In an indicator of the diminishing role of vaccines in preventing transmission of the coronavirus, a little over 25% of the fully vaccinated health-care workers of a Delhi hospital contracted a fresh or ‘breakthrough’ infection. None of the nearly 600 vaccine recipients, however, reportedly required hospitalisation. While previous reports of similar infections have been reported in other studies in India, this is the first time that such a high percentage has been reported as part of a single study.
The study involved health-care workers at the Max group of hospitals in Delhi and Gurugram and was led by scientists at the CSIR-Institute of Genomics and Integrative Biology (CSIR-IGIB). It appears as a preprint and is yet to be peer-reviewed.
The timing between the first and second dose varied, but 482 received the second dose within 42 days of the first dose. About half the recipients had been previously infected with SARS-CoV-2.
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Levels of antibodies
To confirm a reinfection, the researchers relied on levels of antibodies that were directed towards the nucleocapsid region of the coronavirus, which is different from the region (spike protein) that vaccine-generated antibodies normally target. Currently, all the vaccines are designed to produce antibodies against the spike-protein and so high levels of antibodies against the nucleocapsid region were taken to be markers of a fresh coronavirus infection. A breakthrough infection is one where someone tests positive at least two weeks after their second dose.
Shantanu Sengupta of the CSIR-IGIB and one of the scientists who led the study said that 25% was a “conservative estimate” as many of the infections were likely asymptomatic and only a subset of them who manifested symptoms were likely to get themselves tested.
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While the infections were mild it could contribute to healthcare workers unknowingly infecting patients, said Dr. Sengupta. For their analysis, the scientists relied on blood samples taken every week - up to 90 - following vaccination and because this period coincided with India’s second wave, where most infections were due to the Delta variant, it was most likely that these breakthrough infections too were due to the Delta variant. “Two doses of the vaccine weren’t protective against infection but infection followed by vaccination - even a single dose - was significantly protective against fresh infections,” he added.
Currently too, Delta and its associated lineages comprise nearly half the coronavirus infections and are believed to be driving the infection in Kerala and Maharashtra.
Those health-care workers who were previously infected had a reinfection rate of 2.5% over the same period.
In line with emerging evidence from several countries such as the United States and Israel where in spite of half the population being vaccinated, breakthrough infections are being reported, the study underlines that India too may not be immune to the phenomenon.
“The neutralisation of Delta variant by antibodies to non-Delta spike protein is greatly reduced. This means that neither prior infection by non-Delta variants, nor existing vaccines, are individually sufficient for the path to herd immunity. This also implies that masking is an essential part of any rational COVID control strategy, being agnostic to immune escape,” the authors note in their study.
The data indicates an “urgency to explore routes towards more effective use of vaccines,” the authors say. Because a single dose of ChAdOx1-nCoV19 to previously infected subjects induces humoral immunity comparable or better than two doses in naïve subjects, a single dose could be optimally directed to populations with high seropositivity.