Scientists have shown that there are seven "forms of disease" in mild Covid-19, and that the novel coronavirus infection leaves behind significant changes in the immune system even after 10 weeks, findings which may aid in the treatment of patients, and in the development of a potent vaccine.
The study, published in the journal Allergy, involving 109 recovered Covid-19 patients and 98 healthy individuals in the control group, showed that various symptoms related to Covid-19 occur in groups.
In the research, scientists, including those from the Medical University of Vienna in Austria, identified seven groups of symptoms including "flu-like symptoms" with fever, chills, fatigue and cough, "common cold-like symptoms" with rhinitis, sneezing, dry throat and nasal congestion, and "joint and muscle pain".
They said patients may also experience "eye and mucosal inflammation" symptoms, "lung problems" with pneumonia and shortness of breath, "gastrointestinal problems" including diarrhoea, nausea and headache, and "loss of sense of smell and taste and other symptoms”.
"In the latter group we found that loss of smell and taste predominantly affects individuals with a 'young immune system,' measured by the number of immune cells -- T lymphocytes -- that have recently emigrated from the thymus gland," said Winfried F. Pickl, a co-author of the study from the Medical University of Vienna.
"This means that we were able to clearly distinguish systemic (groups one and three) from organ-specific forms (groups six and seven) of primary Covid-19 disease," Pickl said.
From the analysis, the scientists established that Covid-19 leaves behind long detectable changes in the blood of recovered patients, very similar to a fingerprint.
Citing an example, they said the number of granulocytes, which are otherwise responsible in the immune system for fighting bacterial pathogens, is significantly lower than normal in the Covid-19 group.
"However, both the CD4 and CD8 T cell compartment developed memory cells and CD8 T cells remained strongly activated. This indicates that the immune system is still intensively engaged with the disease several weeks after initial infection," Pickl explained.
According to the researchers, increased levels of antibody-producing immune cells were detected in the blood of recovered patients.
The higher the fever of the affected patient during the mild course of the disease, the higher was the antibody levels against the virus, they said.
"Our findings contribute to a better understanding of the disease and help us in the development of potential vaccines since we now have access to promising biomarkers and can perform even better monitoring," the scientists said.
"Above all, the study shows that the human immune system "doubles up" when defending against Covid-19 with the combined action of immune cells and antibodies," they noted.
According to the study, the immune cells are also able to memorise certain "moves" on the part of the virus and respond to them.
"Now it is a matter of implementing these findings and using them for the development of highly-effective Covid-19 vaccines," the researchers said.