The Delta variant of SARS-COV-2 was behind majority of clinical cases of breakthrough infection of Covid-19 but only 9.8 per cent cases required hospitalisation and fatality was observed in only 0.4 per cent cases, according to a new ICMR study.

Breakthrough infection is if one gets infected even after immunisation.

The study, the largest and first nation-wide study of post-vaccination breakthrough infections from India, said its analysis shows that the vaccination does provide reduction in hospital admission and mortality.

"Therefore, enhancing the vaccination drive and immunising the populations quickly would be the most important strategy to prevent further deadly waves of the Covid-19 and would reduce the burden on the health care system," the study said.

Two new SARS-CoV-2 variants Delta AY.1 and AY.2 were also identified in these study samples.

"Delta AY.1 and AY.2 is characterized by the presence of K417N mutation in the spike protein region. K417N, E484K, L452R, and E484Q are the mutations known to disrupt receptor binding domain (RBD) binding capacity and make them more infectious by immune escape mechanisms against the current vaccines. This indicates improved virus fitness to evade immune responses and survive against the vaccines," the study said.

For the study, the Indian Council of Medical Research (ICMR) collected 677 clinical samples (throat swab/nasal swabs) of individuals who had received two doses and one dose of vaccines (Covishield and Covaxin,) and tested positive for COVID-19, from 17 states and Union Territories of the country.

This study indicated that the majority of the clinical cases in the breakthrough were infected with the Delta variant and only 9.8 per cent cases required hospitalization while fatality was observed in only 0.4 per cent cases.

"The overall majority (86.09 per cent) of the breakthrough infections were caused by the Delta variant (B.1.617.2) of SARS-CoV-2 in different regions of India except for the northern region where the Alpha variant predominated," the study said.

The present study revealed the infection among breakthrough cases predominantly occurred through Delta variants indicating its high community transmission during March-June followed by Alpha and Kappa variants.

In the study, 67 cases (9.8 per cent) required hospitalization and fatality was observed in only three cases (0.4 per cent).

The 17 states and Union Territories from where the samples were collected are Maharashtra, Kerala, Gujarat, Uttarakhand, Karnataka, Manipur, Assam, Jammu and Kashmir, Chandigarh, Rajasthan, Madhya Pradesh, Punjab, Puducherry, New Delhi, West Bengal, Tamil Nadu and Jharkhand.

The study found that southern, western, eastern and north-western regions of India predominantly reported breakthrough infections from mainly Delta and then Kappa variant of SARS-CoV-2.

Out of the 677 patients analysed, 85 acquired Covid-19 infection after taking the first dose of the vaccine, while 592 were infected after receiving both doses of the vaccine.

A total of 604 patients had received Covishield vaccine, 71 had received Covaxin and two had received Sinopharm vaccine.

A total of 482 cases (71 per cent) were symptomatic with one or more symptoms, while 29 per cent had asymptomatic SARS-CoV-2 infection.