DH Deciphers | Covishield or Covaxin: Which vaccine is superior?

As India scales up the Covid immunisation, many of us have been wondering which vaccine to take (Covishield or Covaxin) and which one has better protection against the coronavirus. While there's been no proper head-to-head comparison of the two vaccines so far, a preprint publication (not a peer-reviewed paper) released on Sunday showed that Covishield produces more antibodies than Covaxin but clearly mentioned that the research didn't "necessarily predict the efficacy of the vaccine". Here's more on that study:

What does the study show?

First a disclaimer: the study has several limitations. Only a very small number of people received Covaxin as against Covishield (90 vs 515), which makes any attempt to make a head-to-head comparison between the two vaccines statistically invalid. Nonetheless, here's what the study shows:

A single dose of Covishield produces nearly 10 times more antibodies than Covaxin. (Antibodies are proteins that cling to the virus and prevent it from invading the body's cells). The second shot of Covaxin closes the gap with Covishield-triggered antibodies. In people who did not have Covid-19, the seropositivity rates (the proportion of vaccine recipients who develop antibodies) after two complete doses were 97.8 per cent and 79.3 per cent with Covishield and Covaxin, respectively. But after the second dose, Covaxin closed the gap to a large extent. It gained a significant increase in both seropositivity and antibody titre after two completed doses. Contrarily, Covishield was able to generate antibodies in a large proportion of recipients and showed a four-fold rise in antibody counts even after a single dose. "One dose of either vaccine yielded a very high seropositivity and anti-spike antibody titre in SARS-CoV-2 recovered," the authors note. Overall, 97.8 per cent of those who never had the infection and had two complete doses of Covishield had detectable levels of antibodies, or tested seropositive, compared to 79.3 per cent with Covaxin.

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Isn’t that good enough evidence to say Covishield is superior?

Absolutely not, and the researchers said as much in the paper, which experts corroborate. "In conclusion, this cross-sectional study after the completion of two doses of both vaccines suggests that both vaccines induce seropositivity to anti-spike antigen in 95 per cent of SARS-COV-2 naïve and recovered individuals after three weeks. Whether any real difference in inducing immunogenicity exists between two vaccines can only be meaningfully demonstrated through a head-to-head randomised controlled trial," they wrote.

Gagandeep Kang, India's leading vaccine expert and a professor at Christian Medical College, Vellore, observes: "Antibodies are useful to assess the response to vaccines but do not measure protection."

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Virologist Shahid Jameel, who heads the Trivedi School of Biosciences at Ashoka University, says: "Antibody measurements are the easiest way but not the only thing that contributes to efficacy. T cells also play a role in immune responses and so does innate (inbuilt) immunity. Antibodies represent only one class of protective responses."

How does one measure whether a vaccine is giving protection to the recipient?

There are three factors to be taken into account. One needs to measure neutralising antibodies and evaluate the T cell immunity, which shows how the virus-infected cells are killed by the T cells in the body. None was carried out in the present study. Measuring T cells is complicated and needs special assays. The researchers only measured binding antibodies, which is a good surrogate for neutralising antibodies but doesn't directly translate into vaccine efficacy. Besides the T cell and antibody immunity, there is also innate immunity, which, too, has its role in protecting a vaccinated individual. The Centre's principal advisor on Covid-19, Vinod Paul, had also stated that antibody is only one measure of immunity. In the medium to long term, it is a "cell-mediated" immune response of the vaccine that is more critical and requires sophisticated systems of testing.

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Is there any other difference between the two vaccines?

Jameel points out another important difference between the two. "Covaxin (which is) based on killed virus is not expected to raise killer T cells while Covishield is found to raise the best killer T cell responses of all vaccines," he said.

How many doses of Covishield and Covaxin have been used so far?

As per the CoWin dashboard, 20,49,43,388 doses of Covishield have been used so far as against 2,70,16,290 doses of Covaxin. So for every dose of Covaxin, 7.5 doses of Covishield have been used so far. The production of both vaccines is to increase in the coming weeks.