In a recent study posted to the medRxiv* preprint server, a team of researchers from Norway used contact tracing data to measure and compare the household secondary attack rate (SAR) of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron and Delta variants at a time of prominence of both variants in the country.
By the end of 2021, the Omicron variant of concern (VoC) (B.1.1.529 BA.1) had rapidly spread across the globe. In Norway, the Omicron variant took over the Delta variant and by the second week of January 2022, it was detected in >90% of weekly screened national samples.
The immune evasion by the Omicron variant is an important contributor to the high transmissibility seen with this variant. Due to the highly evasive nature of this VoC, there is a need for relevant and timely measures to understand the transmissibility and risk of infection at the population level by the Omicron variant.
In this research work, the researchers conducted a cohort study between December 2021 and January 2022, using data collected from the coronavirus disease 2019 (COVID-19) pandemic preparedness register of the Norwegian government, called Beredt C19. This registry collected information regarding SARS-CoV-2-positive Norwegian residents, testing dates, detection of variants, records of vaccinations, and demographic features.
The register received contact tracing data digitally from Norwegian municipalities aiding the linking of index cases with traced contacts. The study population consisted of registered households and the primary cases were defined as the first person in the households who were SARS-CoV-2 positive either with the Omicron or Delta variant during the study period.
The researchers observed that in households, the overall SAR was higher with the SARS-CoV-2 Omicron variant, 41% (CI95: 38-44) as compared with the Delta variant 35% (CI95: 31-38). In primary cases of children younger than 16 years, there was no significant difference in SAR between the Omicron and Delta variants. Similarly, no significant difference in SAR was observed between the Omicron and Delta variants in the households with primary cases of 40% vs 35% in non-vaccinated, and 47% vs 45% in contacts.
In households with fully vaccinated contacts, the risk of infection was higher with the Omicron variant (RR: 1.37; CI95: 1.15- 1.64) relative to the Delta variant, while a significant difference was not observed in contacts vaccinated with the booster. SAR was lower in households with cases of booster-vaccinated cases and contacts, compared to households with non-vaccinated cases and contacts. In the household contacts, there was a higher risk of SARS-CoV-2 transmission (RR: 5.75; CI95: 1.32-100) with the Omicron variant in the booster-vaccinated primary cases as compared to the Delta variant, with a lower relative risk (RR: 1.18; CI95: 1.04-1.46) in fully vaccinated primary cases.
In adult contacts with a booster vaccination, vaccine effectiveness (VE) was lower for the Omicron variant (47%) as compared to the Delta variant (62%), but it was higher for fully vaccinated cases. In the fully vaccinated cases, protection against infection with the Omicron variant was 16%, while with the Delta variant it was 46%.
While measuring the risk of infection, the researchers observed no significant differences as stratified by gender, age group, and time since the last dose in fully vaccinated cases.
The study findings indicated that a high SAR in household contacts cases infected with the SARS-CoV-2 Omicron variant in Norway is due to the lower effectiveness of the vaccines and not attributable to the high transmissibility of the Omicron variant. The results of the study further suggested that booster vaccination has a limited effect in preventing the transmission of the SARS-CoV-2 Omicron variant.
medRxiv 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.
- Neda Jalali, Hilde Kjelgaard Brustad, Arnoldo Frigessi, Emily Ann MacDonald, Hinta Meijerink, Siri Laura Feruglio, Karin Maria Nygård, Gunnar Øivind Isaksson Rø, Elisabeth Henie Madslien, Birgitte Freiesleben de Blasio. (2022) Increased household transmission and immune escape of the SARS-CoV-2 Omicron variant compared to the Delta variant: evidence from Norwegian contact tracing and vaccination data. medRxiv. doi: https://doi.org/10.1101/2022.02.07.22270437 https://www.medrxiv.org/content/10.1101/2022.02.07.22270437v2