Am I immune to Omicron if I have already become infected with the Delta variant?
Q&A #12 with Geert Vanden Bossche
"Those who became infected with the Delta variant are therefore not immune to the Omicron," says Frank Vandenbroucke, Minister of Public Health Belgium. Is this correct? Will my T cells then not recognize the coronavirus? Or will my antibodies not protect me? Or maybe I will be infected asymptomatically and thus not get sick and then this is equivalent to "after vaccination"?
When you get infected with another variant there is always a chance that you will get sick. However, if you are in good health, the chance that SARS-CoV-2 will make you seriously ill is negligible. We owe this to our innate immunity which - especially in young people - is the first line of defense to clean up and eliminate large amounts of the virus (vacuum cleaner!). Young people, but even all healthy people who are in excellent health (e.g. no excess weight and regular exercise / sport), will often not even get sick or at best develop some vague, mild symptoms. If the first line of defense is broken, then our acquired immune system rushes to the rescue whereby our T cells ensure that the sick, virus-infected cells are eliminated. This allows us to recover from illness.
But whenever our innate immune system is exposed and eliminates the virus (with or without the help of the acquired immune system) it also immediately learns to recognize the virus better in the future. While it continues to recognize all SARS-CoV-2 variants (and even all CoVs), it now does so with more efficiency/affinity. This phenomenon is called "training" of the innate immune system. It is a form of adaptive immunity caused by epigenetic changes that effect a reprogramming of immune cells that secrete innate antibodies. That is, with subsequent exposure to the virus, there is an increasing chance that that person will develop an asymptomatic infection and actually not get sick at all, even if the virus undergoes antigenic drift (antigenic drift). If the virus undergoes an antigenic shift (i.e., severe change due to multiple mutations as in the case of Omicron), then the innate immunity will have to train again for a while before being able to withstand an infection with such a variant without giving rise to illness.
A pandemic is of course an excellent opportunity to train the innate immune system against SARS-CoV-2. However, it also means that if a variant with an antigenic shift (e.g. Omicron) dominates, more people may become ill anyway and within a short period of time the virus will be under pressure due to the induced natural antibodies, which are not able to suppress the virus at high infection pressure. Reducing the infection pressure is possible via (one-time) antiviral chemoprophylaxis. On the contrary, continued vaccination will increase the immune pressure and ensure that the vicious circle of the pandemic is maintained.
Thus, trained innate immunity to SARS-CoV-2 is not equivalent to COVID-19 vaccination but is superior because
It is effective against all variants
It has a sterilizing effect in contrast to vaccine antibodies
Because of its non-varying character it does not lead to the selection of more infectious or resistant variants.
In other words, it benefits both individual and public health. It is the only way to acquire group immunity (independent of the circulating SARS-CoV-2 variant) and thus to move the pandemic into the endemic phase.