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Increased transmissibility of the B.1.1.7 SARS-CoV-2 variant: Evidence from contact tracing data in Oslo, January to February 2021

Our take —

This study, available as a preprint and thus not yet peer reviewed, sought to identify the differences in transmissibility of the B.1.1.7 SARS-CoV-2 variant compared to non-B.1.1.7 variants. They identified primary cases and secondary cases through contact tracing in Oslo, Norway, and found the relative risk of secondary household infection among B.1.1.7 variant-infected cases to be 60% greater than non-B.1.1.7 infected cases. The B.1.1.7 variant also had a higher reproductive number. They were not able to do genetic sequencing on secondary cases to confirm that they became infected from the index case, but these findings suggest that the B.1.1.7 variant has increased transmissibility among close household contacts.

Study design

Prospective Cohort

Study population and setting

The study objective was to determine if there was increased transmissibility of COVID-19 related to the B.1.1.7 variant in Oslo, Norway, compared to other variants. Using contact tracing data from PasInfo in the Oslo municipality from January 4 to February 28, 2021, they identified index cases infected with the B.1.1.7 variant and index cases infected with other lineages, and compared the number of secondary cases among identified close contacts. Close contacts were defined as having direct physical contact or being <2 meters in proximity for 15 minutes or more within 48 hours of symptom onset of the index case or time of positive COVID-19 test for the index case. Only primary cases with virus lineage test results from whole-genome sequencing or Sanger sequencing were eligible. Infections identified as having occurred outside the household were excluded. They used Poisson regression to estimate the number of secondary cases, adjusting for age group.

Summary of Main Findings

The study identified 415 index cases and 2,718 close contacts in total. Of these 2,718, 368 (13.5%) tested positive for SARS-CoV-2 infection. Among primary cases, 146 were infected with the B.1.1.7 lineage (35.2%) compared to 269 with other lineages (64.8%). Among those with B.1.1.7 infection, they had an average of 1.01 secondary infections and 6.83 close contacts. Among other variant-infected cases, they had an average of 6.4 close contacts and a secondary infection rate of 0.82. Within households, the secondary attack rate was estimated as 0.42 for the B.1.1.7 lineage and 0.27 for the non-B.1.1.7 lineages, leading to a relative risk among cases of B.1.1.7 lineage vs. other lineages of 1.60 (95% CI: 1.20 to 2.14). Additionally, the reproduction number for the B.1.1.7 lineage is estimated as 1.01 (95% CI: 0.86 to 1.19) compared to 0.82 (95% CI: 0.72 to 0.93) in other lineages.

Study Strengths

The study had a large number of sequenced samples that they were able to compare across different lineages. They also had contact tracing information, including area of exposure (e.g., household) which allowed them to identify a number of close contacts for their analyses.


One limitation is that the direction of infection (e.g., if an index case was infected by a secondary case or vice versa) could not be discerned, and they did not have information on the lineage of the secondary case and whether the B.1.1.7 variant was transmitted or whether some other lineage infection occurred due to some other exposure. It is challenging to discern which way this may bias results, based on whether individuals infected with a specific variant are more likely to have differences in exposure route or number of contacts. There is no significant evidence of different numbers of contacts based on lineages, however they are not likely powered to determine such a difference, and it cannot be ruled out.

Value added

The study provides real-world evidence of potential increased transmissibility based on the B.1.1.7 variant, which is on the rise in the population.

This review was posted on: 21 May 2021