Effect of Hybrid Immunity, School Reopening, and the Omicron Variant on Trajectory of Covid-19 Epidemic in India: A Modelling Study

ABSTRACT

Intro The concurrent reopening of schools, increasing levels of hybrid immunity, and the emergence of the Omicron variant have affected the trajectory of the pandemic in India. We address related questions using the model Indian state of Andhra Pradesh (pop 53 million). Method(s) A compartmental model which describes the disease progression of COVID-19 with two dose vaccination is employed to understand the effect of vaccination, immunity due to infection and VOC Omicron. This is an age- stratified as well as a contact-structured model. The introduction of the Omicron variant is modelled. We studied disease dynamics in a background of seropositivity gained from an earlier wave of infection as well as an ongoing vaccination program, together called "hybrid immunity". We demonstrate the effect of school reopening as well as of the Omicron (BA.2) variant on cases across different age-groups. Finding(s) Reopening schools increases cases in children as compared to adults, although most such cases are asymptomatic or mild. The height of this peak reduced as the background infection-induced seropositivity was increased from 20% to 40%. At reported values of seropositivity of 64%, no discernable peak was seen. We find that in the absence of vaccination, even at such high levels of seroprevalence, the emergence of the Omicron variant would have resulted in a large rise in cases across all age bands. Discussion(s) In India, the decreasing prevalence of immunologically naive individuals of all ages helped reduce the number of cases reported once schools were reopened. In addition, hybrid immunity, together with the lower intrinsic severity of disease associated with the Omicron variant, contributed to low reported COVID-19 hospitalizations and deaths. Conclusion(s) Mathematical modelling provides a powerful way of addressing central questions regarding the trajectory of the pandemic in India, clarifying the role of hybrid immunity.Copyright © 2023