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%.