Coronavirus prevalence study: vaccinated people may carry the virus but mostly do not fall ill

The recent stage of coronavirus prevalence study led by the University of Tartu confirms that vaccine protects against falling ill but a vaccinated person may still be a virus carrier. Because of a change in the sample, this stage of the study allows to draw reliable conclusions about vaccinated people only, and not about the entire adult population.

According to Ruth Kalda, the head of the prevalence study and Professor of Family Medicine at the University of Tartu, the shift from the random sample was caused by the fact that mostly vaccinated people responded to the invitation to participate in the study. “Compared to official vaccination statistics, our sample included too few unvaccinated people, and also those who had recovered from the disease were underrepresented. Because of the shift, we are not able to give reliable data about virus prevalence and antibodies in the general adult population,” Kalda admitted. She says that to overcome the problem, changes are planned in research methods.

Vaccinated virus carriers are mostly asymptomatic

Of the 2,513 people tested during this stage of the study, 11 gave a positive coronavirus test. Eight of them were carriers of the virus and three had recovered from the disease. The virus carriers included six vaccinated and two unvaccinated people. Most of the people who tested positive had no symptoms; only two had very light symptoms. According to the results, one in every 200 vaccinated adults in Estonia may be currently carrying the virus (0.5%). This stage of the study does not allow to accurately assess the spread of the virus in the entire adult population, but Kalda says that as unvaccinated people get infected more easily, the percentage of the infected may be higher among them.

“Considering the number of the daily new infection cases in Estonia, the amount of vaccinated virus carriers remains quite modest according to our study. Also, their symptoms are either non-existent or very light. This confirms that vaccination is a great relief for the healthcare system,” Kalda explained. Also international studies have shown that vaccines prevent 80–90% of cases associated with the Delta variant, and slightly less of the infections.

The prevalence study revealed that vaccinated people may carry the virus without symptoms, unknowingly, and can therefore be a potential infection risk for others, especially for the unvaccinated who are at least five times more likely to be infected than vaccinated people. “Considering that nearly half of the adult population in Estonia is still unvaccinated and small children do not get the vaccine yet, we should continue to apply the measures preventing the spread of the virus in order to protect them. This is why also vaccinated people should wear masks in crowded rooms and keep safe distance, if possible,” explained Mikk Jürisson, member of the research team and Associate Professor of Public Health at the University of Tartu.

According to Ruth Kalda, in vaccinated people the amount of the virus caused by the Delta strain decreases more quickly in the body than in unvaccinated ones. Thus, they are infectious for a shorter period than the unvaccinated.

The analysis of antibodies within the study showed that nearly all of those vaccinated with two doses (99%), irrespective of age group, had developed antibodies. 81% of those who had received one dose of the vaccine have got the necessary antibodies.

Researchers also examined whether there was a link between the lack of antibodies, the time of vaccination, and the person’s age, gender and other background characteristics. As no such links were found, Kalda says she cannot point out a population group who would currently need a third vaccine dose.

Readiness to use the Covid certificate varies with age

The behavioural study conducted as a part of the prevalence study aimed, among other things, to find out people’s attitudes towards the use of the Covid certificate for travel, participating in entertainment events and day-to-day work. 75% of the respondents generally agree to use the Covid certificate as a prerequisite for travel, and there were no major differences between age groups. On average, 69% of the respondents agree to present the certificate when going to entertainment events, but the amount of those in favour was larger in the older age group.

Slightly more than half of the respondents are ready to use the Covid certificate for working. Among respondents under the age of 39, about 40% were in favour and an equal part was against it. Slightly more than half of those aged 40–64, and nearly 68% of those aged 65 or above were in favour. “While the Covid certificate has been used for travelling for some time already, it is a relatively new requirement at cultural events and workplaces. The Health Board’s infection statistics shows that a lot of outbreaks have in fact started at work and events. All kinds of preventive measures are thus appropriate at such places. It is possible that people still need time to get used to it,” Kalda said.

Two thirds of the participants in the study favoured the vaccination of children.

The coronavirus prevalence study is conducted by a broad-based research team of the University of Tartu in cooperation with Synlab Estonia, Medicum and Kantar Emor.

More information about the study is available on the University of Tartu website.

Further information: Ruth Kalda, Head of Institute of Family Medicine and Public Health, Professor of Family Medicine, University of Tartu, +372 5698 5599, ruth.kalda@ut.ee