Higher B.1.1.7 transmission in households than other SARS-CoV-2 variants
Contact tracing data from a new preprint study on the medRxiv* preprint server provides further evidence that the B.1.1.7 variant — first reported in the United Kingdom — has a high transmission rate. The research, led by Birgitte Freiseleben de Blasio from the Norwegian Institute of Public Health and the University of Oslo in Norway, estimates B.1.1.7 has a 60% greater transmission within households than other severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern.
The researchers write:
Our study suggests that households are major locations for rapid transmission of the lineage B.1.1.7. Therefore, lowering the risk of spread within the families is pivotal to controlling the COVID-19 pandemic with new and more transmissible lineages.”
How they did it
The researchers used data from the contact tracing database PasInfo to find primary positive cases for COVID-19 infection from January 4 to February 28. They used whole-genome sequencing to verify the presence of the B.1.1.7 variant. Primary cases also had to have been infected by a person outside their household.
The team evaluated secondary cases to determine increased coronavirus transmission from B.1.1.7 in Norway. Close contacts were someone with a direct physical or close contact for more than 15 minutes and over 2 meters within 48 hours of the primary case’s symptom onset or time of test for asymptomatic cases. They compared the number of cases from close contacts infected from a person with the B.1.1.7 variant to the number of secondary cases from other SARS-CoV-2 variants.
About 45 cases were excluded from the data analysis leaving a total of 415 primary cases and 2,718 close contacts. About 368 of the 415 tested positive for the COVID-19.
Prevalence of the B.1.1.7 variant in homes
Data results showed 146 of the 415 cases were infected with the B.1.1.7 variant, and the other 269 were infected with different lineages. About 251 cases were in close contact with at least one person in their household. Of the 251, 85 had the B.1.1.7 variant.
People infected with the B.1.1.7 variant had an average of 6.83 contacts with an average of 1.01 secondary infections. Within a household, the number of close contacts was about 1.46, with a 0.61 rate of secondary infections.
Results were low with other variants. People had an average of 6.4 contacts, with about .82 reporting a secondary infection. In the household, people had about 1.36 close contact with an average of .36 secondary infections. The researchers note the high number of close contacts in people over the age of 19 was more likely due to cohorting and school class testing, making the number “overdispersed.”
When the researchers adjusted the number of secondary infections by age, they found it did not change the estimates. The household attack rate for infection was 0.27 for non-B.1.1.7 lineages and 0.42 for B.1.1.7 lineages. B.1.1.7’s reproduction number was 24% greater than other variants. The team estimates the B.1.1.7 lineage to be 16% more transmissible than other variants.
Reason behind decreased transmission outside households
The researchers note B.1.1.7’s transmission rate reduced for secondary infections outside the household. They provide two potential explanations:
“There could be bias in the data when considering all close contacts. By restricting our analysis to contacts within the household, we control for potential selection bias of contacts introduced by the contact tracing, as contacts within households are more likely to be complete in any case and thus comparable across variants. Another possibility is that there is very little difference in the transmissibility for contacts outside households because of stringent and efficient distancing interventions.”
Although the estimates did not vary when the team controlled for age, they strongly suggest overall increases in B.1.1.7 transmission across all age groups.
Study limitations
The researchers acknowledge there’s no way to disprove the possibility that the close contact could have infected the person with the ‘primary infection’ or that one close contact could have been the primary infection for multiple infections. Additionally, people may have been infected but did not report or test for SARS-CoV-2. There’s also the off-chance that close contacts who did not test positive had gained immunity from a prior COVID-19 infection.
*Important Notice
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.
