ABSTRACT Noroviruses are now the leading cause of viral gastroenteritis in children. Development of pediatric norovirus vaccines is underway, yet little is currently known about the development of humoral immunity against norovirus in children. We aim to help close this gap by defining how natural infection primes pediatric humoral immunity at the serological and B cell levels. This work generates new immunological tools to understand the development of norovirus immunity, and leverages samples being collected from an existing birth cohort study in Nicaragua (R01AI127845-03, Becker-Dreps/Vinje: Natural history, immunity and transmission patterns of sapovirus in a Nicaraguan birth cohort). First, we will characterize and compare memory B cell repertoires after first and second norovirus infections of the same genogroup in three previously naïve children. From these primary and secondary infections, we will generate stable populations of B cells to determine frequencies, specificities, and function of corresponding antibodies. Functional assays will be performed by assessing histo-blood group antigen (HBGA) blockade to a panel of diverse noroviruses. These studies will enable us to determine whether primary exposure to norovirus elicits broadly-active antibodies against a range of norovirus genotypes, or if immunity is genotype-specific, and how the breath of the response changes after secondary infection. Following second infections, we will examine whether the predominant response is to pre-existing norovirus strains or to the newly-infecting strain, to better understand how primary infections shape the immune response to subsequent infections. This will provide insight into the question of whether antigenic seniority (boosting of pre- existing immunity to an ancestral strain) may be occurring, as has been shown in previously-infected adults receiving a norovirus vaccine candidate [Lindesmith, et al. 2015 PLoSMed]. Next, we will clone norovirus-specific pediatric mAbs that are found to have HBGA blocking activity from these natural infections. Of highest utility to vaccine development, we will use these mAb to map epitopes targeted in natural GII.4 infections. We posit that epitopes targeted by natural infection may be important to include in pediatric norovirus vaccines, to best mimic the development of natural immunity to norovirus. Overall, a better understanding of priming to norovirus in naïve individuals is now needed to inform the design and schedule of pediatric norovirus vaccines. This project would provide the most in-depth view to date of B cell stimulation and antibody specificity following norovirus infection in naïve children.