Study Shows Efficacy of mRNA COVID-19 Vaccines Against Severe Disease Drops 4 Months After Booster Dose


Although prevention decreased with time, a third booster dose was still highly effective at preventing severe illness with COVID-19.

A new CDC study is the first to show that immunity against severe cases of COVID-19 begins to fall off 4 months following the third dose of an mRNA vaccine (Pfizer or Moderna), according to a Regenstrief Institute press release.

Declining immunity was observed against both the Delta and Omicron variants, which is similar to how the efficacy of mRNA vaccines drops after the second dose, according to the study. Although prevention decreased with time, a third booster dose was still highly effective at preventing severe illness with COVID-19, according to the study.

“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 co-author Brian Dixon, PhD, MPA, Regenstrief Institute and Indiana University Richard M. Fairbanks School of Public Health director of public health informatics, in the press release. “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.”

As of February 8, 2022, the CDC dashboard showed that among Americans 65 years of age and older who received a booster dose, 72.3% were white, 8.9% were Hispanic, and 7.6% were Black. Among the Black and Hispanic population, the rates were lower than the proportion of those groups with 2 doses, which is less than the percentage of the US population of people from those groups, indicating disparities in who has received third doses in the United States. However, in the past 2 weeks, higher rates of vaccination have been observed among these minority groups, according to the researchers.

Among white patients in the emergency department/urgent care (ED/UC), the study showed that 12% had received a third dose compared to 7% of Hispanic patients and 6% of Black patients. Similar disparities were observed in the third dose administration among patients hospitalized for severe COVID-19.

The study reported that individuals who received a second or third dose of an mRNA vaccine had greater protection against hospitalizations than against ED/UC visits, and vaccine efficacy was also lower overall during the Omicron period than during the Delta period.

Following the third dose, protection against Delta variant-associated hospitalization dropped from 96% within 2 months to 76% after 4 months or longer. Additionally, vaccine efficacy against Omicron variant-associated hospitalizations was 91% during the first 2 months but fell to 78% at 4 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 co-author Shaun Grannis, MD, MS, vice president for data and analytics at Regenstrief Institute and professor of family medicine at Indiana University School of Medicine, in the press release. “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.”


First study to show waning effectiveness of 3rd dose of mRNA vaccines. Regenstrief Institute. February 11, 2022. Accessed February 15, 2022.

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