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    Key things to know about COVID-19 immunity

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    Since SARS-CoV-2 spread, causing a worldwide pandemic of disease called Covid-19, It has been studied and learned so much about this virus. Scientists around the world have devoted endless efforts to understand the duration and effects of immunity against this disease, and more and more progress has been made in this discovery, although there are still many questions to answer about the various reactions of the immune system to this virus. But what do we know at this point?

    Immunity occurs when the body protects us from infection by an external pathogen, so that after being exposed to this pathogen, either naturally or through vaccination, our immune system acts by generating antibodies that will defend us against it in the future.

    The first response of our immune system is the innate defence against the intrusion of the virus itself at the time of infection (in the short term, to prevent its advance). The second response is the one that produces antibodies to eliminate the pathogen from our cells and protect us from future infections (producing memory T and B cells).

    What is humoral and cell-mediated immunity?

    Defence against pathogenic organisms (such as viruses) is mediated by coordinated sequential responses: innate immunity and adaptive immunity. Innate immunity is essential to defend against micro-organisms in the first hours or days after infection. After this time, adaptive immune responses are triggered, which develops as a response to an infection to which it is adapted.

    Within so-called adaptive immunity there are two types of responses: humoral immunity and cell-mediated immunity, both of which will act to fight pathogens.  Humoral immunity will do so through antibodies produced by B lymphocytes, which are present in the blood and mucous secretions.

    Cell-mediated immunity, on the other hand, is mediated by T-lymphocytes; in cell-mediated immunity, micro-organisms and their toxins are attacked directly by cells. T lymphocytes, once the immune response has been activated, will give rise to memory T lymphocytes, so that in future infections with the same micro-organism, the organism will be able to recognise it and act more effectively and quickly. Like T-lymphocytes, B-lymphocytes also produce memory cells, which facilitate an immune response to an infection.

    How long immunity against SARS-CoV-2 last?

    Recent studies have shown that people who have overcome Covid-19 by natural infection have developed a good memory of antibodies to the virus for 8 months after infection.

    On the other hand, it is the cell-mediated immunity we have already discussed that protects against severe Covid-19 infection when the antibodies generated by humoral immunity are no longer present. In a recent study, scientists at the Institute for AIDS Research (IrsiCaixa) have Observed how people who have not produced antibodies against the virus despite having passed Covid-19 can fight the virus thanks to cell-mediated immunity.

    In this sense, another study published in the journal Science added that IgG antibodies were more abundant for more than 6 months, and B cells (Cellular Immunity) were more abundant 6 months after the onset of symptoms.

    Vaccinated individuals are contagious?

    As far as we know, a vaccinated person could spread the virus, as a vaccinated person is protected against the disease, but not against the virus.   This means that we can become infected without symptoms and continue to be carriers, and therefore transmit the virus. Therefore, only collective immunity or herd immunity will protect us against the disease and against the virus as well.

    Furthermore, once an individual immunity is activated, in the case of the vaccines currently on the market, it appears that they do not completely inhibit virus replication in the mucous membranes of the respiratory tract.

    And then, what does the vaccine protect against? The vaccines approved by the European Union to tackle the pandemic have been shown to protect against the disease, preventing the severe symptoms caused by the infection and reducing the consequences of the infection on the organism.  These vaccines, regardless of the technology they use, aim to teach our immune system to recognise and fight the virus without making us sick, but they cannot prevent us from get in contact with the virus.

    Natural immunity vs vaccine immune response to covid-19

    When we are vaccinated against Covid-19 we generate antibodies against a specific part of the virus, since vaccines are designed to develop immunity against one or more proteins of the virus.

    This means that assays that are based on identifying antibodies generated against a different part of the virus will not give a positive result in a vaccinated patient. For example, if an assay target the nucleocapsid protein of the virus, and we are vaccinated with the modern vaccine (which incorporates the genetic material of the virus, so that it expresses the spike protein in the body, without expressing the rest of the virus), this test will not identify antibodies, since the nucleocapsid protein has not appeared.

    Therefore, It is especially important to know what information each type of serological test will provide.

    Immunostep carried out a study with individuals vaccinated with Pfizer, Moderna and AstraZeneca vaccines. A comparison of the IgG and IgA responses against the four SARS-CoV-2 antigens of 65 vaccinated individuals was performed and compared with that of naturally infected individuals. As expected, vaccinated individuals only showed reaction to the S and RBD antigens, and IgG was the predominant isotype in vaccinated individuals while sera from naturally infected donors showed antibodies against all four viral antigens.

    Thanks to this assay we can confirm that antibodies are only produced against the protein used in the vaccine, while in a person who has been infected, antibodies are also generated against the other antigens included in this test.

    Can tests detect whether I have been infected with covid-19 again?

    You can find out if you have been infected with covid-19 through antibody tests, or serological tests. Serological tests are those that detect the antibodies generated as a result of an immune response of our organism to fight against a pathogenic organism, in this case, a virus. Some of them can also identify the immune response to vaccination.

    There are two main groups of serological tests that detect different types of antibodies: those for self-diagnosis and those that require laboratory analysis. In both cases, the aim is to identify immunoglobulins of different types (IgG, IgA, or IgM) generated by B lymphocytes in the body. There can also be qualitative tests (which tell you whether you have antibodies) and quantitative tests (which also tell you the number of antibodies generated).

    In this sense, it is important to know that serological tests are created to detect your immune system response against specific proteins of the virus (for example, protein S or protein N in the case of SARS-CoV-2), each of these proteins provides different information about the immune response and depending on whether you have been vaccinated or have suffered from a natural infection, one protein or another should be tested.

    Can I be infected again if I have developed antibodies? How do I know if I have antibodies?

    It is not yet precisely determined how long antibodies protect us, and the role of T-lymphocytes in long-term defence, although it seems that they may act sufficiently to keep us safe, it is important to know that it is possible to become infected again if the number of antibodies we have generated is not sufficient. This information can be found through quantitative serological tests, which will tell us how many antibodies we have and therefore whether we are still protected enough.

    Can I donate blood if I am immunised?

    People who have been vaccinated against covid-19 can donate blood without any problem, donating blood after receiving a vaccination against covid-19 does not reduce the protection of the person donating, and there is no reason to avoid donating blood.