Increased transmissibility of the alpha SARS-CoV-2 variant: evidence from contact tracing data in Oslo, January to February 2021
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Abstract
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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This study, available as a preprint and thus not yet peer reviewed, sought to identify the differences in transmissibility of the B.1.1.7 SARS-CoV-2 variant compared to non-B.1.1.7 variants.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 …
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This study, available as a preprint and thus not yet peer reviewed, sought to identify the differences in transmissibility of the B.1.1.7 SARS-CoV-2 variant compared to non-B.1.1.7 variants.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.
Limitations
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.
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SciScore for 10.1101/2021.03.29.21254122: (What is this?)
Please note, not all rigor criteria are appropriate for all manuscripts.
Table 1: Rigor
Institutional Review Board Statement not detected. Randomization not detected. Blinding not detected. Power Analysis not detected. Sex as a biological variable not detected. Table 2: Resources
No key resources detected.
Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).
Results from LimitationRecognizer: We detected the following sentences addressing limitations in the study:The data are subject to several limitations. We cannot distinguish between the case infecting the contact or vice versa. One contact could be the contact of several cases. There can be undetected cases: close contacts who are …
SciScore for 10.1101/2021.03.29.21254122: (What is this?)
Please note, not all rigor criteria are appropriate for all manuscripts.
Table 1: Rigor
Institutional Review Board Statement not detected. Randomization not detected. Blinding not detected. Power Analysis not detected. Sex as a biological variable not detected. Table 2: Resources
No key resources detected.
Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).
Results from LimitationRecognizer: We detected the following sentences addressing limitations in the study:The data are subject to several limitations. We cannot distinguish between the case infecting the contact or vice versa. One contact could be the contact of several cases. There can be undetected cases: close contacts who are not registered, infected contacts that escape detection, and infected contacts who have not yet developed the infection (censored). Some non-positive contacts could be immune due to a previous infection. However, we do not expect any of these problems to occur often for some lineages than others, and hence the relative estimates should be less affected by these sources of bias. Besides, there are potential sources of bias in the contact tracing data, which could affect our estimates. Even though the definition of close contact and contact tracing instructions were the same for the different lineages, there were likely differences in how the tracing was performed in practice, with an increased focus on tracing and testing contacts of cases infected with the B.1.1.7 variant. Due to the increasing circulation of B.1.1.7 in Oslo, the TICQ (Test-Isolation-Contact tracing-Quarantine) intervention was strengthened. In addition to the general requirement of quarantine for all close contacts of confirmed SARSCov-2 cases, since 5 February, all close contacts were tested at the beginning and end of their quarantine and household members of close contacts of cases confirmed with lineage B.1.1.7 were asked to quarantine until the contact was confirmed negative. Since...
Results from TrialIdentifier: No clinical trial numbers were referenced.
Results from Barzooka: We did not find any issues relating to the usage of bar graphs.
Results from JetFighter: We did not find any issues relating to colormaps.
Results from rtransparent:- Thank you for including a conflict of interest statement. Authors are encouraged to include this statement when submitting to a journal.
- Thank you for including a funding statement. Authors are encouraged to include this statement when submitting to a journal.
- No protocol registration statement was detected.
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