Estonian study: the novel coronavirus is ten times more prevalent than previously thought

The results of the University of Tartu-led study on coronavirus antibodies indicate that the prevalence of the virus notably exceeded the national statistics based on nasal swab testing and a whopping number of people who had the virus did not have any symptoms.

From 11 May to 29 July, the University of Tartu, in cooperation with Estonia’s Kuressaare and Järveotsa family practitioners’ offices, invited over 3,600 people to take part in a study on coronavirus antibodies, called KoroSero‑EST-1. Of those invited, almost 2,000 people agreed to participate, give a venous blood sample and fill in a questionnaire.

The researchers aimed to detect the presence of coronavirus antibodies (that indicate a person has been exposed to the novel coronavirus) in the blood of people invited to the study based on a randomised sample compiled by the Estonian Health Insurance Fund.

Before the study started, the researchers hypothesised that the number of people who have had COVID-19 in Estonia is larger than indicated by national statistics based on nasopharyngeal swab testing. Indeed, the results of the test-study in two regions confirmed this hypothesis. 

The prevalence of the coronavirus exceeding the national statistics

Based on the study, it is safe to say that three people out of 200 in Tallinn and six people out of 100 in Saaremaa island have been exposed to the coronavirus. The analysis of the data revealed that the prevalence of the coronavirus exceeded the national statistics by ten times in Tallinn and three and a half times in Saaremaa.

“For instance, the official registered number of coronavirus infections in Saaremaa is 167 cases per 10,000 residents, but based on the study on antibodies, it is 597 cases per 10,000 residents,” Piia Jõgi, the head of the research group, said in a statement.

People in Saaremaa staged a silent protest on 26 April to persuade the government to abolish the restrictions in the island that were stricter than in the mainland Estonia; the island was particularly hard hit by the coronavirus crisis. Photo by Krista Tito.

The study showed that 80% of seropositive people (those with antibodies) did not have any symptoms and 56% of seropositive people had not had any known contact with a COVID-19 patient. Thus only 20% of people with coronavirus antibodies reported having had a symptom of COVID-19, such as high fever, runny nose, nausea, sore throat, diarrhoea or chest pain.

Safety measures important

According to Jõgi, the incidence of coronavirus in Estonia is low, similarly to many other European countries. “A large proportion of our population is still susceptible to SARS-CoV-2. This means that if we do not follow the general rules such as to properly wash our hands, keep distance, stay home when ill and after returning from abroad if required to, we are at great risk of a new wave of the virus, which can lead to new restrictions,” Jõgi stated.

KoroSero-EST-1 was a test-study, helping plan the second, population-based study on coronavirus antibodies – KoroSero-EST-2 – as efficiently as possible. The first results of KoroSero-EST-2 can be expected at the end of September.

Cover: A man in Estonia wearing a surgical mask during the coronavirus pandemic. Photo by Pille-Riin Priske.

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