ICMR study shows immune evasion potential of Omicron

People infected with Delta variant of SARS-CoV-2 remain susceptible to Omicron which proves the latter's immune evasion potential post natural infection and vaccination, a new ICMR study has shown.

In the study done on a healthcare professional infected during all three Covid waves, ICMR scientists stressed that although vaccine boosters augment the immune response against the Omicron variant, it wanes over time and hence, non pharmacological interventions such as wearing masks, maintaining hand hygiene and infection control remain the most reliable weapons to curb the direct transmission of SARS-CoV-2.

The study which has been published in the Journal of Infection on Monday reported a case with primary SARS-CoV-2 infection, breakthrough infection with Delta and reinfection with Omicron in a healthcare professional from India.

A 38-year-old healthcare professional from Delhi who developed symptoms of fever, severe bodyache and sore throat tested positive for SARS-CoV-2 on October 9, 2020 through RT-PCR test.

This was a primary SARS-CoV-2 infection of this case which occurred during the first wave of pandemic, the study led by Dr Pragya Yadav, a senior scientist at NIV Pune, stated.

The body aches and fever persisted for the next four days and dry cough was present for a week. There were no complaints of breathlessness, vitals and SpO2 were normal. Post recovery, the patient experienced symptoms of long Covid including generalised weakness for 2-3 weeks and insomnia for 2-3 months.

The patient received the first dose of Covishield vaccine on January 31, 2021 and the second dose on March 3.

After a period of over a year, the person developed bodyache and headache in November 2021. Subsequently, he was found positive for SARS-COV-2 by RT-PCR, the study said.

The bodyache persisted for 2-3 days, but the case remained afebrile with no other symptoms. Recovery was uneventful except forgetfulness ("brain fog" of COVID-19). The case represented the breakthrough infection post primary SARS-CoV-2 infection and two doses of Covishield vaccination.

During the third wave of pandemic, the healthcare professional developed headache on January 24 2022 after being asymptomatic for a period of 2 months. His samples were again screened for SARS-CoV-2 by RT-PCR and found to be positive, the ICMR study stated.

He was in home isolation for 7 days; the recovery was uneventful except for increased forgetfulness post reinfection. The case observed SARS-CoV-2 reinfection post the incidences of primary and breakthrough infection.

During these three episodes of COVID-19 infection, he didn't require hospitalisation and received treatment for symptomatic relief under home isolation, the study highlighted.

The genomic sequence analysis clearly demonstrates that the case had a breakthrough infection with Delta derivative (AY.112) and reinfection with the Omicron sub lineage BA.2, it stated.

Although the clinical specimens of the primary infection couldn't be characterised, the probability of B.1 as an infecting variant would be higher as infection occurred in October 2020 where emerging variants were not detected in India, it said.

A serum sample collected post 8 days re-infection was tested by Anti-SARS-CoV-2 human IgG ELISAs namely, whole virion inactivated antigen, receptor Binding Domain of Spike Protein (S1RBD) specific IgG and Nucleocapsid (N) protein IgG capture ELISA as described earlier.

The IgG antibody titre was found 1:400, 1:1300 and 1:80 for whole antigen, S1-RBD and N protein ELISA. Although the case had a breakthrough infection followed by a reinfection, the antibody titres were lower than expected.

"With all these findings, we confirm the case of a breakthrough infection with Delta despite the hybrid immunity followed by a reinfection with Omicron post breakthrough infection," the study said.

A recent study has shown neutralisation for Omicron variant being reduced to 25.9 fold after a previous delta infection. It is reported that individuals with a prior infection with delta variant, remain susceptible to a repeat infection with Omicron, the study said.

The study highlighted that the findings of this study prove the immune evasion potential of the Omicron post natural infection and vaccination.

"The fight against COVID-19 is far from over. Although, vaccine boosters or third dose have shown an augmentation in the immune response against Omicron variant, it has been found to wane over time. In such a situation, non pharmacological interventions in the form of mask, hand hygiene and infection control remain the most reliable weapons to curb the direct transmission of SARS-CoV-2," the study stated.