A new study published in the journal Communications Medicine has found that people with long COVID develop a specific “immune signature” in their blood, which can distinguish them from those who fully recovered or were never infected.

Researchers analyzed blood samples from participants in Australia and Norway, dividing them into three groups: healthy individuals with no history of SARS-CoV-2 infection, patients who had COVID-19 but recovered without long-term effects, and individuals with long COVID symptoms.

The study examined levels of proteins related to inflammation and nervous system function in order to identify stable biological differences between the groups.

The results showed that patients with long COVID retain a distinctive blood protein profile even months after infection. These differences involve both inflammatory processes and molecules associated with neural activity. This “signature” set of markers could potentially form the basis for future diagnostic tests that objectively confirm long COVID.

Researchers also noted that in some patients the immune system remains in a state of prolonged activation, which may explain symptoms such as chronic fatigue, cognitive impairment, and “brain fog.” However, there is still no single biomarker that fully explains the condition, and scientists consider it a multifactorial disorder involving several overlapping immune processes.

Another important finding was that booster vaccination did not increase inflammatory profiles in people with long COVID, which reduces concerns that vaccination might worsen their condition.

The authors emphasize that the identified immune signature not only improves understanding of long COVID, but may also pave the way for more precise diagnostic methods and personalized treatments.