Two shots of Covishield gave 63% protection against symptomatic SARS-CoV2 infection and 85% protection against moderate or severe disease, say the results of a study conducted in Faridabad, Haryana to assess the effectiveness of vaccine. The study notably was conducted during the second wave when the Delta variant was dominant.
While several studies — as did the latest study — have shown that vaccination elicited fewer neutralising antibodies against the Delta variant compared to the wild type variant, the Haryana study found that there wasn't any significant reduction in cellular immunity.
This category of immunity, also called T cell immunity, results from the body learning to destroy the coronavirus after having being taught to do so from either a vaccination or a previous infection and is considered more long lasting than the protection conferred from neutralising antibodies which are produced within weeks of an inoculation or infection.
The Haryana study thus implies that the vaccines continued to be effective at their primary promise of protecting against severe disease and death regardless of the variants.
Though India is now seeing a downswing in confirmed cases, several parts of Europe, Africa, the United States are seeing a surge in breakthrough infections, or those following full vaccination, prompting the roll out of booster doses in some countries. Concerns of a new variant, B.1.1.591 first discovered in South Africa have also emerged.
The results from this study are similar to the results from a phase-3 trial of the vaccine (AstraZeneca) in November, 2020 that showed a 62.1% efficacy when two standard doses of the vaccine were administered. Effectiveness differs from efficacy and is a measure of the real-world performance of a vaccine compared to its results in a clinical trial, the latter being conducted under relatively ideal conditions. Importantly, the clinical trial results were from when the Delta variant — that is at least twice as transmissible as the wild type variant used to make vaccines — wasn’t prevalent.
The findings of ChAdOx1 effectiveness of the Faridabad study, that appears online in Lancet Infectious Diseases on Friday, are comparable to recent studies in England and Scotland that have reported effectiveness of 60–67% for the vaccine against infection by the Delta variant.
To estimate the effectiveness of two doses of the ChAdOx1 vaccine, 2,379 confirmed cases and 1,981 controls (healthy individuals) were analysed. 85 (3·6%) cases were fully vaccinated compared with 168 (8·5%) controls. 157(6·4%) of 2,451 cases and 181(9·1%) of 1,994 controls had received only one dose giving a vaccine effectiveness of a single dose against SARS-CoV-2 infection of 46·2%.
Among the 84 cases of moderate-to-severe COVID-19, only one (1·2%) was completely vaccinated compared with 84 (3·7%) of 2,295 cases with mild COVID-19. 16 deaths were reported in the unvaccinated or incomplete vaccination group but none in the completely vaccinated group.
The study was conducted at Employee State Insurance Corporation Medical College (ESICMC) Hospital (Faridabad, India) and the Translational Health Science and Technology Institute (Faridabad, India) for RT-PCR testing for suspected SARS-CoV-2 infection between April 1 and May 31, 2021. The two centres account for nearly 90% of all tests in Faridabad.
The Covishield study follows a similar study analysing the effectiveness of Covaxin conducted at the All India Institute of Medical Sciences, Delhi and also published in the same journal on Tuesday. That study found that the effectiveness of Covaxin was 50% and significantly lower than the 65.2% efficacy in its phase-3 efficacy trial.
While both studies were comparable in that they were done during the second wave, the Covaxin trial involved only healthcare workers, a high-risk group. Moreover the Covaxin study had a much bigger population of 500 participants who'd got at least one dose of the vaccine compared to the 262 in the Covishied study. Significantly, the Covaxin study didn't report on how well the vaccine protected against hospitalisation and death.
“COVID-19 vaccination policies should encourage complete vaccination in addition to other epidemiological public health measures to overcome the threat posed by the delta variant and other emerging variants of SARS-CoV-2,” the authors noted.
