A nationwide study from the US Centers for Disease Control and Prevention (CDC) is the first to show that immunity against severe COVID-19 disease begins to wane four months after a third dose of an mRNA vaccine (Pfizer or Moderna).
Waning immunity was observed during both the Delta and Omicron variant waves in 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, said the researchers.
Until this study, published on the Morbidity and Mortality Weekly Report, little was known about durability of protection after three doses, especially during Delta or Omicron predominance.
“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.
“We also found that people who are Hispanic or black are half as likely to have a third vaccine dose than white people, making them more vulnerable to severe COVID and highlighting the need for public health officials to increase efforts to protect these vulnerable populations.”
According to a CDC dashboard, as of 8 February 2022, among Americans 65 years or older who received a booster dose: 72,3% were white, 8,9% were Hispanic, and 7,6% were black.
The rates among black or Hispanic groups are lower than the proportion of those groups with two doses, and these proportions are lower than the percentage of the population comprising people from those groups, indicating disparities in who has received third doses.
In the past two weeks, however, higher rates of vaccination have been observed among these minority groups (16,9% of recent boosters among Hispanics; 12,7% among blacks). In the study, among white patients, 12% had received a third dose compared with 7% of Hispanic patients and 6% of black patients. Similar disparities in third dose administration were observed among patients hospitalised for severe COVID-19.
Overall, the study reported that people with second and third doses of an mRNA vaccine had greater protection against hospitalisations (severe disease) than against emergency department (ED) /urgent care (UC) visits (symptoms which may not require hospitalisation). 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 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 hospitalisation declined from 96% within two months to 76% after four months or longer. Vaccine effectiveness against Omicron variant-associated hospitalisations 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 co-author Dr Shaun Grannis, 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 after a third vaccine dose supports further consideration of booster doses to sustain protection against moderate-to-severe COVID-19 illness.”
Effectiveness of a Third Dose of 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
Mark G. Thompson, Karthik Natarajan, Stephanie Irving, Elizabeth Rowley, Eric Griggs, Manjusha Gaglani, Nicola Klein, Shaun Grannis, Malini DeSilva, Edward Stenehjem, Sarah Reese, Monica Dickerson, Allison Naleway, Jungmi Han, Deepika Konatham, Charlene McEvoy, Suchitra Rao, Brian Dixon, Kristin Dascomb, Ned Lewis, Matthew Levy, Palak Patel, I-Chia Liao, Anupam Kharbanda, Michelle Barron, William Fadel, Nancy Grisel, Kristin Goddard, Duck-Hye Yang, Mehiret Wondimu, Kempapura Murthy, Nimish Valvi, Julie Arndorfer, Bruce Fireman, Margaret Dunne, Peter Embi, Eduardo Azziz-Baumgartner, Ousseny Zerbo, Catherine Bozio, Sue Reynolds, Jill Ferdinands, Jeremiah Williams, Ruth Link-Gelles, Stephanie Schrag, Jennifer Verani, Sarah Ball, Toan Ong.
Published in Morbidity and Mortality Weekly Report on 28 January 2022
What is already known about this topic?
COVID-19 mRNA vaccine effectiveness (VE) in preventing COVID-19 might decline because of waning of vaccine-induced immunity or variant immune evasion.
What is added by this report?
VE was significantly higher among patients who received their second mRNA COVID-19 vaccine dose <180 days before medical encounters compared with those vaccinated ≥180 days earlier. During both Delta- and Omicron-predominant periods, receipt of a third vaccine dose was highly effective at preventing COVID-19–associated emergency department and urgent care encounters (94% and 82%, respectively) and preventing COVID-19–associated hospitalisations (94% and 90%, respectively).
What are the implications for public health practice?
All unvaccinated persons should start vaccination as soon as possible. All adults who have received mRNA vaccines during their primary COVID-19 vaccination series should receive a third dose when eligible, and eligible persons should stay up to date with COVID-19 vaccinations.
MMWR article – Effectiveness of a Third Dose of 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 (Open access)
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