The prevalence of COVID-19 in Tallinn is ten times higher than previously thought and three-and-a-half times higher on Saaremaa, the university's study showed. The island of Saaremaa was by far the worst hit region of Estonia during the peak of the pandemic in the spring.

The study compared the incidence of the KoroSero-EST-1 coronavirus antibody study with official statistics, which are based on nasopharynx test samples.

Rates still comparatively low, second wave risk depends on public responsibility

Piia Jõgi, assistant to pediatric infectious diseases at the University of Tartu and a lecturer in paediatrics at the University of Tartu Hospital, says that in comparison with many other European nations, coronavirus rates in Estonia are still low.

At the same time, the population should be on guard against a potential second wave.

"We still have a large proportion of the population susceptible to SARS-CoV-2," Jõgi said. "This means that if we do not follow general rules, such as washing our hands carefully, maintaining social distancing, staying at home when sick or after a foreign trip [if the country visited was above the 16 per 100,000 cases threshold] we run a very high risk of a new coronavirus wave, which may bring with it its own restrictions."

The basis on which the first number is compiled is different, however, and uses nasopharynx samples, rather than serological tests, as in the case of the new study.

Serologic tests are blood tests that look for antibodies in your blood, while nasopharynx samples are swabs from the upper part of the throat behind the nose.

1.4 percent of Tallinn, and 6 percent of Saaremaa, subjects had COVID-19 antibodies

The University of Tartu, Kuressaare Hospital on Saaremaa and Järveots family medical center in Tallinn surveyed a total of 2,000 individuals over the period May 11 - July 29. The study randomly sampled in conjunction with the Health Insurance Fund (Haigekassa) with the aim of detecting the presence of coronavirus antibodies in the blood.

If antibodies are present it means the individual has been exposed to the virus.

The prevalence of coronavirus stood at 1.4 percent in the Järveots group of 1,006 people and at 6 percent among the Kuressaare group of 954 people, Jõgi said. 

When compared with official data this gave a COVID-19 presence 10 times higher than previously thought for Tallinn and 3.5 times higher for Saaremaa.

The findings mean that Saaremaa, which has an officially registered figure of 167 cases per 10,000 inhabitants, actually has a rate of 597 per 10,000, Jõgi said.

80 percent of subjects with antibodies were symptom-free

The study showed 80 percent of subjects who presented with antibodies displayed no symptoms, the remainder said they had experienced a high fever, runny nose, nausea, sore throat, diarrhea or chest pain – all symptoms of COVID-19.

Over half of those with antibodies had had no known contact with a coronavirus carrier.

KoroSero-EST-1 was a pilot study ed by Marje Oona, associate professor of family medicine at the University of Tartu.

A forthcoming survey, KoroSero-EST-2, will use analyze blood samples from private sector lab diagnostic firm SYNLAB to see if antibodies are present. Samples will be distributed to researchers for analysis anonymously with only the region and age of individual given.

The University of Tartu will also conduct a coronavirus prevalence study based on samples from the nasopharynx which will include 16,000–20,000 people across Estonia selected at random. They will be interviewed and tested in order to provide the country with better evidence-based data when making decisions related to COVID-19.

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