mRNA vaccines significantly reduce severity of Delta, Omicron COVID-19 infections

People who have received two or three doses of an mRNA COVID-19 vaccine are significantly more likely to have milder illnesses if infected with the Delta or Omicron coronavirus variants than those who are unvaccinated, according to a US-wide study involving a team of University of Utah researchers.

The study, which examined health care personnel, first responders and other frontline workers in Utah and five other states, builds on previous research that indicates mRNA vaccines provide protection against severe health outcomes associated with COVID-19 despite the variants’ increased transmissibility.

“It’s encouraging that the mRNA vaccines hold up rather well against these variants,” said Sarang Yoon, DO, assistant professor in the Department of Family and Preventive Medicine at University of Utah Health. She is a study co-author who leads the Utah portion of the research and is part of the Rocky Mountain Center for Occupational and Environmental Health, a partnership between the University of Utah and Weber State University.

“We know that breakthrough cases are more likely with Delta and Omicron than the initial strain, but the vaccines still do a good job of limiting the severity of the infection.”

The study was published in the Journal of the American Medical Association (JAMA), the latest of several peer-reviewed papers resulting from the nationwide HEROES-RECOVER project funded by the US Centers for Disease Control and Prevention.

Researchers examined 1,199 participants who developed COVID-19 infections. Of the participants, 24% were infected with Delta and 62% contracted Omicron, while 14% had the original virus strain.

Delta highlights:

• Participants who had received two vaccine doses were significantly less likely to be symptomatic than those who were unvaccinated (77.8% vs. 96.1%)
• Symptomatic participants with a third dose were far less likely to experience fever or chills than those who were unvaccinated (38.5% vs. 84.9%) and experienced symptoms for an average of six fewer days (10.2 days vs. 16.4 days)

Omicron highlights:

• The risk of symptomatic infection was similar between participants with two vaccine doses and those who were unvaccinated, while those with three doses experienced a higher risk than the unvaccinated (88.4% vs. 79.4%)
• Symptomatic participants with three doses were significantly less likely to experience fever or chills (51.5% vs 79%) or seek medical care (14.6% vs 24.7%) than the unvaccinated

The authors noted that, while the study is among the largest of its kind examining COVID-19 vaccines over time and across variants, grouping participants by variant and vaccine status resulted in some combinations with relatively few people, affecting the precision of the findings. They also indicated that the study was not able to account for all factors influencing COVID-19 severity, which may skew the results. There were also results the authors characterized as “unexpected” among participants who received three doses and had symptomatic Omicron infections.

Researchers gathered data between December 14, 2020, and April 19, 2022. Participants submitted self-collected nasal swabs weekly regardless of COVID-19 symptoms, as well as at the beginning of experiencing signs of illness. Participants were excluded from the study if they had infections before the study start date, or if their infections occurred: sooner than 14 days after their second vaccine dose, sooner than seven days after their third dose or more than 149 days after their third dose.