NEW DELHI: The decline in the effectiveness of Covaxin, India's indigenous COVID-19 vaccine, from 77.8 per cent to 50 per cent during a Delta-driven case surge in April and May this year is neither bad nor surprising, say scientists.
The differing figures led to some concern, particularly among those who had received Covaxin, but several scientists allayed the misgivings, pointing to the potency of the Delta strain, the intensity of the second Covid wave in India and the level of exposure among healthcare workers.
The results of the first real-world assessment of Covaxin published in The Lancet Infectious Diseases journal on Wednesday showed that two doses of the vaccine, also known as BBV152, are 50 per cent effective against the symptomatic disease.
The study assessed 2,714 hospital workers at the All India Institute of Medical Sciences (AIIMS) in Delhi, from 15 April to 15 May, who were symptomatic and underwent RT-PCR tests.
Earlier, an interim study based on Phase 3 clinical trials showed that two Covaxin doses had 77.8 per cent efficacy against symptomatic disease and present no serious safety concerns.
"One possible reason for this drop is also infection period when Delta variant was the most prominent. The original 77 per cent figure is for Wuhan strain. In general, all vaccines are at least marginally less effective against the Delta variant as compared to Wuhan strain," said Vineeta Bal from Pune's Indian Institute of Science Education and Research.
Immunologist Satyajit Rath added that it is not clear whether the decrease in protection between the two studies is a "real difference".
"Even if it is, there are so many different possible contributory factors that it is difficult to offer even a semblance of likelihood. We should note that the earlier one was a trial of 'efficacy', while this is a study of 'effectiveness'. Typically, the latter is commonly less than the former," Rath, from the National Institute of Immunology (NII) in New Delhi, told PTI.
"That said, this is not a bad level of protection at all it seems to me," Rath added.
Efficacy is the degree to which a vaccine prevents disease, and possibly also transmission, under ideal and controlled conditions, while effectiveness refers to how well it performs in the real world.
Although a vaccine that has high efficacy would be expected to be highly effective in the real world, it is unlikely to translate into the same effectiveness in practice.
Immunologist Bal noted that vaccine effectiveness is also about the extent of morbidity encountered during infectious disease.
"If there is a significant reduction in the severity of cases, 50 per cent is still a useful efficacy, it can decrease the load on poor healthcare infrastructure," Bal told PTI.
The original 77.8 per cent efficacy, she added, is based on short-term data collection to get emergency use approval (EUA).
The post-vaccination observation period, she added, was very short.
Researchers noted that 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.
Covaxin, developed by Hyderabad-based Bharat Biotech in collaboration with the National Institute of Virology-Indian Council of Medical Research (NIV-ICMR), Pune, is an inactivated whole virus vaccine administered in a two-dose regimen, 28 days apart.
Bharat Biotech has said the study shows that Covaxin meets the World Health Organisation's criteria for vaccines against the Delta variant.
"The results compared well with the 65.2 per cent efficacy against the Delta variant obtained during the controlled phase 3 clinical trials of Covaxin conducted among the general population. This study also shows that Covaxin meets the WHO's criteria for vaccines against the dreaded Delta variant," the company tweeted on Thursday.
In January this year, Covaxin was given emergency use approval (EUA) in India for people aged 18 and above.
The WHO added the vaccine to its list of approved emergency use COVID-19 vaccines earlier this month.
Results from a recent study, which included researchers from the Indian Council of Medical Research, indicate that both Covishield, the other vaccine being used in India, and Covaxin significantly reduce the risk of severe COVID-19 and are effective against the Delta variant among Indians aged 45 years and above.
The yet-to-be peer-reviewed study found that overall effectiveness against severe COVID was 80 per cent with two doses of Covishield, and 69 per cent with two doses of Covaxin.
"The vaccine effectiveness estimates were similar against the Delta strain and sub-lineages," the authors of the study noted.
Another report based on data from The Centers for Disease Control and Prevention (CDC) using mRNA vaccines also shows that in health workers efficacy of vaccine-mediated protection was less with the emergence of Delta variant.
Writing in a linked Comment of The Lancet Infectious Diseases study, Ramachandran Thiruvengadam, Akshay Binayke, and Amit Awasthi of the Translational Health Science and Technology Institute, Faridabad said 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.
"The Delta variant has high transmissibility, infectivity, and virulence, which causes severe disease. These attributes might have contributed to 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," the scientists, who were not involved in the study, noted.
"Nevertheless, faced with the challenge of protecting as much of the population as possible, the ongoing vaccination drive should be continued as a public health intervention against SARS-CoV-2, along with strict adherence to other non-pharmacological interventions, particularly in the context of variant-driven surges," they added.
A study published in August in the New England Journal of Medicine found the effectiveness of two doses of Pfizer vaccine to be 93.7 per cent against the Alpha variant and 88 per cent against the Delta variant.
With the AstraZeneca vaccine, the effectiveness of two doses was 74.5 per cent against Alpha and 67 per cent against the Delta variant.