Current coronavirus prevalence is more than double the peaks of previous infection waves

The recently completed wave of the coronavirus prevalence study led by the University of Tartu shows that coronavirus prevalence in the Estonian adult population has tripled since January. The infection is widespread in all age groups, but researchers say the peak of the disease wave is over. Nearly 90% of adults have developed antibodies against coronavirus.

In the study, 2,369 random-sampled* adults were tested from 9 to 24 March. Of these, 10.5%, or one in nine on average, tested positive. Half of them had recently recovered from coronavirus and no longer posed a risk of infection to others, but a half were still infectious. The majority of those potentially infectious also had symptoms. According to the leader of the prevalence study, Professor of Family Medicine of the University of Tartu Ruth Kalda, the high proportion of people who have recovered from the disease indicates that the peak of this infection wave is over.

The study also assessed the prevalence of infection in groups of different vaccination statuses. It showed that the infection rate among those who had received the booster dose was a fifth lower than among those vaccinated with two doses and twice lower than among unvaccinated people who had never contracted the virus. Therefore, among unvaccinated adults who have not suffered from the disease before, an estimated 21%, or one in five on average, are currently infected.

According to Kalda, these are the highest infection rates ever during the prevalence study. Fortunately, however, the current wave has not exceeded the record hospital admission numbers dating from March-April 2021. "This is due to the somewhat milder pattern of cases caused by the Omicron strain, as well as the fact that after two years of the coronavirus pandemic, the prevalence of antiviral antibodies in the community is higher, and the vaccination rate is also better than during the previous waves of infection," Kalda explained.

This study wave showed that nearly 90% of adults have antibodies against coronavirus. The prevalence of antibodies is similar across age and sex. However, the results reveal that while almost all vaccinated persons had antibodies, only 77% of unvaccinated persons who had recovered from coronavirus had antibodies. According to Kalda, these results suggest that the immunity gained from suffering from coronavirus tends to decline over time. "To keep the immune response stronger and more stable, vaccination is necessary even after recovering from the disease," Kalda explained. The study found that nearly 40% of those who were neither vaccinated nor known to have suffered from coronavirus still had antibodies.

Despite the particularly wide spread of the virus, people's behaviour shows a decrease in caution. For example, one in six respondents had been in close contact with someone who might have been infected with coronavirus before the survey wave, but nearly three quarters of them did nothing to prevent the possible spread of infection.

The prevalence study is carried out by a broad-based research group of the University of Tartu in cooperation with Synlab, Medicum and Kantar Emor. According to Mikk Jürisson, the chief executive of the prevalence study and Associate Professor of Public Health at the University of Tartu, the prevalence study has a crucial role in providing an overview of the prevalence of infection and assessing the dynamics of outbreaks now that the national testing strategy has changed. "Many thanks to all who have agreed to participate in the study and helped the research team to provide a regular evidence-based overview of the virus situation," said Jürisson.

For more information about the coronavirus prevalence study, see the University of Tartu web page.

The study is funded by the European Regional Development Fund from the EU measure to respond to the Covid-19 pandemic.

* As the sample is stratified by region, age group, gender and education level, the estimation of prevalence is not based on the simple arithmetic mean but the corresponding weighted mean.