The Omicron variant that caused a huge surge in COVID-19 infections worldwide has been studied in terms of prevalence, infectivity, and virulency. The first study conducted in South Africa showed the low pathogenic nature of the Omicron variant compared to the Delta and original Wuhan variants. The deaths, ICU admissions, length of the hospital stay, and also the need of oxygen support all reduced greatly in Omicron infected patients as compared to the earlier variants. The share of the Omicron variant in Delhi (India) rose rapidly. In two weeks (December 21, 2021-January 3, 2022), the omicron prevalence increased from 38% to 81%. Because of the Omicron variant, in the last week of January 2022, Delhi’s caseload surged nine folds. The cases analysed in Delhi, Mumbai, and throughout India revealed that the number of deaths during the Omicron wave was much lower compared to the Delta and Wuhan variants.

The dynamics of the Omicron variant in India, Mumbai, and Delhi have been studied. The % share of the Omicron variant in overall India rose from 0.35% to 97.12% during the period November 29, 2021, to February 7, 2022. Due to the emergence of the Omicron variant, the daily new infections in India rose from a baseline tally of below 8,000 cases to the highest number of 347,063 cases recorded on January 20, 2022. In Mumbai, the Omicron variant completely replaced the Delta and other lineages as reported on March 3, 2022 genome sequencing report. In Mumbai because of the Omicron variant surge, the new infections rose to 20,971 on January 7, 2022, from a baseline tally of 191 daily cases recorded after receding the second Delta variant wave. In real-time, the vaccine effectiveness among the Indian population during the Omicron variant surge was studied and noticed that the two-dose regimen was 99.3% effective in preventing death. The death rate among the hospitalized patients was reduced by 12% among the vaccinated individuals compared to the unvaccinated ones. During the Omicron variant wave, the average age of the COVID-19 patients shifted by 11 years towards the younger age which was because the vaccination among the younger population was low compared to the elderly population.

The results of the fourth COVID-19 national serosurvey (June-July 2021) of India have been analysed. Much needed data of vaccine generated antibodies have been reported world first as recorded in Indian population on the ground in real. More than two-thirds of (67.6%) Indian population developed antibodies against the infection. This includes natural immunity build up due to infection and vaccine generated antibodies. Eleven states: Madhya Pradesh, Rajasthan, Bihar, Gujrat, Chhattisgarh, Uttarakhand, Uttar Pradesh, Andhra Pradesh, Karnataka, Tamil Nadu, and Odisha had sero positivity of >67.6%which was the national average. Ten other states registered < 67.6% seroprevalence were: Punjab, Telangana, Jammu and Kashmir, Himachal Pradesh, Jharkhand, West Bengal, Haryana, Maharashtra, Assam, and Kerala. Single dose vaccination of 24% population added 19% seroprevalence in the population whereas 13% full vaccination increased 28% individuals registered antibodies. The highest (90%) seroprevalence was registered among the individuals who have taken both vaccine doses followed by HCWs (85%). Seroprevalence in 85-90% population might be the threshold for herd immunity which delayed or possibly stopped the third COVID-19 wave in India.

The results of all six serosurveys of Delhi (India) have been analysed. The first serosurvey held in June-July 2020 found seroprevalence in 22.9% population. The second, third, and fourth survey conducted in August, September, and October registered 29.1%, 25.1%, and 25.5% seropositivity, respectively. The seroprevalence was increased to a larger extent of 56.1% in the fifth (January 2021) survey due to the accumulation of antibodies among the residents in three pandemic waves that hit the city in June, September, and November 2020. The last and the fourth bigger wave that hit the city in April 2021 caused almost all (97%) citizens to develop antibodies against the natural COVID-19 infection. The seroprevalence in women was marginally higher (90%) than in men (88%). The seroprevalence was highest (92%) among the individuals above 50 years of age followed by 18-49 years of age (90%) and 82% in below 18 years of age. The residents who were vaccinated with Covishield produced slightly higher antibodies (95%) than Covaxin (93%). In Delhi,one dose or two doses vaccination could increase seroprevalence to 95% that was the maximum which vaccination could generate. The unvaccinated population had seroprevalence of 85%. A maximum enhancement of 13% seroprevalence was recorded upon vaccination with one or two doses.

The vaccine breakthrough infections (BTI) among healthcare workers (HCWs) in various hospital networks in India have been studied. Apollo Hospital, PGIMER, Medanta, Civil Hospital, Park Hospital, Max, Fortis, Narayana, and Maulana Azad hospitals were included in the study. During the Delta variant surge, the BTI were reported in the range of 6-25%. By comparing the BTI rates during the severe Delta variant wave and less severe Original (Wuhan, Alpha, Beta, and Gamma) strain wave, it was worked out that the BTI were more among HCWs who were exposed to high viral load. In the initial days of the pandemic in the first wave when the total active cases were less, the rates of BTI were in the range of 1.6- 2.6%. When the viral load increased in the second deadly Delta wave, the BTI rose to 6-25%. The real-time data collected has established that the vaccination gave a protective shield against the novel coronavirus infection. Though the BTI were reported but the severity of the infection remained mild with a low hospitalisation rate and oxygen support requirement. No BTI infected HCWs succumbed to the SARS-CoV-2 infections.