During the period March 2021-April 2022, a randomly selected group of the adult population in Belgium received a health questionnaire and saliva test kit to collect a saliva sample from themselves. Sciensano then examined samples to determine the prevalence of antibodies against the coronavirus in the general population.
The aim of the study was to provide information on the prevalence of people with antibodies against the coronavirus in the general Belgian adult population over time and to study to what extent this prevalence varied in function of health status and lifestyle characteristics (e.g. chronic diseases, smoking behavior, BMI, …). In order to follow the evolution of the presence of antibodies over time, the data collection was repeated after about 3 and 6 months with the same individuals. In this way, it was also possible to study possible changes in the presence of antibodies against the coronavirus, taking into account the vaccination status of the individuals.
The prevalence of antibodies against the coronavirus was determined in a random sample of the general adult population in Belgium, including people who had no contact with health services. The study was complementary to other seroprevalence studies in Belgium which focused on specific groups (blood donors, school children, health care workers, rest home residents).
If the study participants had given their consent, Sciensano stored their saliva samples in its biobank. People could also indicate if the data collected in this study could be linked in an encrypted way with data from administrative and clinical databases for further scientific research.
The three main research questions of the SalivaHIS study were:
- How did the prevalence of anti-SARS-CoV-2 antibodies evolve during the study period in the general population, the vaccinated and the unvaccinated population?
- What socio-demographic and health-related characteristics were associated with seropositivity, in the general population, the vaccinated population and the unvaccinated population?
- To what extent did people with a positive test result serorevert to a negative test in a following data collection point and what factors were associated with seroreversion?
The results of this study provided useful information to Belgian policy makers to estimate the impact of the vaccination strategy and the different COVID-19 waves on the presence of antibodies to the coronavirus in the population. In addition, these results provided useful research data to understand different aspects of the epidemic.
The main results of the study are summarised below.
- The prevalence of salivary antibodies against the coronavirus in the community-dwelling adult population in Belgium increased from 25.1% to 92.3% between April 2021 and March 2022. This was primarily the result of the vaccination campaign, which had reached a global coverage for primary vaccination of 89% in the adult population by the 31st of March 2022.
- The antibody prevalence obtained in this study addressing the general population directly where somewhat lower than the antibody prevalence in the blood donor population for the same study period.
- Among people who had received the vaccine, the antibody prevalence rates increased with time, from 92.5% to 99.9% between April 2021 and March 2022.
- In line with the surge in infections during the 4th and 5th COVID-19 waves identified by the conventional surveillance systems in Belgium, the antibody seroprevalence peaked at end of March 2022 and even surpassed the level of population vaccination coverage. This is probably the result of the high numbers of infections, leading to the development of antibodies among the unvaccinated people.
- In the unvaccinated population, an initial increase of the prevalence antibodies against the coronavirus was observed in the early period of April-July 2021 (from 16.6% to 21.9%), but no further increase was noted in the following study periods. However, the number of unvaccinated people in our study had become too small as from the second data collection point (wave 2) to assess this with precision.
- In the total (vaccinated and unvaccinated) population, the prevalence of antibodies against the coronavirus was significantly higher among women than among men. This difference was particularly discernible among the vaccinated individuals.
- In the vaccinated population, the prevalence of antibodies against the coronavirus was significantly lower among people aged 65 years and older and among people with a chronic disease than among younger people and people without chronic diseases respectively.
- Among vaccinated people, higher prevalence rates of antibodies against the coronavirus were found in people with at least a bachelor diploma (compared to those with secondary education or lower) and people living together with others (compared to those living alone).
- In the unvaccinated population, smokers had a lower prevalence of antibodies against the coronavirus than non-smokers, including after correction for past known COVID-19 infection.
- Seroreversion, the loss of measurable antibodies against the coronavirus was rare in vaccinated people, even in those who received their last dose of vaccine more than 3 months ago. In contrast, in about 1 in 3 people who were not fully vaccinated, antibodies became undetectable over time.
- Further monitoring of antibodies against the coronavirus in the general population should allow the distinction between antibodies generated by vaccination and antibodies generated by infection and be able to assess the level of protection at population level and in specific population groups. From a global public health perspective the monitoring should preferably be integrated in a more general serosurveillance system in which antibodies against other pathogens are also included.