A new study published in The Lancet Infectious Diseases has found that Covishield vaccine was effective against SARS-CoV-2 in fully vaccinated individuals during the second wave of the pandemic.¹

A multi-institutional team of researchers examined the effectiveness of Covishield (ChAdOx1) vaccine, particularly against the delta (B.1.617.2) variant, the key driver of the deadly second wave in the country.

Two centers in Faridabad (a city in the state of Haryana) were selected as the study sites. The study was conducted in April and May this year. All persons who tested positive for SARS-CoV-2 infection on RT PCR were included in the study group, while those who had a negative RT PCR were taken as the controls. Effectiveness of both doses of Covishield against Covid-19 was the primary end point. The secondary endpoints were the “effectiveness of a single dose against SARS-CoV-2 infection and effectiveness of a single dose and complete vaccination against moderate-to-severe disease among infected individuals”.

In addition, the in vitro live-virus neutralizing activity and cellular immune responses against the variants and the wild-type SARS-CoV-2 were also assessed in healthy vaccinated persons to find out how the vaccines worked to exert their protective effect.

The study analyzed the vaccine effectiveness in 2379 confirmed cases and 1981 controls. Out of the 2379 confirmed infections, 85 (3·6%) were fully vaccinated as against 168 (8·5%) in the control group.

In fully vaccinated individuals, the vaccine effectiveness against SARS-CoV-2 infection was found to be 63%. In participants who had taken a single dose of the vaccine, the vaccine effectiveness was 46·2%.

Compared with 84 of 2295 cases with mild COVID-19, one of 84 cases with moderate-to-severe COVID-19 was fully vaccinated, the vaccine effectiveness in fully vaccinated individuals against moderate-to-severe disease was much higher at 81.5%. While, the effectiveness of a single dose against moderate-to-severe infection was found to be 79.2%.

Compared to the wild-type SARS-CoV-2, neutralizing antibody response was reduced against different variants among 49 healthy and fully vaccinated persons. However, a notable finding was the preservation of the spike-specific CD4 and CD8 T-cell responses against both the delta variant and wild-type SARS-CoV-2.

Vaccine-mediated immunity is a combination of humoral and cellular immune response. Several studies have demonstrated waning antibody levels following vaccination with Covid-19 vaccines.

This study has shown that Covishield vaccine was effective against moderate-to-severe COVID-19 even during the second wave caused predominantly by the highly infectious delta variant, which is still the predominant strain globally.  This study has also demonstrated cellular immune response of Covishield against SARS-CoV-2 include the delta variant, which could offset the declining protective effect of humoral immunity.

Reference

1. Thiruvengadam R, et al. Effectiveness of ChAdOx1 nCoV-19 vaccine against SARS-CoV-2 infection during the delta (B.1.617.2) variant surge in India: a test-negative, case-control study and a mechanistic study of post-vaccination immune responses. Lancet Infect Dis. 2021 Nov 25;S1473-3099(21)00680-0. doi: 10.1016/S1473-3099(21)00680-0.