Coronavirus is ten times more prevalent than previously thought

The results of the study on coronavirus antibodies KoroSero-EST-1 led by the University of Tartu indicate that the prevalence of the virus notably exceeded the national statistics based on nasal swab testing: by ten times in Tallinn and three and a half times in Saaremaa. Only 20% of people who had coronavirus antibodies in their blood had had a symptom of a viral disease. The population-based study on coronavirus antibodies KoroSero-EST-2 has already started.

From 11 May to 29 July, the University of Tartu in cooperation with Kuressaare and Järveotsa family practitioners’ offices invited 3,608 people to take part in the study on coronavirus antibodies KoroSero‑EST-1. Of those invited, 1,006 people from the practice list of Järveotsa family practitioners’ office in Tallinn and 954 from that of Kuressaare family practitioners’ office in Saaremaa agreed to participate, give a venous blood sample and fill in a questionnaire.

The researchers aimed to detect the presence of coronavirus antibodies in the blood of people invited to the study based on a randomised sample compiled by the Estonian Health Insurance Fund. Antibodies were measured at SYNLAB using the ELISA method. The presence of antibodies indicates that a person has been exposed to the coronavirus. 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.

The results of the test-study in two regions confirmed this hypothesis. According to Piia Jõgi, the head of the research group of KoroSero-EST-1, Assistant of Paediatric Infectious Diseases at the University of Tartu and Teaching Physician in Paediatrics at Tartu University Hospital, the prevalence of coronavirus was 1.4% among the patients of Järveotsa family practitioners’ office and 6% among the patients of Kuressaare family practitioners’ office. This means that three people out of 200 in Tallinn and six people out of 100 in Saaremaa have been exposed to the coronavirus. The analysis of the data revealed that the prevalence of 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,” explained Jõgi. It should be noted that officially, cases of infection are registered based on PCR testing of the nasopharyngeal swab, which allows detecting persons currently infected with the virus. Serological tests indicate whether a person has been exposed to coronavirus some time ago and, as a result, has developed antibodies.

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.

Risk of the second wave of the virus

According to Jõgi, the incidence of SARS-CoV-2 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,” said Jõgi.

Population-based study

KoroSero-EST-1 was a test-study, helping to plan the population-based study on coronavirus antibodies KoroSero-EST-2 as efficiently as possible. This study is led by Marje Oona, Associate Professor in Family Medicine of the University of Tartu, and the collection of blood samples for the study has already started. KoroSero-EST-2 is easier to organise: instead of family physicians inviting people to participate in the study, the surplus of blood samples sent to SYNLAB are used for the detection of antibodies. The lab provides the researchers with anonymous samples for analysis. The researchers will only know the region where the blood was taken and the age of the patient. The first results of KoroSero-EST-2 can be expected already at the end of September.

Study on coronavirus prevalence

The University of Tartu is also conducting a study on the prevalence of coronavirus, in which the prevalence and progress of the pandemic in Estonia is determined by PCR test of nasopharyngeal swab sample. A random statistical sample of 16,000 to 20,000 people will be interviewed and tested across Estonia to give the state evidence-based data for making decisions related to COVID-19.

For more information on KoroSero-EST, see the home page of the study. The study is financed by the Government of the Republic.