Two doses of Covaxin 50% effective against Delta strain of COVID-19: Scientists

Earlier, an interim study based on Phase three clinical trials showed that two Covaxin doses had 77.8 percent efficacy against symptomatic disease and present no serious safety concerns.

One possible reason for this drop is also the infection period when the Delta variant was the most prominent. The original 77 percent 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 percent is still a useful efficacy, it can decrease the load on poor healthcare infrastructure, Bal told PTI.

The original 77.8 percent 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 percent 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 Organization's criteria for vaccines against the Delta variant.

The results compared well with the 65.2 percent efficacy against the Delta variant obtained during the controlled phase three 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 November 25.

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 percent with two doses of Covishield, and 69 percent 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 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.

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 percent 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 percent against the Alpha variant and 88 percent against the Delta variant.

With the AstraZeneca vaccine, the effectiveness of two doses was 74.5 percent against Alpha and 67 percent against the Delta variant.