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 study.

The study, involving a team of University of Utah researchers, examined healthcare personnel, first responders, and other frontline workers in the US.

"It's encouraging that the mRNA vaccines hold up rather well against these variants," said Sarang Yoon, assistant professor in the Department of Family and Preventive Medicine at University of Utah Health.

"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," Yoon said in a study published in the Journal of the American Medical Association (JAMA).

Researchers examined 1,199 participants who developed COVID-19 infections.

Of the participants, 24 per cent were infected with Delta and 62 per cent contracted Omicron, and 14 per cent had the original virus strain.

In the case of Delta, participants who had received two vaccine doses were significantly less likely to be symptomatic than those who were unvaccinated.

In the case of Omicron, 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.

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.

There were also results from the authors characterised as "unexpected" among participants who received three doses and had symptomatic Omicron infections.