Vaccination may not have a significant impact on superspreading events

PURPOSE: Vaccination for SARS-CoV-2 is highly effective in preventing severe disease and death but its impact on reducing transmission outside households in reducing super spreading events are not so clear. METHODS & MATERIALS: We reviewed Singapore's Ministry of Health (MOH) daily detailed reports on cases and clusters from April - June 2021. Cases were classified as: Fully vaccinated- completed two doses of vaccine at least fourteen days prior or not fully vaccinated. Cases were also categorized by whether they were diagnosed while under quarantine or by symptomatic or routine surveillance testing. Index cases were defined as the first cases identified in each cluster. Secondary cases were classified as non-index cases. Stratified analysis was performed on VassarStats (Vassar College, USA). RESULTS: 803 cases were reviewed; 187 (23%) were fully vaccinated, 56 (7%) were partially vaccinated and 560 (70%) were completely unvaccinated. There were a total of 77 clusters reported during this period, ranging from 3 to 108 cases primarily in malls, markets and bars. The proportion of index cases for these clusters who were fully vaccinated (20.8%) was similar (RR 0.864, 95% CI 0.511-1.46) to the secondary cases (23.6%). In contrast, the majority of index cases for clusters were identified through surveillance (96.1%) with only three of the 77 (3.9%) detected in quarantine while the majority of 726 secondary cases (68.2%) were detected in quarantine. CONCLUSION: Although many of the clusters may not represent true superspreading events, it is still concerning that vaccinated individuals formed such a significant proportion of the index cases for clusters. More studies are clearly needed to better understand super-spreading events and to devise better vaccination strategies to prevent single cases from sparking clusters. As vaccination rates increase globally, preventing superspreading events will be a major part of controlling the pandemic.