From the Document: "Over the past year, the global vaccination campaign to end the COVID-19 [coronavirus disease 2019] pandemic has progressed from tests of vaccine efficacy to over six billion (and counting) administered doses. Highly structured clinical trials facilitated strong statistical examinations of the vaccines because randomization accounts for unobserved confounders that could bias the analysis. Post-approval assessments have limitations because they rely on data from less structured, real-world settings. Several post-approval studies show the existing mRNA [messenger ribonucleic acid] vaccines launched initially in the US effectively prevent, as predicted, the worst outcomes of COVID-19, while the vaccines many other countries use are less effective but still invaluable in fighting the pandemic. An observed erosion in vaccine efficacy this year, relative to the clinical trials of 2020, may be a function of the new variants of the virus, waning acquired immunity against infection over time, or other factors, such as the higher incidence of prior infections in the unvaccinated population and the tendency for random error (such as false-positive COVID-19 tests) to attenuate measured efficacy rates. Taken together, the studies make it plain that vaccinating the unvaccinated with any of the globally approved vaccines remains the most important step for lessening the pandemic's severity."
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