January 7, 2022

Boosters encouraged to prevent breakthrough cases

Vials of the COVID-19 vaccine.

Madison, Wis. – As the COVID-19 variant omicron continues to spread rapidly, healthcare providers and vaccinated individuals are reporting more breakthrough cases; COVID-19 cases detected in a person who is vaccinated or vaccinated and has had a vaccine booster.

The omicron variant is very contagious and likely spreading faster than delta, according to the Centers for Disease Control and Prevention. Based on early data, breakthrough cases are expected but tend to be less severe than cases in those who are unvaccinated.

The surge of cases from omicron shows how important vaccination and boosters are in preventing severe illness, hospitalizations and death, according to Dr. Dan Shirley, interim medical director of infection control, UW Health, and associate professor of medicine (infectious disease) at the UW School of Medicine and Public Health.

“Data also suggest that receiving a booster dose increases the vaccine effectiveness against omicron to similar levels that were previously seen against delta,” he said. “This includes increasing the protection dramatically for those who had previous infection.”

The CDC has stated that data from South Africa and the United Kingdom demonstrate that vaccine effectiveness against infection for two doses of an mRNA vaccine is approximately 35 percent. A COVID-19 vaccine booster dose restores vaccine effectiveness against infection to 75 percent.

Scientists are still learning about omicron transmissibility and severity of disease. The most recent data from the Department of Health Services in Wisconsin is from November 2021, so it predates omicron, but many experts believe it is possible omicron could make disparities between the vaccinated and the unvaccinated worse when it comes to outcomes from COVID-19 infections.

Wisconsin COVID-19 case rates

November 2021

  • Unvaccinated: 3,348.2 cases, 184 hospitalizations, 35 deaths per 100,000

  • Vaccinated: 722.5 cases, 17.1 hospitalizations, 2.8 deaths per 100,000