The recommendation was based on a monovalent non-adapted vaccine. BA.2, BA.5 and more recently, BQ.1.1, BA.2.75.1 and XBB/XBB.1 are characterized by an enormous escape potential due to destruction/deletion of a variety of epitopes recognized by neutralizing antibodies2,3. Therapeutic monoclonal antibodies are hardly effective especially against the recent Omicron variants. BMS-935177 Moreover, the number of breakthrough infections in vaccinated individuals has significantly increased since the emergence of the Omicron variant4C6. Due to the numerous mutations in the Omicron spike protein, which mediates the entry into the cell by binding to the human ACE2 receptor, many of the antibodies elicited by vaccination and/or contamination fail to bind to the mutated spike and thereby cannot exert their neutralizing potential7. Under these conditions, efficient neutralization of the Omicron variants by the remaining neutralizing antibodies requires high affinity and titers. Booster vaccinations are given for several reasons. The antibody titer rises rapidly within the first 2?weeks after a booster vaccination, providing the best protection against the virus, but drops back to a baseline level within the first few months8. This baseline antibody level is built up by the immunologic memory. Immunological memory should also be brought on by booster vaccination, which increases memory B and long-lived plasma cells9. Furthermore, booster vaccination stimulates a broader immune response formed by somatic hypermutation and antibody affinity maturation10,11. In the case of mRNA vaccination, Paul Naaber’s study described that this decrease of the neutralizing titers after booster vaccination occurs more slowly as compared to the titer after two vaccinations, indicating immunological memory and a positive long-term effect of the third vaccination12. In Germany, the fourth vaccination (second booster) has been recommended for certain groups at risk since February 202213. BMS-935177 The recommendation was based on a monovalent non-adapted vaccine. This raises the questions (1) whether the second booster has a further impact on the breadth of the humoral immune response and (2) whether a longer persisting humoral immune response can be induced. Results Study design Healthcare workers who had received the third vaccination 6?months before were recruited for this study. This time point corresponds to the first blood collection (6m3V). At this time, subjects received the fourth vaccination (second boost). Two weeks (2w4V) and 6?months (6m4V) after this vaccination, BMS-935177 blood samples were collected again. None of the study participants had a previous SARS-CoV-2 contamination. The first vaccinations were performed with the original vaccine BNT162b2. Participants were vaccinated for the fourth time in mid-February 2022. At that time, no Omicron-matched vaccines were available. However, studies have revealed that a half dose of Spikevax as a fourth vaccination results in a higher titer and better cellular response than a full dose Rabbit Polyclonal to JNKK of BNT162b214. Therefore, the fourth vaccination was performed with 50?g Spikevax (Moderna). Comparable antibody titers 6?months after 3rd and 6?months after 4th vaccination Antibody levels (Fig.?1, Table ?Table1)1) against the SARS-CoV-2 Wuhan-Hu1 RBD protein before the fourth vaccination (6?months after the third vaccination, 6m3V) were still high for IgG [median: IgG 9243 AU/ml (IQR 8306C12,982 AU/ml)] and in a similar range of previously published data form Paul Naaber12. In addition to Naaber et al., we also analyzed the IgA level in BMS-935177 the sera [IgA 4259 AU/ml (IQR 3166C6449 AU/ml)]. After the second boost (fourth vaccination, 2w4V) with Spikevax, the titers were significantly increased by 3.1-fold and 2.4-fold for IgA [median: 13,405 AU/ml (IQR 9852C22,645 AU/ml)] and IgG [median 22,487 AU/ml (IQR 19,553C25,107 AU/ml)], respectively. The titers at 6?months after the fourth vaccination (6m4V) were decreased for both subclasses to 5977 AU/ml (IQR BMS-935177 2912C10,787 AU/ml) (IgA) and 8918 AU/ml (IQR 7190C15,349 AU/ml) (IgG). Interestingly, the values 6?months after the fourth vaccination are not significantly different from the titers 6?months after the third vaccination, suggesting that this antibody repertoire at 6?months after the fourth vaccination is almost the same as before. Interestingly, the same pattern of increase and decrease is usually observed for titers against the SARS-CoV-2 Delta (IgG: 6m3V: 7739 AU/ml, 2w4V: 20,313 AU/ml, 6m4V: 8330 AU/ml; IgA: 6m3V: 3133 AU/ml,.