Explained: Covishield vaccine study shows breakthrough impact

The results of the study (‘Covishield (AZD1222) Vaccine effectiveness among healthcare and frontline Workers of Indian Armed Forces: Interim results of VIN-WIN cohort study’), published in a special issue of the peer-reviewed Medical Journal Armed Forces India on Tuesday, underline the strong benefits of vaccination against breakthrough infections and deaths, and reiterate the message ‘Get Vaccinated, Stay Safe’, the researchers said.

A very large study

“This is the largest study from India evaluating Covid vaccine effectiveness so far,” the researchers have said. Air Cmde Subramanian Shankar, corresponding author of the study, told The Indian Express: “Other studies have a sample size under 1 million. Hence we believe that VIN-WIN cohort is possibly one of the largest studies worldwide on vaccine effectiveness, if not the largest.”

Study and findings

Healthcare workers and frontline workers of the armed forces were among the first to get their jabs after India started vaccinating on January 16 this year. The study presents an interim analysis of vaccine effectiveness estimates of 1.59 million recipients until May 30.

“Data of 1,595,630 individuals (mean age 27.6 years; 99% male) over 135 days was analysed. Till 30 May, 95.4% and 82.2% were partially and fully vaccinated (respectively),” says the study.

“The UV (unvaccinated), PV (partially vaccinated) and FV (fully vaccinated) compartments comprised 106.6, 46.7 and 58.7 million person-days respectively. The number of breakthrough cases in the UV, PV and FV groups were 10,061, 1,159 and 2,512; while the deaths were 37, 16, and 7 respectively. Corrected VE (vaccine effectiveness) was 91.8-94.9% against infections.”

The study used anonymised data from the existing Armed Forces Health Surveillance system which had been enhanced for monitoring Covid-19. The system had data for daily vaccinations with first and second doses, dates of testing positive for Covid-19, and Covid-related deaths. As the shift occurred from unvaccinated to partially vaccinated and then fully vaccinated, the numbers in each group changed daily. Since each individual stayed in the three groups (UV, PV, and FV) for varying lengths of time, the population at risk was measured in person days (100 person-days could be either one person for 100 days or 10 persons for 10 days each).

The crude rates were calculated by dividing infections/deaths by the population at risk, and corrections were made for the force of the pandemic’s second wave in April-May 2021, which was 600-1,000 times higher than in January, Air Cmde Shankar said.

Other studies

A study from Scotland published in The Lancet in April this year analysed a cohort of 1.33 million people who were vaccinated between December and February to gauge the “real-world” effectiveness of first doses of the Pfizer-BioNTech and Oxford-AstraZeneca vaccines against hospital admissions. The results showed a vaccine effect of 91 per cent for Pfizer-BioNTech and 88 per cent for Oxford-AZ.

The study concluded that the “mass roll-out of the first doses of the…vaccines was associated with substantial reductions in the risk of hospital admission due to Covid-19”.

The VIN-WIN study mentions results of other Covishield vaccine effectiveness studies as well.

A case control study in the UK among 1.57 lakh people older than 70 years found reduced odds of 73 per cent in cases and 43 per cent in hospital admissions; an RCT study of 11,000-odd individuals older than 18 years in the UK, Brazil, and South Africa reported a reduction of 62% in cases; and an RCT study of 2,026 HIV-negative individuals aged 18-65 in South Africa reported a reduction of 22 per cent cases against the B.1.351 (Beta) variant of the virus.

Earlier this month the Indian Council of Medical Research reported the findings of a study done by the police department of Tamil Nadu, ICMR-National Institute of Epidemiology, and Christian Medical College, Vellore, that showed 82 per cent effectiveness on personnel who had received a single dose, and 95 per cent on those administered both jabs.

In Maharashtra, a study across 20 government Covid centres under the director of medical education and research showed that 87.5 per cent of those hospitalised were not vaccinated.

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Challenges and limitations

The VIN-WIN study was carried out at a time when the country was reeling from the second wave of the pandemic. “The study cohort, belonging to the tri services, was spread across the nation. Apart from constraints of terrain and location, data had to be collated at a central facility and updated on a daily basis. This required creating a novel surveillance system,” Air Cmde Shankar said.

“A conventional cohort study incurs significant cost. So the Armed Forces Medical Services team decided to innovatively use results of the natural experiment that was created with 1.59 million individuals moved from unvaccinated to partially to fully vaccinated groups. By tracking them in detail on a daily basis, researchers could use the individuals as their own ‘internal comparison’,” he said. “Researchers had to also take into account the changing dynamics of disease transmission in the form of the pandemic’s second wave.”

Among the limitations of the study, the authors have noted that “the cohort differed from the Indian population”. “While the mean age of this cohort (27.6 years) was similar to that of the Indian population, the confidence interval was much narrower as it did not represent almost 50% of the population (age <18 years ~40% and &gt;60 years~10% ).”

Also, it was a “predominantly male cohort comprising individuals with minimal co-morbidities”. Thus, the results “may not generalize across the entire population”, and “vaccine effectiveness may or may not be similar…”

Biggest takeaway

Surg Vice Admiral Rajat Datta, Director General, Armed Forces Medical Services, and co-author of the study, said in a statement: “The study sends a clear message of vaccine efficacy… It would be an important step to help overcome vaccine hesitancy backed by scientific evidence.”

Air Cmde Shankar said the team would now follow this cohort over time to answer other questions; “one of the important ones would be to determine the ideal time for a third/booster dose”.

On Tuesday, Dr V K Paul, Member NITI Aayog (Health), spoke about the importance of the armed Forces study. “While no vaccine can guarantee against infection, it can prevent serious disease,” he said.