CHANGES IN SARS-COV-2 ANTIBODY PREVALENCE IN PRIMARY AND SECONDARY SCHOOLS STUDENTS DURING THE EMERGENCE OF THE OMICRON VARIANT: THE UK SCHOOLS INFECTION SURVEYS

INTRO: Children and Young people were the last group in England be offered COVID-19 vaccination (from September 2021), thus were the largest susceptible group when SARS-CoV-2 Omicron variants emerged. We monitored vaccine and naturally-derived antibodies in schools between November 2021 and March 2022. METHODS: We conducted three large surveys (November 2021, January and March 2022) in a nationally representative random sample of primary and secondary schools, stratified by regions. Oral fluid samples were tested for IgG antibodies against SARS-CoV-2 nucleocapsid (anti-NP) and spike (anti-S1) proteins using novel validated ELISAs; vaccines used in England elicit anti-S1 antibodies only. We calculated weighted prevalences for each survey, and used multilevel logistic regression to investigate associations with socio-demographic factors. FINDINGS: Overall 11311 students contributed 22478 biological samples (respectively 4840, 7549 and 10089 in rounds 1, 2 and 3, with similar socio- demographic characteristics). In 4-11 year olds, not eligible for vaccination, anti- S1 and anti-NP antibody prevalences were 31.3% and 26.6%, 46.2% and 43.8%, and 53.4% and 58.7% respectively over the three rounds. The corresponding estimates in 12 to 18 year olds) were 70.7% and 34.6%, 85.6% and 45.9%, 89.0 and 53.9%. In November 2021 (before Omicron dominance), higher anti-S1 antibody positivity was associated with older age and Black ethnicity, and non- eligibility for free school meals in 4-11 year olds. In 12-18 years it was associated with non-eligibility for free school meals. In March 2022 when Omicron dominated, these associations remained, together with urban location in 4-11 years old. CONCLUSION: The steep increase in 4-11 year olds in both sets of antibodies reflected the emergence and spread of highly infectious Omicron variants whilst high and increasing anti-S1 prevalence in secondary students was consistent with greater vaccine uptake. Socioeconomically deprived 12-18 year olds had lower odds of anti-S1, suggesting lower vaccine uptake or access.