Hybrid immunity after BNT162b2 Covid-19 vaccine administration in children aged 5 to 11 years

Background: The immune response to coronavirus disease 2019 (COVID-19) vaccination is stronger among adults with prior infection (hybrid immunity). It is important to understand if children demonstrate a similar response to better inform vaccination strategies. Our study investigated the humoral response after BNT162b2 COVID-19 vaccine doses in SARS-CoV-2 na & iuml;ve and recovered children (5-11 years). Methods: A multi-institutional, longitudinal, prospective cohort study was conducted. Children were enrolled in a case-ascertained antibody surveillance study in Ottawa, Ontario from September/2020-March/2021; at least one household member was severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) positive on RT-PCR. In November 2021, BNT162b2 COVID-19 vaccine was authorized for children aged 5-11 in Canada. Children enrolled in the surveillance study intending to receive two vaccine doses were invited to participate in this study from November 2021-April 2022. Main exposure was prior SARS-CoV-2 infection, defined by positive RT-PCR or SARS-CoV-2 anti-N IgG antibody presence. Primary outcome was spike IgG antibody levels measured following the first vaccine dose (2-3 weeks) and second vaccine dose (3-4 weeks). Results: Of the 153 eligible children, 75 participants (median age 8.9 IQR (7.4, 10.2) years; 38 (50.7 %) female; 59 (78.7 %) Caucasian) had complete follow-up. Fifty-four (72 %) children had prior SARS-COV-2 infection. Spike IgG antibody levels are significantly higher in SARS-CoV-2 recovered participants after receiving the first dose (p < 0.001) and the second (p = 0.01) compared to infection na & iuml;ve children. Conclusions and Relevance: SARS-CoV-2 recovered children (5-11 years) demonstrated higher antibody levels following first BNT162b2 vaccine dose compared with na & iuml;ve children. Most reached antibody saturation two to three weeks after the first dose; a second dose didn't change the saturation level. A single vaccine dose in SARSCoV-2 recovered children may be equivalent or superior to a 2-dose primary series in na & iuml;ve children. Further research is needed on the durability and quality of a single vaccine dose in this population.