Single dose of Covishield offers no protection against Covid-19, says new study

This, the doctors say, could be due to emergence of the Delta variant, which is now the predominant strain in several countries

A large hospital-based study in India has found that a single dose of Covishield barely offers any protection from Covid-19 symptoms or severe disease, though experts are still not fully convinced with such a conclusion.

Doctors at Delhi’s Gangaram Hospital used the hospital employees’ Covid-19 vaccination data to show a single dose of Covishield offers no protection against symptomatic outcome or any outcome of interest, but there is significant protection with both doses and previous infection.

This, the doctors say, could be due to emergence of the Delta variant, which is now the predominant strain not only in India but also in several other countries.

“With the emergence of the Delta variant, previous studies showing 50-70 per cent protection with a single dose of Covid vaccine needs to be revisited. Vaccines are less effective against the Delta variant,” Ruma Satwik, lead investigator of the study told DH.

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She said because of the Delta variant, new UK data in June 2021 showed a single dose had offered only 30 per cent protection as against 60 per cent seen last year. Similarly, the Public Health Scotland data shows only 18 per cent protection with a single dose beyond 28 days.

The result from the Gangaram study is in contrast to previous research showing a modestly reduced but significant protection offered by a single dose as seen in the studies from Christian Medical College, Vellore (50%) and Public Health England (33%).

Eminent virologist and CMC professor Gagandeep Kang observed that one of the drawbacks of the new study was its small sample size of 623 people getting a single dose who were compared with 2,716 persons receiving both the shots. This increases the uncertainty factor (known as confidence interval) in the study result, making them unreliable.

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Satwik admitted having such a flaw in the study, but argued that data collection and statistical analysis were robust in her research, which was carried out with three other doctors from the same hospital. “Moreover, the world is now divided into post Delta and before Delta periods,” she said.

The findings come at a time when India is close to vaccinating half of its target population of 94.47 crore individuals with a single dose of a Covid vaccine, the lion’s share of which are Covishield.

The Centre’s plan is to inoculate as many people with a single dose hoping that such a strategy would offer at least some protection in case of a third wave.

"But the policy has to evolve for which more data is needed,” Satwik added.