Medical researchers of the University of Tartu published the first Estonian data on SARS-CoV-2 antibody persistence after asymptomatic or mild infection – 80% of subjects still had antibodies eight months later.
Coronavirus antibody persistence was examined as a follow-up of the SARS-CoV-2 seroprevalence study KoroSero-EST-1. The subjects of the study were patients from Kuressaare family medicine centre (Saaremaa) and Järveotsa family medicine centre (Tallinn) who were found to have coronavirus antibodies from May to July 2020. These patients were examined again in November 2020.
KoroSero-EST-1 showed that 80% of people with antibodies had no symptoms of the disease and 56% of them had had no known exposure to COVID-19. Only 20% of people with antibodies reported having had any symptoms of coronavirus, such as high fever, runny nose, nausea, sore throat, diarrhoea or chest pain.
According to researchers of the University of Tartu, asymptomatic people form an important group from the perspective of understanding the SARS-CoV-2 infection and its spread, as it is still unclear why the infection goes unnoticed in some people. It is, therefore, necessary to examine whether asymptomatic individuals have developed immune memory for antibodies and T-cell immunity and to compare it to that of patients who suffered from mild and severe forms of COVID-19.
Persistence of antibodies
In the follow-up study, researchers looked at antibody persistence in people who had had no or mild symptoms of COVID-19. The preliminary data indicated that in about 80% of subjects, SARS-CoV-2 antibodies persisted for at least eight months after the infection. The prevalence of virus-neutralising antibodies was similar. Longer-term data is not yet available.
These results reveal that people who have a mild or asymptomatic form of the disease also develop immunity against the coronavirus. “Currently, there is no reason to believe that those with mild COVID-19 would lose immunity faster than those who suffer from a severe form of the disease,” said Piia Jõgi, head of the KoroSero-EST-1 follow-up study, Lecturer of Paediatric Infectious Diseases at the University of Tartu and teaching physician in paediatrics at Tartu University Hospital.
How the immunity develops and why the antibodies disappear faster in 20% of subjects is still being studied. Jõgi said that the development of immunity depends on several factors: “So far, we have focused on the so-called antibody-mediated immunity. Now we have started to explore the second type of immunity – cellular immunity – in the same study.”
Next, researchers will assemble the results of studies on cellular immunity and inflammatory markers that could provide more information about the persistence of immunity as well as the reasons why some people suffer from a severe form of COVID-19, while others have only mild or no symptoms.
In March, a new study – KoroSero-EST-3 – led by medical researchers of the University of Tartu will be launched to examine population immunity across Estonia in different age groups. The study aims to determine how many Estonian residents have antibodies one year after the pandemic reached Estonia.
The KoroSero-EST-1 follow-up study is led by Piia Jõgi, Lecturer of Paediatric Infectious Diseases at the University of Tartu and teaching physician in paediatrics at Tartu University Hospital, and Pärt Peterson, Professor of Molecular Immunology at the University of Tartu. The research team includes family doctors from Järveotsa and Kuressaare family medicine centres, researchers from the Institute of Biomedicine and Transitional Medicine, the Institute of Genomics, the Institute of Family Medicine and Public Health and the Institute of Clinical Medicine of the University of Tartu as well as specialists from SYNLAB Estonia.