COVID-19 damages immune response for some: 'A big deal'

The study, published last week in the journal Immunity, reported a "major reduction" in the body's quantity and quality of CD8+ T cells — known as "killer T cells" for their ability to kill infected cells — in people who had survived a bout with COVID-19.

"You have damage that, even in recovery from the infection, you haven't really recovered your ability to make those CD8+ cells. So something happened in the course of infection to prevent that, to damage your response," Stanford professor Mark Davis, head of the university's Institute for Immunity, Transplantation and Infection, told CBS News.

Researchers analyzed how CD8+ T cells and CD4+ T cells, known as "helper T cells," respond to COVID-19 infection and vaccination by studying blood samples from three groups of volunteers.

The first group had never been infected with COVID-19 and received two doses of the Pfizer-BioNTech vaccine.

The second group had previously been infected with the virus and also received two vaccine doses.

The third group had COVID-19 and was unvaccinated.

The researchers found that vaccination of people who had never been infected with COVID-19 sparked robust CD4+ and CD8+ T-cell responses to the virus' spike protein.

But people who had survived a COVID-19 infection before vaccination produced spike-specific CD8+ T cells at considerably lower levels — and with less functionality — compared to vaccinated people who had never been infected. 

Researchers said they also found substantially lower levels of spike-specific CD8+ T cells in unvaccinated people with COVID-19.

"Looking at the broader swath of data, we came up with a seven-fold average reduction. But it was much larger in several of the cases. So that's a big deal," Davis told CBS News, stressing the cell drop-off could result in the immune system taking longer to tackle infected cells.

Researchers emphasized people who recover from COVID-19 infection and then get vaccinated are more protected than people who are unvaccinated. 

The CD4+ T cells appeared undamaged by the virus.

The scientists said the findings highlight the need to develop vaccination strategies to specifically boost antiviral CD8+ T-cell responses in people previously infected with COVID-19.

"Technology-wise, looking at the T-cell response and quantitatively measuring it is harder than antibody detection," Chao Jiang, a program officer for the National Institutes of Health, told CBS News. "That's why there's a need to improve that technology and also relate it to vaccine development."

The study was co-funded by the National Institute of Allergy and Infectious Diseases, which is part of the National Institutes of Health.

Around 81% of the US has received at least one COVID-19 vaccine dose, with nearly 70% completing the primary dose series, according to the Centers for Disease Control and Prevention.

The agency is reporting over 103 million US cases of the illness, causing more than 1.1 million deaths since the pandemic began three-plus years ago.