Covishield unable to halt breakthrough Delta infections: Study

Fresh evidence on Covishield’s inability to halt “breakthrough infections” caused by the Delta variant of SARS-CoV-2 in fully vaccinated individuals emerged on Sunday with a group of Indian researchers reporting an unexpectedly large proportion of Covid-19 infections among the vaccine recipients.

In a study carried out on healthcare workers from five hospitals in Delhi and its satellite cities, they found signatures of Delta infection in 25% of 92 individuals who received two shots and in 48% of 82 recipients immunized with a single dose of Covishield.

The study carried out jointly by scientists at the Institute of Genomics and Integrative Biology and doctors at Max Hospital came days after doctors at Delhi’s Gangaram Hospital reported that a single dose of Covishield offered no protection against “symptomatic outcome or any outcome of interest.”

“Vaccination breakthroughs with Delta are far more common than realised so far. A corollary of very frequent asymptomatic vaccination breakthroughs in healthcare workers is that masking will remain critical to reduce risks of transmission in high risk settings,” tweeted IGIB director Anurag Agrawal.

The infection rates -- reported by the IGIB-Max team -- are higher than 9% observed at the Christian Medical College, Vellore and 1.6-2.6% reported by the Postgraduate Institute of Medical Education and Research, Chandigarh, earlier this year.

Also read: 84-day gap between two Covishield jabs for efficiency: Centre tells Kerala HC

“The primary purpose of protection from severe disease is well met by vaccines -- here Covishield. There was no severe disease or death among this cohort (group) of vaccinated healthcare workers even during one of the worst delta surges,” Agrawal said referring to India’s ferocious second Covid-19 wave in the April-May period.

Worryingly, they found 48% of persons getting a single dose developing the infection. “This is unacceptably high,” the team reported in a paper, which is yet to be peer reviewed but has been posted in an online archive.

The large proportion of breakthrough infections and 48% infection rate among the single dose recipients underlines the importance of having a relook at the current vaccination policies including the gap between the two doses of Covishield. Currently the gap is 12-16 weeks.

Such a high level of breakthrough infections bolsters the evidence for suggestions that existing vaccines might not help achieve the so-called herd immunity threshold and stop the epidemic, now fuelled predominantly by the Delta variant.

But the data indicates an urgency to explore routes towards more effective use of vaccines. In populations with high seropositivity, a single dose can offer considerable protection but in areas where the Delta variant is circulating, it might help to shorten the gap as two doses offer much higher level protection.