Tennessee's COVID-19 vaccine breakthrough case percentage remains stable
Oct. 28—After increasing each month since May, updated state data shows the proportion of serious and fatal COVID-19 infections among vaccinated Tennesseans fell slightly in September.
The trend signals that vaccines continue to provide strong protection against the coronavirus despite some waning efficacy since the initial rollout.
The vaccinated made up 13% of Tennessee's COVID-19 hospitalizations and 15% of Tennessee's COVID-19 deaths in September, down from 14% and 17%, respectively, in August, according to the most recent COVID-19 Critical Indicators Report from the Tennessee Department of Health.
The proportion of overall breakthrough cases increased slightly, making up 16% of all cases in September, compared to 14% in August.
Although data on "breakthrough" infections — which occur when fully vaccinated people are infected by the coronavirus — is useful to help understand in part how well vaccines are working, it should not be used to draw broad conclusions about vaccine efficacy.
That's because raw breakthrough case reports do not control for factors such as age and medical conditions the way large-scale studies do, and vaccination rates are highest among older adults and people with underlying conditions — groups at high risk for serious COVID-19 illness and death, regardless of vaccination status.
Breakthrough infections are also more likely to occur as more people become vaccinated and during periods of high community spread, which increased significantly from early July through August due to the highly contagious delta variant but decreased throughout September.
Dr. Mark Anderson, an infectious disease specialist at CHI Memorial, said the slowing of spread is one factor that helps explain why the percentage of breakthrough cases remained relatively stable from August to September.
"The changes are minimal and within the margin of error and shows the consistency of the vaccines' efficacy," Anderson said in an email sent by a hospital spokesperson. "The surge in immunizations in late August/early September, fueled by the fear of the delta variant, helped protect more people from the virus. We saw a rapid decline in the total number of cases in our community starting around mid-September, which can be attributed to fewer people being susceptible to the virus — whether through vaccine or natural immunity."
Since May, 558 breakthrough deaths and 1,366 hospitalizations have been reported in Tennessee, according to the report. As of Wednesday, more than 3.2 million Tennesseans have been fully vaccinated against COVID-19, according to the department of health.
The department notes in its report that COVID-19 hospitalizations in vaccinated people are investigated more rigorously than unvaccinated people, so the number of hospitalizations in unvaccinated people is likely an undercount.
Nationally, new COVID-19 vaccine data from the U.S. Centers for Disease Control and Prevention shows that in August people who were unvaccinated were at six times greater risk of testing positive for the disease, 12 times more likely to be hospitalized for it and 11 times more likely to die from COVID-19 than people who were fully vaccinated.
Contact Elizabeth Fite at efite@timesfreepress.com or follow her on Twitter @ecfite.
