Immunity against severe COVID-19 infection begins to wane four months after receiving the third dose, according to a new study from the U.S. Centers for Disease Control and Prevention (CDC). As Omicron and Delta continue to spread and new variants emerge, understanding the level of immunity granted by vaccines and boosters over time is increasingly important.
This study marks the first assessment of the durability of the protection provided by three doses of Pfizer or Moderna vaccines during both Delta and Omicron waves. The researchers reported that individuals with second and third doses of mRNA vaccines showed greater protection against severe disease (hospitalization) than against emergency department/urgent care (ED/UC) visits. The study also showed that vaccine effectiveness was higher during the Delta period than the Omicron period.
Vaccine effectiveness against ED/UC visits during the Delta period declined from 97 percent within the first two months of receiving a booster to 89 percent effectiveness at four months. During the Omicron period, these fell from 87 percent within the first two months to 66 percent at four months.
The study’s co-author Brian Dixon, PhD, MPA added: “The mRNA vaccines, including the booster shot, are very effective, but effectiveness declines over time. Our findings suggest that additional doses may be necessary to maintain protection against COVID-19, especially for high-risk populations.”