The study assessed 2,714 hospital workers from the All India Institute of Medical Sciences (AIIMS) in New Delhi, from April 15 to May 15, 2021, who were symptomatic and underwent RT-PCR testing for Covid-19. 

 

The Delta variant was the dominant strain in India during the study period, accounting for approximately 80 per cent of all confirmed Covid-19 cases.

 

"Our study offers a more complete picture of how BBV152 performs in the field and should be considered in the context of Covid-19 surge conditions in India, combined with the possible immune evasive potential of the Delta variant," said Dr Manish Soneja, Additional Professor of Medicine at AIIMS in New Delhi.

 

Covaxin (BBV152) is a vero cell-derived, inactivated whole-virion vaccine formulated with a novel adjuvant and administered in a two-dose regimen, 28 days apart.

 

In January 2021, BBV152 was approved for emergency use in India for people aged 18, while earlier this month, the World Health Organization (WHO) also added BBV152 to its list of approved emergency use Covid-19 vaccines

 

Covaxin shot developed by Bharat Biotech in collaboration with the Indian Council of Medical Research (ICMR)-National Institute of Virology (NIV), in July announced the final results of its Phase-III clinical trials. 

 

According to the company, the vaccine showed an efficacy rate of 77.8 per cent against symptomatic Covid-19, which went up to 93.4 per cent against severe symptomatic infection.

 

In a study published in the journal The Lancet, earlier this month, Bharat Biotech demonstrated Covaxin to be 77.8 per cent effective against Covid-19, and 65.2 per cent against Delta variant.

 

However, in the latest study researchers at AIIMS acknowledged that the vaccine effectiveness is lower than the efficacy reported by a recently published phase-III randomised control study of BBV152.

 

Of the 2,714 employees in the study population, 1,617 people tested positive for SARS-CoV-2 infection, and 1,097 tested negative. The adjusted vaccine effectiveness against symptomatic Covid-19 after two doses of BBV152 with the second dose administered 14 or more days before undergoing RT-PCR testing was 50 per cent.

 

Higher risk of exposure to Covid-19 infection among the hospital employees than the general population, as well as the prevalence of circulating variants of concern, especially Delta, may have contributed to BBV152's lower effectiveness, the researchers said.

 

"Our study took place when the Covid-19 test positivity rate in Delhi was around 35 per cent a" the highest since the beginning of the pandemic. It is an important factor to consider while interpreting the impact of any vaccine." added Professor Naveet Wig, Head of Medicine at AIIMS, New Delhi.

 

The researchers stressed that the findings also add to the "growing body of evidence that rapid vaccine rollout programmes remain the most promising path to pandemic control. However, they also urged to continue additional protective measures, such as mask-wearing and social distancing".

 

Further, the authors also acknowledged several limitations in the study such as the study does not estimate the vaccine effectiveness against hospitalisation, severe disease, and death, which require further assessment. In addition, the study was not designed to estimate vaccine effectiveness for different time intervals after vaccination or to determine if vaccine effectiveness changed over time.

 

"The decline in vaccine effectiveness against SARS-CoV-2 infection during a delta-driven surge in cases is neither surprising nor exclusive to inactivated SARS-CoV-2 vaccines including BBV152," said researchers including Dr Ramachandran Thiruvengadam, Dr Akshay Binayke, and Dr Amit Awasthi of the Translational Health Science and Technology Institute in a linked comment. 

 

They were not involved in the study.

 

"The Delta variant has high transmissibility, infectivity, and virulence, which causes severe disease. These attributes might have contributed to a reduced vaccine effectiveness against symptomatic infections, which has been reported to be as low as 56 per cent for other vaccines in multiple studies worldwide. Future studies should be designed with the emphasis to evaluate protection against moderate to-severe Covid-19," they added.