Prevalence of anti-SARS-CoV-2 antibodies in the general population in Belgium : Results of the second data collection (28/09/21-23/02/22)

KEY FINDINGS Between the last period of wave 1 (14/06-11/07) and the first period of wave 2 (27/09-24/10) the percentage of adults (18 years or older) with anti-SARS-CoV-2 antibodies has increased from 69.5% to 90.4%. This substantial increase is in line with the intensive vaccination campaign during this time period. During the second period of wave 2 (25/10-21/11) the prevalence increased further to 92.9%. Among the fully vaccinated having received the last dose (eventually a booster) in the past 3 months (but more than 2 weeks ago), the prevalence of anti-SARS-CoV-2 antibodies was 97.9% in the first period of wave 2 (27/09-24/10) and 99.1% in the second period (25/10-21/11). Among the fully vaccinated having received the last dose (eventually a booster) longer than 3 months ago, these percentages were respectively 93.8% and 95.7% Among people who had not received any COVID-19 vaccine at all the prevalence of anti-SARS-CoV-2 antibodies increased in wave 2 from 22.5% (27/09-24/10) to 36.5% (25/10-21/11). However, caution is needed to interpret these figures, because in wave 2 the number of unvaccinated people was quite low. Most of those who had a negative test in wave 1 (87.8%) became positive in wave 2. This is mainly explained by the vaccination campaign: 94.8% of these people received a COVID-19 vaccine between their participation in wave 1 and wave 2. Only 9.7% of the unvaccinated people with a negative test in wave 1 became positive in wave 2. In total, 3.1% of those who had anti-SARS-CoV-2-antibodies in wave 1 seroconverted in wave 2. Among the unvaccinated and the partially vaccinated people, the risk of becoming seronegative was 10 times higher than among the fully vaccinated. Among the total study population, older age was associated with a higher risk of not having antibodies in wave 2. Among the fully vaccinated, older people and people with at least one chronic disease were at higher risk of not carrying antibodies. The risk of being seronegative in wave 2 increased significantly as the number of days since the last vaccination advanced. Furthermore, having received at least one dose of mRNA vaccine compared to adenoviral-vectored vaccine reduced the risk of being seronegative by 90%.