(HealthDay News) — COVID-19 vaccine effectiveness (VE) is increased after three doses but wanes with time since vaccination after receipt of two or three doses, according to research published in the Feb. 11 early-release issue of the U.S. Centers for Disease Control and Prevention Morbidity and Mortality Weekly Report.

Jill M. Ferdinands, Ph.D., from the CDC COVID-19 Emergency Response Team, and colleagues conducted a test-negative case-control study to examine vaccine effectiveness against COVID-19 emergency department/urgent care (ED/UC) visits and hospitalizations among U.S. adults at various points after receipt of a second or third vaccine dose during periods of delta and omicron variant predominance.

A total of 241,204 ED/UC encounters and 93,408 hospitalizations were examined across 10 states during Aug. 26, 2021, to Jan. 22, 2022.


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The researchers found that after receipt of both two and three doses, VE was lower during the omicron-predominant versus the delta-predominant period at all time points. VE was higher after receipt of a third dose versus a second dose during both periods; however, with increasing time since vaccination, VE waned.

VE against ED/UC visits was 87 percent during the first two months after the third dose during the omicron period and decreased to 66 percent among those vaccinated four to five months earlier; the corresponding VE against hospitalization was 91 and 78 percent, respectively. VE was generally higher for protection against hospitalization than ED/UC visits in both periods.

“The finding that protection conferred by mRNA vaccines waned in the months after receipt of a third vaccine dose reinforces the importance of further consideration of additional doses to sustain or improve protection against COVID-19-associated ED/UC encounters and COVID-19 hospitalizations,” the authors write.

Several authors disclosed ties to the pharmaceutical industry.

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