A new report shows that COVID-19 mRNA vaccine effectiveness (VE) in preventing COVID-19 might decline because of waning of vaccine-induced immunity or variant immune evasion, according to CDC MMWR data.
In a multistate analysis of 222,772 Emergency Departments (ED) and Urgent Care (UC) encounters and 87,904 hospitalizations among adults with COVID-19–like illness during August 26, 2021–January 5, 2022, estimates of VE against laboratory-confirmed COVID-19 declined during the Omicron-predominant period compared with VE during the Delta-predominant period.
During both periods, VE was significantly lower among patients who received their second mRNA COVID-19 vaccine dose ≥180 days before the medical encounters compared with those vaccinated more recently. VE increased following a third dose and was highly effective during both the Delta- and Omicron-predominant periods at preventing COVID-19–associated ED and UC encounters (94% and 82%, respectively) and preventing COVID-19–associated hospitalizations (94% and 90%, respectively).
Estimates of VE for 2 doses of an mRNA vaccine were higher against COVID-19–associated hospitalizations than against COVID-19–associated ED or UC encounters, especially during the Omicron period, which is consistent with possible vaccine attenuation of severity of COVID-19 disease but was not observed in this network previously.
This study also found that immunocompromised adults had lower third dose VE against COVID-19–associated ED and UC encounters and hospitalization, which is consistent with trends observed for VE following a second dose and is consistent with recommendations for a booster dose for this group 5 months after the additional primary dose.
These findings underscore the importance of receiving a third dose of mRNA COVID-19 vaccine to prevent both moderately severe and severe COVID-19, especially while the Omicron variant is the predominant circulating variant and when the effectiveness of 2 doses of mRNA vaccines is significantly reduced against this variant.