In a letter to the editor of the New England Journal of Medicine, researchers from Abt Global and their partners reported that a study of essential and frontline workers showed that three doses of COVID-19 vaccine are more effective than two. The vaccines are less effective against Omicron than against the Delta variant but are effective against both. 

The study for the Centers for Disease Control and Prevention (CDC) involved 3,241 essential and frontline workers who sent in nasal swabs weekly from August 26, 2021 until January 22, 2022, regardless of whether they had symptoms. Effectiveness against the Delta variant was 65 percent after two doses and 91 percent after three. The results were similar to a study in which participants received medical care because they had symptoms.

Vaccine effectiveness against Omicron infection for three doses (60 percent) was higher than for two doses (46 percent), but still substantially lower than three-dose effectiveness against Omicron reported elsewhere where individuals sought medical attention for their symptoms (82-90 percent). Lower effectiveness against mild or asymptomatic Omicron infection dovetails with data suggesting the Omicron variant is less severe. The results show continued effectiveness against clinically severe outcomes related to both variants.

The authors’ findings and conclusions do not necessarily represent the CDC’s views.