Findings from a new study reveals that immunity against severe COVID-19 disease starts to wane 4 months after receipt of the third dose of an mRNA vaccine. Waning immunity was detected during both the Delta and Omicron variant waves, which is comparable to how mRNA vaccine effectiveness wanes after a second dose. Data also revealed that although protection decreased with time, a third dose was still highly effective at preventing severe illness with COVID-19.

The study, entitled "Waning Effectiveness 2-dose and 3-dose mRNA Vaccines Against COVID-19-Associated Emergency Department and Urgent Care Encounters and Hospitalizations Among Adults During Periods of Delta and Omicron Variant Predominance—VISION Network, 10 States, August 2021-January 2022," was published in the CDC's Morbidity and Mortality Weekly Report.

Study coauthor Brian Dixon, PhD, MPA, Regenstrief Institute and Indiana University Richard M. Fairbanks School of Public Health director of public health informatics, stated, "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. 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."

The report also noted that according to a CDC dashboard, as of February 8, 2022, among Americans aged 65 years or older who received a booster dose: 72.3% were Caucasian, 8.9% were Hispanic, and 7.6% were African American. The rates among individuals who are African American or Hispanic are lower than the percentage of those groups with two doses, and these percentages are lower than the percentages of the U.S. population composed of individuals from those groups, indicating disparities in who has received third doses in the U.S.

The CDC indicated that by late February 2022, however, higher rates of vaccination have been observed among these minority groups (16.9% of recent boosters are among individuals who are Hispanic and 12.7% of recent boosters are among individuals who are African American). In the study, among patients who are Caucasian in the emergency department/urgent care (ED/UC), 12% had received a third dose compared with 7% of patients who are Hispanic and 6% of patients who are African American. Comparable 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 ED/UC visits (symptoms that 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 2 months of receipt of a booster to 89% effectiveness at 4 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 2 months after a third dose, decreasing to 66 % at 4 months after a third dose.

After the third dose, protection against Delta variant—associated hospitalization dropped from 96% within 2 months to 76% after 4 months or longer. Vaccine effectiveness against Omicron variant—related hospitalizations was 91% during the first 2 months declining to 78% at 4 months.

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, stated, "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. 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."

The CDC collaborated with several U.S. healthcare systems and the Regenstrief Institute to set up the VISION network to evaluate COVID-19 vaccine effectiveness. In addition to the Regenstrief Institute, other members are Columbia University Irving Medical Center, HealthPartners, Intermountain Healthcare, Kaiser Permanente Northern California, Kaiser Permanente Northwest, and the University of Colorado. Regenstrief contributes data and expertise to the VISION Network.

The content contained in this article is for informational purposes only. The content is not intended to be a substitute for professional advice. Reliance on any information provided in this article is solely at your own risk.

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