An unprecedented surge in SARS-CoV-2 infections driven by the Delta variant was reported from India recently [1]. The total reported new cases from its capital, New Delhi, in April and May 2021 were more than those reported in a 13-month period since the onset of the pandemic (765,117 vs 661,123)[2]. We estimated ChAdOx1 nCoV-19 effectiveness during this surge.Sir Gangaram Hospital is a tertiary care private hospital in New Delhi, having 4296 employees with equitable access to medical benefits, including investigations, medicines and hospitalization. Of these, between 16.1.21 to 30.4.21, 2716 received two doses, and 623 received a single dose of ChAdOx1-nCoV2. 927 remained unvaccinated till 30.4.21. 20 received the BBV152 vaccine or the BNT162b2 vaccine and were excluded from our analysis. We studied infection rates, moderate to severe disease rates, supplemental oxygen therapy rates and death rates in the ChAdOx1 2-doses and single-dose cohorts against the unvaccinated cohort in the period from 1.3.21 to 31.5.21. Disease severity was assigned as per the Indian Council of Medical Research guidelines[3]. Cumulative event rates were calculated using the Kaplan Meier estimator. Cox Proportional Hazard regression model was used to calculate Hazard Ratios (aHR) adjusting for age, gender, health-worker role, previous SARS-CoV-2 infections, active or retired status and comorbidities. Vaccine effectiveness was calculated as (1-aHR)x100.In total, 560 (13.1%) employees out 4276 tested positive for COVID-RTPCR between 1.3.21 to 31.5.21. Of these, 9 (1.6%) were lost to follow-up, 10 (1.8%) were asymptomatic, 458 (81.79%) had mild disease, 57 (10.2%) had moderate disease, and 26 (4.64%) had severe disease. There were six deaths totally in the study population (1.06%).Of those testing positive, 61 were hospitalized, and 499 remained in home quarantine(HQ). Notably, 57.5% of those with moderate to severe disease were in home quarantine (due to non-availability of hospital beds) and would have been missed had it not been through strict disease surveillance and follow up. Since hospitalization rates alone would not have been entirely indicative of disease severity, we did not take it as an outcome measure in our study.Marginal lowering of the incidence of symptomatic infections [12.0% (327/2716) vs 14.2% (133/937) aHR:0.76 (95%CI: 0.62-0.94)] and significant lowering of moderate to severe disease [1.2% (33/2716) vs. 3.4% (32/937); aHR: 0.35 (95% CI: 0.21-0.58)] and supplemental oxygen therapy [0.4% (11/2716) vs. 1.8% (17/937); aHR: 0.25 (95%CI: 0.11-0.58)] was observed in the 2-dose as compared to the unvaccinated cohort. This effect persisted for all events occurring beyond 14 days from dose 2.However, when analyzing for events beyond 21 days from the single dose, the incidence of symptomatic infections [12.3% (75/607) vs.
