JAMA : the journal of the American Medical Association
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Monitoring COVID-19 vaccine performance over time since vaccination and against emerging variants informs control measures and vaccine policies. ⋯ Among adults, the OR for the association between symptomatic SARS-CoV-2 infection and COVID-19 vaccination (as an estimate of vaccine effectiveness) was higher during Delta variant predominance, suggesting lower protection. For mRNA vaccination, the steady increase in OR by month since vaccination was consistent with attenuation of estimated effectiveness over time; attenuation related to time was greater than that related to variant.