of IgG are used, which are called later-phase antibodies, and whose elevated level lasts longer than IgM. For this purpose, only quantitative tests should be selected, which are characterized by high reliability of the results obtained. The expert also emphasizes that such tests should be carried out in certified laboratories.
In patients who have been infected with SARS-CoV-2, IgG antibodies may be directed against various components of the virus, including nucleocapsid (N) protein. In vaccinated persons, only the production of those directed against the S protein is stimulated. Therefore, in order to assess the effect of the vaccine, one can choose one of several tests.
One of them is a quantitative test assessing the concentration of IgG antibodies directed against the S1 subunit and the nucleocapsid (N) protein. This makes it possible to distinguish vaccinated people who have not been infected with SARS-CoV-2 in the past (IgG antiN – negative, IgG S1 – positive) from those who have been in contact with the virus (IgG antiN – positive, IgG S1 – positive). The second type of test allows the quantitative measurement of the concentration of IgG antibodies specific for the S1 and S2 subunits of the coronavirus S protein. The third allows the quantitative measurement of the concentration of IgM and IgG antibodies recognizing the receptor binding domain (RBD) located in the S1 subunit of the coronavirus S protein.
You do not need to be prepared for the test, you do not need to fast. The collected material is venous blood.
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When to take the test «According to clinical trials of COVID-19 vaccines that have been approved for use in the EU, IgG antibodies should be detectable no earlier than 14 days after the 1st dose. There is no point in doing an immunoassay beforehand. For the available SARS-CoV-2 vaccinations, as a rule, two doses of vaccine are needed to achieve vaccine immunity. In addition, a number of a person’s characteristics, including age, can influence the later appearance of IgG antibodies. Therefore, it is recommended to carry out the test 7-14 days after receiving the second dose – then we should get full immunity »- says prof. Krzysztof Simon, a specialist in the field of infectious diseases.
How to interpret test results? «It should be remembered that there is no specific upper limit of the norm regarding the level of IgG antibodies to SARS-CoV-2. In the above-mentioned tests, only the threshold above which the result is considered positive is set. If the result is positive, it should be assumed that the administration of the vaccine has effectively stimulated the immune system »- she adds.
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When do we get full protection against the virus?
Prof. Andrzej Fal from the Cardinal Stefan Wyszyński University in Warsaw also points out that this does not mean that we are completely protected against SARS-CoV-2 infection. Full protection against COVID-19 is achieved 1-2 weeks after the second dose. Second, it is currently unknown whether vaccination protects against infection. In clinical trials of vaccines, not only against COVID-19, the endpoint for efficacy evaluation is symptomatic infection. Therefore, both between the administration of the first and second dose, as well as after the completion of the vaccination course, the sanitary recommendations should still be adhered to.
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Prof. Fal points out that the result obtained at the same time and with the same test may differ from person to person. This does not necessarily mean different levels of protection. The results obtained in the three above-mentioned tests are not comparable with each other because they define antibodies directed against different regions of the S protein.
As he points out, the administration of the COVID-19 vaccine stimulates various processes in the immune system, not only related to the production of antibodies. For this reason, one should not make too far-reaching conclusions on the basis of the results of the serological test alone.
A separately initiated response is of a cellular nature, related to cytotoxic T lymphocytes – explains Prof. Simon. They secrete a number of antiviral cytokines and have the ability to identify virus-infected cells and destroy them, which prevents the virus from multiplying and spreading in the body. The cellular type response is of particular importance in suppressing the progression of the disease to a severe form. In addition, vaccinated people develop memory B cells that store information about the coronavirus S protein. Thanks to them, it is possible to immediately resume the production of antibodies in a situation when the body of the vaccinated person is in contact with SARS-CoV-2. For this reason, the possible detection of a decrease in serum IgG levels some time after inoculation does not amount to a decrease in immunity.
Commercial serological tests are not able to assess the activity of T and B cells – this requires specialized tests that are not widely available – says Prof. Throw. Therefore, the determination of the level of antibodies after vaccination, while important, only partially assesses the developed immunity.
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