SARS-CoV-2 Omicron variant rapidly takes over Delta viral strain in Norwegian households

In a recent pre-print study posted to the Research Square* server and currently under review at a Nature Portfolio journal, researchers compared the secondary attack rates (SARs) of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron and Delta variants of concern (VoCs) in Norwegian households.

The rapid and massive global increase in coronavirus disease 2019 (COVID-19) cases due to the emergence of the new SARS-CoV-2 Omicron strain, in fully vaccinated Norwegian individuals, warrants an assessment of its transmission capabilities to enable the development of protective health strategies, policies, and site-targeted therapeutic measures for the prevention and control of Omicron-induced COVID-19.

To this end, the authors of this study conducted a national cohort study to compare SARs of the new Omicron variant against the earlier reported Delta variant to gain insights into the higher Omicron-induced viral transmission in Norway.

About the study

In the present study, data of Norwegian male and female residents who tested positive for COVID-19 between December 14, 2021, and January 23, 2022, was obtained from the National Beredt C19 register via contact tracing method, inclusive of their vaccination status: booster vaccinated, full vaccinated, partially vaccinated, or unvaccinated. The Beredt C19 register comprised information from several national municipalities, registries, and counties of Norway about the COVID-19-positive resident demographic and personal details, testing date, causative variant, and vaccination status. SARS-CoV-2 exposure details were also obtained from household contacts based on personal interviews.

This study included two to six-member households with contacts registered under 64 Norwegian municipalities. The first individual who tested positive for COVID-19 caused due to SARS-CoV-2 Delta or Omicron VoCs based on diagnosis by genomic sequencing or polymerase chain reaction (PCR) was considered as a primary case. Secondary household contacts were identified based on matching their identification numbers with those of primary cases. Institutions, healthcare facilities, and households with individuals of multiple generations were excluded from the study. Six primary case individuals and 36 secondary household contacts with previously diagnosed COVID-19 infections were also excluded.

Results and discussion

A total of 1,122 Omicron- (59%) or Delta- (41%) variant-induced primary COVID-19 infections and 2,169 secondary household contacts for the Delta (40%) and Omicron (60%) cases were identified. Overall, the 10-day SARs were higher for the Omicron variant (51%) compared to the Delta variant (36%) across all vaccination groups. Individuals vaccinated with booster doses showed reduced SARs for the Omicron (46%) as well as the Delta (11%) VOCs. However, vaccine effectiveness (VE) for booster-vaccinated individuals was also lower against the Omicron (45%) variant compared to the Delta (65%) variant.

The overall protection against acquiring infection in vaccinated groups was 42% and 27% for the Delta and Omicron VOCs, respectively. However, fully vaccinated, booster vaccinated, and unvaccinated primary cases were at equal risk of transmitting Omicron-induced COVID-19 infections to their household contacts, although this risk was 80% lower for the booster-vaccinated group against the Delta variant. These findings indicate substantially higher transmissibility and risk of acquiring COVID-19 infections with the Omicron variant compared to the Delta strain and definite protection against infection by booster doses in Norwegian households.

Higher SARs seen in households could be due to repeated and prolonged exposure in homes. The high transmissibility and infectivity rates of the Omicron variant are due to multiple mutations in the viral spike (S) glycoproteins in the molecular structure that enhance viral protein binding with human angiotensin-converting enzyme 2 (ACE2) receptors thereby facilitating the evasion of protective host immune responses. Although the Omicron genetically mutated variant is highly transmissible, it causes less severe disease with lower associated hospitalizations and deaths.

Conclusion

The present study results indicate that the Omicron variant was more transmissible than the Delta VoC among all vaccination groups. Vaccines confer definitive protection against COVID-19 infections, with the least risk of infection observed in booster-vaccinated individuals. Thus, although viral transmission is inevitable, individuals must be booster vaccinated to decrease the risk of acquiring COVID-19 infections.

Future studies with age-adjusted and vaccination time-adjusted data, representative of nationwide municipalities and counties, inclusive of non-household close social contacts, and diagnosed and registered with uniform procedures, are required to develop more standardized and protective healthcare measures, targeted against specific immune pathways to improve the standard of care for COVID-19 patients.  

*Important notice

Research Square publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Susha Cheriyedath

Susha has a Bachelor of Science (B.Sc.) degree in Chemistry and Master of Science (M.Sc) degree in Biochemistry from the University of Calicut, India. She always had a keen interest in medical and health science. As part of her masters degree, she specialized in Biochemistry, with an emphasis on Microbiology, Physiology, Biotechnology, and Nutrition. In her spare time, she loves to cook up a storm in the kitchen with her super-messy baking experiments.