Third dose of COVID-19 mRNA vaccine significantly restores waning immunity, according to new CDC study

Co-authored by HealthPartners Institute, study highlights how a third dose can provide protection against still-high COVID-19 transmission

Bloomington, Minn. – A new study shows that although protection against COVID-19 decreased with time after getting a third dose of mRNA vaccine, the third dose was still highly effective at preventing severe illness caused by both Delta and Omicron variants in adults.

The study was co-authored by HealthPartners Institute and published in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report.

“In Minnesota and across the U.S., fewer than 50 percent of vaccine-eligible people have received their third dose, and many people are wondering if it’s really necessary,” said Malini DeSilva, researcher with HealthPartners Institute and co-author on the study. “This study shows that even now during Omicron predominance, third doses can significantly improve protection against severe disease. Transmission is still high so people should get their third doses if they haven’t already.”

The researchers analyzed more than 241,000 emergency department and urgent care visits and more than 93,000 hospitalizations across 10 states between Aug. 26, 2021 and Jan. 22, 2022. While data showed that vaccine effectiveness for two and three doses was lower during Omicron predominance, vaccines continued to offer protection against severe COVID-19.

When Omicron was the predominant variant, two doses of mRNA vaccines were 69 percent effective at preventing emergency department and urgent care visits within two months of vaccination, and 37 percent effective after five months.

But, after a third dose, the vaccines were 87 percent effective at preventing emergency department and urgent care visits within two months of vaccination, and 66 percent after four months.

Protection against severe disease was also boosted by a third dose. The mRNA vaccines were 91 percent effective at preventing hospitalizations during the first two months after a third dose and decreased to 78 percent after four months.

“Omicron is a highly transmissible variant, but these data underscore the benefits of vaccines,” DeSilva said. “In spite of waning vaccine immunity, three doses help keep people out of the hospital.”