Seroprevalence in serum samples was around 10C36.67% to IgA and 23.3C60% to IgG. Seroprevalence in serum samples was around 10C36.67% to IgA and 23.3C60% to IgG. Finally, we detected the presence of the subclasses IgG1, IgG2, and IgG4 against all the structural proteins of SARS-CoV-2. Conclusions: This work provides evidence of the presence of IgA and IgG antibodies against the four structural proteins of SARS-CoV-2 in breast milk and serum samples derived from breastfeeding women, which can confer immunity to the newborn. Keywords: antibodies, IgA, IgG, breastfeeding, COVID-19, structural proteins, SARS-CoV-2 1. Introduction COVID-19, or BM-131246 coronavirus disease 2019, is highly contagious and affects the human respiratory system. The incubation period of COVID-19 is 5 to 7 days [1] and includes symptoms such as cough, fever, dyspnea, fatigue, and others [2]. COVID-19 promotes severe pulmonary damage and progressive respiratory failure. This disease is associated with infection by SARS-CoV-2, which is transmitted to humans by exposure to infectious respiratory fluids and fomites [3,4]. SARS-CoV-2 BM-131246 has a spherical shape with a diameter of around 80C120 nm, contains spike-like structures, has a single-stranded positive-sense RNA (+RNA) genome that encodes for accessory proteins (ORF; open read frames), 16 non-structural proteins (nsp1C16), and four structural proteins (S, N, M, and E) associated with the formation of the viral particle [5,6,7]. The spike protein (S) has 1273 amino acids and two sub-units (S1 and S2); its primary function is binding the virus with the host cell. The nucleocapsid protein (N) has 419 amino acids and two domains (RNA-binding domain and dimerization domain), allowing for the packing of the viral RNA. The membrane protein (M) has 222 amino acids and is the most abundant structural protein; it contains three structural domains and is the only protein that interacts with all the structural proteins. Finally, the envelope protein (E) has 75 amino acids and is involved in the viral cycle (assembly, budding, envelope formation, and pathogenesis) [8,9]. Vaccination against COVID-19 in women during pregnancy or breastfeeding is essential to providing immunity to the newborn and increasing the available data about the effectiveness of vaccines in this specific risk group [10]. Breastfeeding is an exclusive characteristic of mammals and consists of the secretion of milk from the mammary glands with a high nutritional value that allows for development and confers protection against several diseases during the first weeks or months after birth [11,12]. The conferred protection BM-131246 of breast milk is associated with the presence of IgA and IgG capable of recognizing proteins of SARS-CoV-2 [13]. Previous studies have described the presence of antibodies against SARS-CoV-2 in breast milk and serum samples derived from breastfeeding women either with natural infection or who are vaccinated and have reported a correlation between the effectivity of vaccination and the neutralizing activity of antibodies [14,15,16]. IgA and IgG antibodies in breast milk and serum samples of breastfeeding women were associated with protecting the newborn against the transmission or the development of severe COVID-19 [17]. To our knowledge, there are only a BM-131246 few studies describing the presence of antibodies against all the structural proteins of SARS-CoV-2 in breastfeeding women, and this communication aimed to provide evidence of the presence of IgA and IgG against S, N, M, and E proteins of SARS-CoV-2 in breast milk and serum samples derived from naturally infected or vaccinated breastfeeding women. 2. Materials and Methods 2.1. Study Design and Participants We conducted an observational study aimed at analyzing the presence of IgA and IgG antibodies against all the structural proteins of SARS-CoV-2 in Rabbit Polyclonal to STK10 breast milk and serum samples derived from 30 vaccinated/naturally infected breastfeeding mothers. Samples were collected from October to December 2022. The inclusion criteria were: COVID-19 vaccination (at least one dose) or natural infection by SARS-CoV-2, breastfeeding (exclusive or mixed), and a number of children (single child or more). A survey was used for the collection of clinical and epidemiological data. It included: age; diagnosis of COVID-19 via RT-PCR, antigen, or clinical/unknown (not reported by the patient); serological status; symptomatology (fever, headache, loss of smell, loss of taste, dyspnea, chest pain, cough, sore throat, burning eyes, congested nose, muscle pain, joint pain, fatigue, chills, vomit, and diarrhea); number of infections; vaccine doses; and type of breastfeeding. 2.2. Collection of Samples Lactating mothers provided breast milk and serum samples. The breast milk was self-collected, manually or by breast pumping in sterile conical tubes. Later, a blood sample was obtained by venipuncture by trained personnel. We centrifuged the blood sample and placed the serum in a new sterile tube. Collected samples were immediately processed and remains were stored at ?20 C for additional analysis. 2.3. IgA.