Background SARS-CoV-2 variants of concern (VOCs) have threatened COVID-19 vaccine effectiveness. We aimed to assess the effectiveness of the ChAdOx1 nCoV-19 vaccine, predominantly against the delta (B.1.617.2) variant, in addition to the cellular immune response to vaccination. Methods We did a test-negative, case-control study at two medical research centres in Faridabad, India. All individuals who had a positive RT-PCR test for SARS-CoV-2 infection between April 1, 2021, and May 31, 2021, were included as cases and individuals who had a negative RT-PCR test were included as controls after matching with cases on calendar week of RT-PCR test. The primary outcome was effectiveness of complete vaccination with the ChAdOx1 nCoV-19 vaccine against laboratory-confirmed SARS-CoV-2 infection. The secondary outcomes were effectiveness of a single dose against SARS-CoV-2 infection and effectiveness of a single dose and complete vaccination against moderate-tosevere disease among infected individuals. Additionally, we tested in-vitro live-virus neutralisation and T-cell immune responses to the spike protein of the wild-type SARS-CoV-2 and VOCs among healthy (anti-nucleocapsid antibody negative) recipients of the ChAdOx1 nCoV-19 vaccine. Findings Of 2379 cases of confirmed SARS-CoV-2 infection, 85 (3•6%) were fully vaccinated compared with 168 (8•5%) of 1981 controls (adjusted OR [aOR] 0•37 [95% CI 0•28-0•48]), giving a vaccine effectiveness against SARS-CoV-2 infection of 63•1% (95% CI 51•5-72•1). 157 (6•4%) of 2451 of cases and 181 (9•1%) of 1994) controls had received a single dose of the ChAdOx1 nCoV-19 vaccine (aOR 0•54 [95% CI 0•42-0•68]), thus vaccine effectiveness of a single dose against SARS-CoV-2 infection was 46•2% (95% CI 31•6-57•7). One of 84 cases with moderate-to-severe COVID-19 was fully vaccinated compared with 84 of 2295 cases with mild COVID-19 (aOR 0•19 [95% CI 0•01-0•90]), giving a vaccine effectiveness of complete vaccination against moderate-to-severe disease of 81•5% (95% CI 9•9-99•0). The effectiveness of a single dose against moderate-to-severe disease was 79•2% (95% CI 46•1-94•0); four of 87 individuals with moderate-to-severe COVID-19 had received a single dose compared with 153 of 2364 participants with mild disease (aOR 0•20 [95% CI 0•06-0•54]). Among 49 healthy, fully vaccinated individuals, neutralising antibody responses were lower against the alpha (B.1.1.7; geometric mean titre 244•7 [95% CI 151•8-394•4]), beta (B.1.351; 97•6 [61•2-155•8]), kappa (B.1.617.1; 112•8 [72•7-175•0]), and delta (88•4 [61•2-127•8]) variants than against wild-type SARS-CoV-2 (599•4 [376•9-953•2]). However, the antigen-specific CD4 and CD8 T-cell responses were conserved against both the delta variant and wild-type SARS-CoV-2. Interpretation The ChAdOx1 nCoV-19 vaccine remained effective against moderate-to-severe COVID-19, even during a surge that was dominated by the highly transmissible delta variant of SARS-CoV-2. Spike-specific T-cell responses were maintained against the delta variant. Such cellular immune p...

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Background: Rapid expansion of the omicron SARS-CoV-2 variant of concern despite extensive vaccine coverage might be related to decreased neutralising ability of vaccine induced antibodies. The neutralising ability of different vaccines with or without natural SARS-CoV-2 infection against omicron is however not well known. Methods: We tested the ability of vaccine and natural infection induced antibodies to neutralise omicron variant in a live virus neutralisation assay. Four groups of individuals were included: (i) complete vaccination with ChAdOx1 nCoV-19 (n=20), (ii) complete vaccination with ChAdOx1 nCoV-19 plus prior SARS-CoV-2 infection during the delta variant driven surge (n=20), (iii) complete vaccination with inactivated whole virus vaccine (BBV152) (n=20), (iv) complete vaccination with BBV152 plus prior SARS-CoV-2 infection (n=20). Primary outcome was fold-change in the virus neutralisation ability of plasma against the omicron variant compared with ancestral and delta variant. Findings: The neutralisation geometric mean titre (GMT) was 384 (95% CI: 662, 223) against the ancestral virus with BBV152 vaccination alone and 383 (95% CI: 709, 207) with ChAdOx1 nCov-19 vaccination alone. The corresponding values for hybrid immunity groups were 795 (95% CI: 1302, 486) and 1424 (95% CI: 2581,786) respectively. Against the omicron variant, only 5 out of 20 in both BBV152 and ChAdOx1 nCoV-19 vaccine only groups, 5 out of 19 in BBV152 plus SARS-CoV-2 infection group, and 9 out of 20 in ChAdOx1 nCoV-19 plus SARS-CoV-2 infection group exhibited neutralisation titres above the lower limit of quantification (1:20). The 50% neutralization titre against ancestral strain and omicron demonstrated strong correlation with anti-RBD IgG levels [Pearson r: 0.94 (0.91, 0.96) p: <0.001 and 0.92 (0.88, 0.95) p:<0.001 respectively]. Interpretation: Omicron variant shows significant reduction in neutralising ability of both vaccine induced and hybrid immunity induced antibodies which might explain immune escape and high transmission even in the presence of widespread vaccine coverage.

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Over 90% of the COVID-19 patients manifest mild/moderate symptoms or are asymptomatic. Although comorbidities and dysregulation of immune response have been implicated in severe COVID-19, the host factors that associate with asymptomatic or mild infections have not been characterized. We have collected serial samples from 23 hospitalized COVID-19 patients with mild symptoms and measured the kinetics of SARS-CoV-2 viral load in respiratory samples and markers of inflammation in serum samples. We monitored seroconversion during the acute phase of illness and quantitated the amount of total IgG against the receptor-binding domain of SARS-CoV-2 and estimated the virus neutralization potential of these antibodies. Viral load decreased by day 8 in all the patients but the detection of viral RNA in saliva samples did not correlate well with viral RNA detection in nasopharyngeal/oropharyngeal swab samples. 25% of the virus-positive patients had no detectable neutralizing antibodies in the serum and in other cases, the efficiency of antibodies to neutralize SARS-CoV-2 B1.1.7 strain was lower as compared to the circulating virus isolate. Decrease in viral load coincided with increase in neutralizing antibodies and interferon levels in serum. Most patients showed no increase in inflammatory cytokines such as IL-1β or IL-6, however, elevated levels of IL-7 and other inflammatory mediators such as TNF-α and IL-8 was observed. These data suggest that most mild infections are associated with absence of inflammation coupled with an active innate immune response, T-cell activation and neutralizing antibodies.