Breakthrough COVID-19 cases are becoming more common. Here's what you need to know.

Staff photo by C.B. Schmelter / Dr. Elizabeth Forrester works inside the Baylor Esoteric and Molecular Lab at the Tennessee Aquarium Conservation Institute on the campus of Baylor School on Friday, Feb. 19, 2021 in Chattanooga, Tenn.

COVID-19 vaccines were never promised to be 100% effective, and experts are united in saying they remain the best way to combat the pandemic.

But a growing number of breakthrough cases - the term used to describe people who become infected by the coronavirus despite being fully vaccinated - is drawing attention to how much research is still needed to better understand the virus.

Breakthrough infections were thrust further into the spotlight this past week as more high-profile, fully vaccinated individuals across the nation - including Chattanooga Mayor Tim Kelly - announced they had tested positive for COVID-19. Kelly, 54, tweeted Wednesday that he had "mild, allergy-like symptoms" thanks to the vaccine preventing a more serious case.

"We knew we were going to see these breakthrough infections, but the important thing is to look at what's happening to these people," said Dr.