This is not easy to quantify how contagious Omicron is. One of the world’s leading infectious disease experts, Dr. Antonio Fauci, says, “Omicron, with its extraordinary, unprecedented degree of efficiency of transmissibility, will ultimately find just about everybody” . On January 10, 2022, the U.S reported a record 1.35 million new coronavirus infections, almost five times higher than the peak points of all other waves . Almost similar patterns were observed in many countries in Europe. Such an unprecedented increase in cases might be attributed to the latest Covid variant Omicron, either due to its greater transmissibility or other factors.
In Norway, a study found that among 117 participants in a party, 74% got infected with Omicron. In the party, there was one participant from South Africa (the country where Omicron was first detected and later spread) . Among the partygoers, about 96% were fully vaccinated. The study concludes that Omicron is highly contagious even among vaccinated people.
Researchers in Denmark analyzed the transmission of Omicron and Delta variants among household members (11,937 households) and determined that Omicron was approximately 2.6 times (95%-CI: 2.34-2.90) to 3.7 times (95%-CI: 2.65-5.05) more infectious than the Delta variant among vaccinated and boosted individuals . However, among unvaccinated people, Omicron was only 1.17 times (95%-CI: 0.99-1.38) more infectious than the Delta, which was statistically insignificant.
Immune Evasion and Reinfection
The most significant matter about the new variant is whether it can evade immunity developed by vaccinations and previous infections. Several studies have been found in the literature that inspected the evasion ability of Omicron compared to other variants. Between November 30, 2021 and January 1, 2022, one study in the United States investigated clinical and epidemiologic data from cases testing positive for SARS-CoV-2 infection inside the Kaiser Permanente Southern California healthcare system. The study included 52,297 cases of SGTF (Omicron) infection and 16,982 cases of non-SGTF (Delta [B.1.617.2]) infection . The adjusted odds of having previously been infected with SARS-CoV-2 were 4.45 (95% CI: 3.24-6.12) fold greater in individuals with Omicron variant infections than in cases with Delta variant infections. Likewise, the adjusted odds of receiving any vaccine series (1, 2, or 3 doses of BNT162b2/mRNA-1973, or Ad.26.COV2.S with or without a booster dose of any vaccine) were significantly greater (2-6 times) in cases with Omicron variant infections than in cases with Delta variant infections.
Lyngse and his colleagues, in a large-scale study in Denmark, found that among vaccinated people, Omicron was 2.6-3.7 times more infectious than the Delta variant. However, they found no significant differene in transmissibility for the two variants among unvaccinated people . They summarize that the Omicron VOC's rapid spread can be attributed to its immune evasiveness, rather than an inherent increase in fundamental transmissibility.
Pullium et al. conducted a study in South Africa with routine surveillance data of 2,796,982 persons who tested positive for SARS-CoV-2 in a laboratory at least 90 days prior to 27 November 2021 to examine reinfection risk. The analysis found that, in contrast to the Beta and Delta, the Omicron variant of SARS-CoV-2 demonstrates substantial population-level evidence for evasion of immunity from prior infection .
Reduced neutralization of the Omicron variant has been seen in investigations using plasma specimens from persons who received the entire (two- or three-dose) mRNA vaccination series  and from patients who had previously been infected with SARS-CoV-2 . Besides, several early observational studies revealed that COVID-19 vaccinations were substantially less efficacious in preventing Omicron variant infection –.
Researchers from the University of Edinburgh, UK, found that the Omicron variant had a 10 times greater risk of reinfection than the Delta variant . Also, Brandal et al. demonstrate that Omicron has a more remarkable ability to evade immunity developed by vaccines. They showed 74% of people got infected in a party where 96% of party participants were fully vaccinated for SARS-CoV-2 .
The severity for Covid-19 is measured through the requirement of hospital admission, length of hospital stay, the requirement of ventilators, the time required to recover, mortality, etc. There is some unanimity regarding the severity of the new Omicron variant per the existing literature we have found. The Southern California study that included 52,297 cases of SGTF (Omicron) infection and 16,982 non-SGTF (Delta) infection reported a significantly decreased risk of severe clinical outcomes and shorter lengths of stay in the hospital . Hospitalizations of Omicron and Delta variant infections occurred in 235 (0.5%) and 222 (1.3%) cases, respectively. Throughout the follow-up period, zero instances of Omicron variant infection required mechanical ventilation, compared to 11 cases of Delta variant infection ( p<0.001). In addition, the median length of stay in the hospital was 3.4 (2.8-4.1) days shorter for hospitalized individuals with Omicron variant infections than for hospitalized cases with Delta variant infections, indicating a 70% (95% CI: 64.0-74.5%) reduction in hospital length of stay.
Researchers in South Africa assessed the clinical severity of the SARS-CoV-2 Omicron variant using nationwide data. After adjusting for confounding variables, they found that persons with Omicron (SGTF) infection had an 80% lower risk of hospitalization than those without Omicron infection (aOR 0.2, 95% CI: 0.1-0.3) . Moreover, compared to earlier Delta infections, after adjusting for risk factors for severe disease, those infected with Omicron had a 70% decreased risk of severe disease (aOR 0.3, 95% CI: 0.2-0.5).
The study in Scotland conducted by researchers from the University of Edinburgh indicates that when compared to Delta, Omicron is related with a two-thirds reduction in the probability of COVID-19 hospitalization . They also found that while vaccination provides the best protection against Delta, the third/booster dose provides significant protection against the risk of symptomatic COVID-19 infection in Omicron when compared to ≥25 weeks after the second vaccine dose .
Clinical characteristics of 466 patients infected with Omicron variant admitted to a large hospital in Tshwane, South Africa, were compared to 3962 hospital admissions from earlier waves. Deaths and ICU admissions were 4.5% vs 21.3% (p< 0.0001) and 1% vs 4.3% (p< 0.0001) for the Omicron and preceding waves, respectively; length of stay was 4.0 days vs 8.8 days .
A study conducted in England observed a reduction in the risk of hospitalization for Omicron infections when compared to Delta infections during the study period (Dec 1 to Dec 14, 2021). The magnitude of the reduction varied according to the inclusion criteria for cases and hospitalization, ranging from 20–25% when any hospitalization is used as the endpoint to 40–45% when hospitalizations lasting 1 day or more is used .
A retrospective cohort study (Omicron cohort and Delta cohort) was carried out using electronic health record (EHR) data from 577,938 first-time SARS-CoV-2 infection patients in the United States . After adjustment for demographics, socioeconomic determinants of health, comorbidities, medications, and immunization status, the 3-day risks in the Emergent Omicron cohort were consistently less than half of those in the Delta cohort: Emergency Department (ED) visit: 4.55% vs. 15.22% (risk ratio or RR: 0.30, 95% CI: 0.28-0.33); hospitalization: 1.75% vs. 3.95% (RR: 0.44, 95% CI: 0.38-0.52]); ICU admission: 0.26% vs. 0.78% (RR: 0.33, 95% CI:0.23-0.48); mechanical ventilation: 0.07% vs. 0.43% (RR: 0.16, 95% CI: 0.08-0.32) .