How do the anti-Covid vaccines, administered in the arm, raise a wall between the nose and mouth which are the entrance doors of Sars-CoV-2 into our body? A group of Humanitas scientists tried to answer this question, revealing new information on the immune response activated by Pfizer / BioNTech’s vaccine. Elements that would explain precisely why, at least in the first 3 months following the intramuscular injection, the vaccinated are protected not only from serious illness and hospitalization, but also from contagion. The findings are published in ‘Embo Molecular Medicine’, signed by a team coordinated by Maria Rescigno, head of the Humanitas Mucosal and Microbiota Immunology Laboratory and Professor of General Pathology at Humanitas University, and by Chiara Pozzi, Humanitas biologist and researcher. . The researchers analyzed blood and saliva samples taken from around 100 people vaccinated with Comirnaty *. The work is part of a larger project in which, since 2020, more than 4 thousand professionals from Humanitas hospitals have voluntarily participated in the ‘Covid Care Program’. This is one of the first and most extensive epidemiological and immunological studies launched in Humanitas – a note recalls – to evaluate the immune response to Sars-CoV-2 and find answers to the many questions raised by the pandemic. “Systemic vaccines usually don’t protect mucous membranes as well,” Rescigno points out. For this reason “we went to see if and which antibodies present in the blood could be transferred into saliva following vaccination. Usually it is IgA, mucosal immunoglobulins, which protect the barriers exposed to the external environment. IgG, that is the systemic immunoglobulins which, permeating the saliva, also provide protection at the level of the mucous membranes. These immunoglobulins are reduced over time “, and” this could justify why the mRna vaccine is capable to a certain extent, within 3 months after administration, to protect against infections “.” The level of IgG specific for Sars-CoV-2 in saliva – Pozzi remarks – is correlated to their level in the peripheral blood: the more a person has antibodies in the blood, the more he has the chance that these permeate the saliva. Since the vaccine induces a very high quantity of antibodies, we also find IgG in saliva. When “however” the antibodies are lowered in the blood eg referico, they also lower in the most external mucous membranes “. This included, it remained to be understood why IgA is present in saliva in very low levels after vaccination. “The specific IgA for Sars-CoV-2 found in plasma and in very low levels in saliva are classically produced in the peripheral blood (IgA1) – explains Rescigno – and have the characteristic of being very unstable if they are found in the presence of bacterial proteases, such as those present in the saliva. They therefore risk being degraded in that habitat by the salivary microbiota. Hence we can assume why we see IgA in the blood and not in the mucous membranes “. According to the authors,” the study reconfirms the efficacy of the vaccine that not only protects from serious illness and death, but it is also useful for protecting the mucous membranes, preventing infection. This effect is not lasting, because the antibodies produced, once they arrive in the saliva, are easily dispersed “. This is why, “in the future, it could be useful to study a vaccine intended precisely for the protection of the mucous membranes, from where the virus enters the organism”.