Even mild COVID can have ‘lasting’ effects on immune system: Study

Jolyon Attwooll

11/08/2021 4:24:19 PM

A pre-print study conducted in South Australia has found immune dysregulation in COVID-19 patients several months after infection.

Young woman with long COVID
Researchers could not find any direct correlation between the disease severity and the extent of immune dysregulation.

The study, which is yet to be peer-reviewed, suggests COVID-19 can have an impact on the immune system many months after the virus is contracted – even among these who have mild symptoms.
In a collaboration, including scientists from Flinders University and the University of Adelaide, researchers used a cohort of 69 people who had contracted COVID-19. Of those, 50 had mild symptoms, six had moderate infection, with seven classed as severe and a further six categorised as critical.
They found the immune system of participants was ‘significantly altered’ for up to six months after infection took place.
‘The study found substantial dysregulation of immune cell numbers that was strongest at 12 weeks post infection, but was still evident in most cases for up to six months and potentially even longer,’ Professor David Lynn said.
As well as an increased number of immune cells and antibodies, the authors also found a strong dysregulation of gene expression – the information stored in DNA that governs the way cells react to changing environments.
Researchers said they were ‘surprised’ that they could not find any direct correlation between the disease severity and the extent of immune dysregulation, while Professor Lynn said there were no clear indications as to why people recovered at different rates.
‘The level of disease severity doesn’t translate directly to the level of immune dysregulation and we haven’t been able to find any patterns indicating that an individual’s age or sex is a differentiating factor governing differences in recovery,’ he said.
‘Clearly there are other factors at play that need to be explored.’
All the samples in the study were gathered in South Australia in the earlier stages of the pandemic before the vaccination program had begun.
The authors describe the cohort as ‘uniquely placed’ to be assessed for their immune responses due to their very limited risk of re-infection, given that South Australia effectively suppressed the virus for many months, and that no vaccine had affected their system.
The analysis examined antibody responses, the expression of thousands of genes in the blood, and around 130 different types of immune cells. Researchers took blood samples at 12, 16 and 24-weeks after infection and compared the results to individuals from a healthy control group.
According to the authors, the study builds on previously published works, which they say mostly assessed immune responses from 2–12 weeks post-infection.
In assessing the limitations of their work, the authors note the small sample size of the study, most notably for participants with more severe disease.
‘This is particularly important given the apparently highly heterogenous recovery in immune dysregulation over time,’ they wrote.
‘Further larger studies will be needed to more fully assess differences due to disease severity, treatment and other confounders.’
The authors also highlighted a lack of information about the symptoms experienced by the participants, which meant the study was unable to pinpoint any link between types of immune dysregulation and symptoms of long COVID.
‘One could logically infer that this dysregulation is linked to the physical symptoms of long COVID; however, further research is needed to prove this,’ Professor Lynn said.
The researchers say they will follow participants for three years to study how their immune systems respond longer term.
Understanding of long COVID is still evolving. Earlier cases showed symptoms extending several months after infection, including tiredness, muscle weakness, sleeplessness and anxiety.
A study released in March found that more than half of those admitted to hospital with COVID-19 had symptoms four months after being discharged.
Another study, published in The Lancet last month, suggested that almost half of those admitted to hospital developed health complications beyond the recognised symptoms of COVID-19 during their stay.
Recent statistics published in the UK indicate that more than 1% of the population is still reporting ongoing symptoms more than four weeks after their initial COVID-19 infection.
The impact of long COVID on children is also still being assessed, with estimates of its prevalence ranging widely from 0–27%.
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