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We report a significant cluster of SARS-CoV-2 (B.1.617.2 variant) among young healthcare workers (HCW) (median age of 27years) living in the Mamoudzou interns apartment complex, belonging to the Hospital Center in Mayotte. Among them, 18 developed SARS-CoV-2 infection (62.1%) and all were symptomatic. The infection rate was higher for people who had had a second dose more than 6 months before than for those who had had a second dose less than 6 months before (P=0.05). This epidemic had no individual consequence, but the hospital functioning in Mayotte was significant impacted. This report reinforces the need for a third dose of vaccine among HCWs, in addition to non-pharmaceutical measures.

PT-COVID-19 Group: Ausenda Machado, Ana Paula Rodrigues, Carlos Matias Dias, Rita Ferreira, Joana Isidro, Miguel Pinto, Sílvia Duarte, Luís Vieira, Pedro Pinto Leite, António Maia Gonçalves, Luís Menezes Information on COVID-19 vaccines' effectiveness and viral loads in vaccine infection breakthrough cases are critical to inform decision-makers. Few studies have compared the effectiveness of mRNA vaccines against Delta versus Alpha variant of concern (VOC) and estimated variant-specific viral loads in vaccine infection breakthroughs cases. We developed a case-case study to compare mRNA vaccines' effectiveness against Delta versus Alpha variant using data on RT-PCR SARS-CoV-2 positive cases notified in Portugal during May-July 2021. The odds of vaccine infection breakthrough in Delta compared to Alpha cases were estimated by conditional logistic regression adjusted for age group, sex, matched by the week of diagnosis. RT-PCR cycle threshold values (Ct values) were compared by vaccination status and variant as an indirect measure of viral load and infectiousness. We report a statistically significant higher odds of vaccine infection breakthrough complete vaccination (OR=1.96; CI95% 1.22 to 3.14) in the Delta cases when compared to the Alpha cases. We observed that Delta variant cases revealed a Ct-value mean increase of 2.24 (CI95% 0.85 to 3.64) between unvaccinated and fully vaccinated breakthrough cases contrasting with 4.49 (CI95% 2.07 to 6.91) in the Alpha VOC. We found significantly higher odds of vaccine infection breakthrough in Delta when compared to Alpha cases, suggesting lower effectiveness of the mRNA vaccines in preventing Delta variant infections. Additionally, the vaccine breakthrough cases are estimated to be of higher mean Ct values, suggesting higher infectiousness with the Delta variant infection. These findings can help decision-makers weigh on the application or lifting of control measures and adjusting vaccine roll-out depending on the predominance of the Delta variant and the coverage of partial and complete mRNA vaccination. N/A

We investigated a COVID-19 outbreak of the SARS-CoV-2 Delta variant of concern in a London care home, where 8/21 residents and 14/21 staff had received a single dose of Vaxzevria (ChAdOx1-S; AstraZeneca) vaccine. We identified 24 SARS-CoV-2 infections (16 residents, 8 staff) among 40 individuals (19 residents, 21 staff); four (3 residents, 1 staff) were hospitalised, and none died. The attack rate after one vaccine dose was 35.7% (5/14) for staff and 81.3% (13/16) for residents.