Study: Majority of COVID-19 patients had antibodies eight months later

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A social distancing sticker on a door at Viru Keskus in Tallinn. Source: Helen Wright / ERR

According to a study conducted by medical researchers at the University of Tartu, nearly 80 percent of COVID-19 patients who were either asymptomatic or had mild symptoms has antibodies eight months after being cleared from the virus.

As a follow-up to a previous KoroSero-EST-1 study, the University of Tartu conducted a seroprevalence study looking into the persistence of coronavirus antibodies, the university announced on Monday. The study also involved patients from Kuressaare and Tallinn, who had antibodies in the period of May-July in 2020. The patients were compared now with their status in November 2020.

The initial KoroSero-EST-1 study showed that 80 percent of people with antibodies did not show any symptoms and 56 percent of them did not come in contact with a patient knowingly. Only 20 percent of people with antibodies noted that they had any symptoms, such as a fever, runny nose, nausea, sore throat, diarrhea or chest pain.

According to University of Tartu researches, asymptomatic patients make up an important group into studying the spread of the coronavirus, as it is still unclear why the infection goes unnoticed in some people. Therefore it is necessary to study if asymptomatic patients have developed an immune memory against antibodies and T-cell immunity and to compare that to patients who were asymptomatic or had mild symptoms.

Persistence of antibodies

Results of the follow-up study show that asymptomatic patients or patients with mild symptoms also develop an immunity to the coronavirus. "There is no reason to believe at this time that people who had mild symptoms would lose immunity faster than those who had severe symptoms," said Piia Jõgi, a doctoral lecturer at the Tartu University Hospital and head of the KoroSero-EST-1 study.

The development of immunity and why 20 percent of studied antibodies remain or do so for a short period is still being studied. Jõgi noted that the development of immunity depends on many factors. "There is a so-called guaranteed immunity to antibodies that we have studied so far. There are ongoing studies about another type - cellular immunity," Jõgi said.

As a next step, researchers will conclude the results of cellular immunity and inflammatory marker studies that could give more insight into the persistence of antibodies, in addition to why some people go through COVID-19 with severe symptoms and others with milder or none at all.

Another study is set to being in March, which will look into the population immunity across Estonia in different age groups. The goal of the study is to make sure how many Estonians are seropositive a year after the virus first appeared in Estonia.

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Editor: Kristjan Kallaste

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