A nationwide study from the U.S. Centers for Disease Control and Prevention (CDC) shows that immunity against severe COVID-19 disease begins to wane four months after receipt of the third dose of an mRNA vaccine (Pfizer or Moderna). Waning immunity was observed during both the Delta and Omicron variant waves in a similar fashion to how mRNA vaccine effectiveness wanes after a second dose. Although protection decreased with time, a third dose was still highly effective at preventing severe illness with COVID-19.

“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,” said study coauthor Brian Dixon, PhD, MPA, Regenstrief Institute and Indiana University Richard M. Fairbanks School of Public Health director of public health informatics. “We also found that people who are Hispanic or Black are half as likely to have a third vaccine dose than people who are white, making people who are Hispanic or Black more vulnerable to severe COVID and highlighting the need for public health officials to double down on efforts to protect these vulnerable populations.”

According to a CDC dashboard, as of February 8, 2022, among Americans 65 years or older who received a booster dose: 72.3% were people who are white, 8.9% were people who are Hispanic, and 7.6% were people who are Black. The rates among people who are Black or Hispanic are lower than the proportion of those groups with two doses, and these proportions are lower than the percentage of the U.S. population composed of people from those groups, indicating disparities in who has received third doses in the U.S. In the last two weeks, however, higher rates of vaccination have been observed among these minority groups (16.9% of recent boosters are among people who are Hispanic; 12.7% of recent boosters are among people who are Black). In the study, among patients who are white in the ED/UC, 12% had received a third dose compared to 7% of patients who are Hispanic and 6% of patients who are Black. Similar disparities in third dose administration were observed among those patients hospitalized for severe COVID-19.

Overall, the study reported that individuals with second and third doses of an mRNA vaccine had greater protection against hospitalizations (severe disease) than against emergency department/urgent care (ED/UC) visits (symptoms which may not require hospitalization). Vaccine effectiveness was also lower overall during the Omicron period than during the Delta period.

Vaccine effectiveness against ED/UC visits declined from 97% within the first two months of receipt of a booster to 89% effectiveness at four months or more during the Delta-predominant period (summer/early fall 2021). During the Omicron-predominant period (late fall 2021/winter 2021-22), vaccine effectiveness against ED/UC visits was 87% during the first two months after a third dose, decreasing to 66% at four months after a third dose.

After the third dose, protection against Delta variant-associated hospitalization declined from 96% within two months to 76% after four months or longer. Vaccine effectiveness against Omicron variant-associated hospitalizations was 91% during the first two months declining to 78% at four months.

“Our findings confirm the importance of receiving a third dose of mRNA COVID-19 vaccine to prevent moderate-to-severe COVID-19 illness, especially among those with comorbidities,” said study coauthor Shaun Grannis, MD, MS, vice president for data and analytics at Regenstrief Institute and professor of family medicine at Indiana University School of Medicine. “That protection conferred by mRNA vaccines waned in the months following a third vaccine dose supports further consideration of booster doses to sustain protection against moderate-to-severe COVID-19 illness.”