Since the first reported case of COVID-19 in the U.S. on January 20, 2020, researchers have come to understand that the SARS-CoV-2 virus can have long-term impacts on the immune system. A recent Swedish study has shown the effects on immune system cells months after severe infection. Learn what the researchers found and what it could mean for you.*

COVID-19 and the immune system 

The immune system is divided into an innate (general) response to foreign bodies and an adaptive (specialized) response to specific pathogens. The innate immune system is the body’s first line of defense and starts working as soon as any foreign bodies are detected. The adaptive response happens after infection, as the cells learn to attack pathogens that the body has already encountered and dealt with.

For both these responses to work, the researchers explain in Frontiers in Immunology, myeloid cells (blood cells formed in bone marrow) must function properly. These include cells like monocytes (large white blood cells) and dendritic cells (messenger cells between the innate and adaptive immune systems). 

Monocytes signal other immune cells to kill foreign invaders, triggering inflammation. Dendritic cells activate immune system T-cells that kill infected cells, signaling B-cells to make antibodies that will neutralize the pathogen.

The recent study was based on the theory that impaired levels and functioning of these immune cells from severe SARS-CoV-2 infection is likely behind the immune system dysfunction seen in severe COVID.

What the researchers did

The researchers examined blood samples from 21 people hospitalized for COVID-19 at Norrköping’s Vrinnevi Hospital.

• For comparison, blood samples were taken from 16 healthy individuals acting as controls.
• Four samples were taken, the first upon hospital admission and the last after six-eight months.

The blood samples were analyzed for immune cell content and activity.

What they found

The team discovered that, the greater the severity of COVID-19, the more time that was needed for immune cell levels to be replenished. Levels of dendritic cells were found to be “extremely low” at the outset of infection, according to co-author Marie Larsson, Ph.D., professor of virology at Linköping. Though cell levels later recovered, their activity was higher than normal. 

• The greater the C-reactive protein (CRP) levels (a marker of inflammation), the longer it took for dendritic cells to recover. Therefore, CRP may hamper new dendritic cell production.
• Monocyte distribution still differed between patients and healthy controls after six months, which could indicate that the body is still recovering.

“The main finding in our study is that many cell types are activated six months after the onset of the illness,” said Larsson. “This is similar to what happens to the immune cells in settings such as cancer and HIV, when there is a chronic infection in the body, and ongoing immune activation.” 

What it means

Overall, the researchers say, their study findings show changes in the immune system that persist for longer than six months after severe COVID-19. This could indicate that viral antigens still remain, and/or indicate a less-than-optimal activation is occurring that can silence an immune response rather than enhance it.

“The conclusion we draw from this,” Larsson said, “is that antiviral drugs should be administered at the earliest possible stage of the infection, to reduce the inflammation, so as to prevent the SARS-CoV-2 virus from running wild.”

*Söderlund Leifler, K. & Linköping University. (2023, January 20). Examining COVID-19’s long-term effects on the innate immune system. Medical Xpress. https://medicalxpress.com/news/2023-01-covid-long-term-effects-innate-immune.html