Date of Award

January 2023

Document Type

Open Access Thesis

Degree Name

Master of Public Health (MPH)

Department

School of Public Health

First Advisor

Saad Omer

Second Advisor

Krishna C. Poudel

Abstract

Background: First identified in Wuhan, China in late 2019, the infectious viral disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has affected millions of lives worldwide and poses the largest public health crisis in the past century. The development and deployment of vaccines have been a key public health measure to mitigate the subsequent harm associated with the viral infection. Despite the mRNA vaccine platform being in development for 15+ years, the mass vaccination campaigns were the first large-scale use of this technology, leaving much to be learned about their effectiveness and longevity. Mass vaccination campaigns influence the course of the pandemic by protecting the vaccine recipient directly, but also protecting unvaccinated individuals by reducing the transmission probability. As a result, investigating the indirect effects of vaccinations can uncover the impact of vaccines on viral dynamics and safeguarding of unvaccinated individuals.

Objectives: The rapid review and meta-analysis aims to assess the indirect protection afforded by COVID-19 vaccination against SARS-CoV-2 infections and severe COVID-19 since the Omicron variant's debut and the introduction of vaccination in various settings around the world.

Methods: A systematic review of the literature was conducted by searching Medline (n=2,530), EuropePMC (n=569), and Embase (n=4,014) databases for relevant publications on the indirect effects of COVID-19 vaccination (n=7,124). Duplicates were removed (n=2,444). Exclusion criteria were applied and 30 articles investigating factors related to the indirect effects were ultimately reviewed.

Results: Meta-analyses used generalized linear mixed models to obtain SAR logit estimates and 95% CIs. SARs stratified by covariates according to index case and contact vaccination status. Household SARs was 31% (95% CI, 16% - 51%). SARs for booster vaccinated index cases were 38% (95% CI, 22% to 58%), 50% (95% CI, 46% to 54%) for fully vaccinated index cases, 77% (95% CI, 0% to 100%) for partially vaccinated index cases, and 63% (95% CI, 37% to 83%) among unvaccinated index cases. SARs among booster vaccinated contacts were 32% (95% CI, 22% to 44%), 50% (95% CI, 45% to 55%) for fully vaccinated contacts, 57% (95% CI, 13% to 92%) for partially vaccinated contacts, and 55% (95% CI, 31% to 77%) among unvaccinated contacts. For newborns of vaccinated individuals, the SARs were 4% (95% CI, 2% to 7%). For newborns of unvaccinated individuals, the SARs were 4% (95% CI, 0% to 40%). The estimated vaccine effectiveness for susceptibility was 13.76%, while the estimated vaccine effectiveness for infectiousness was 45.27%. The combined effect of direct vaccine protection and indirect vaccine effectiveness was estimated to be 52.80%. Moreover, mean and median cycle threshold (Ct) values were found to be higher in vaccinated individuals, especially among those who are boosted. Higher Ct values are indicative of a lower transmission potential due to a lower viral load.

Conclusions: These results suggest a significant impact that vaccination has on transmission and secondary attack rates, highlighting the crucial role that vaccination plays in curbing the spread of infectious diseases. Elevated Ct values observed in vaccinated individuals can serve as a valuable surrogate measure of the effect of vaccination in reducing viral load and infectiousness. Despite challenges posed by viral evolution, these findings underscore the importance of indirect effects and emphasize the need to continue to develop and administer effective vaccines to protect public health.

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This is an Open Access Thesis.

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