Estimation of secondary household attack rates for emergent spike L452R SARS-CoV-2 variants detected by genomic surveillance at a community-based testing site in San Francisco

James Peng, Jamin Liu, Sabrina A Mann, Anthea M Mitchell, Matthew T Laurie, Sara Sunshine, Genay Pilarowski, Patrick Ayscue, Amy Kistler, Manu Vanaerschot, Lucy M Li, Aaron McGeever, Eric D Chow, Carina Marquez, Robert Nakamura, Luis Rubio, Gabriel Chamie, Diane Jones, Jon Jacobo, Susana Rojas, Susy Rojas, Valerie Tulier-Laiwa, Douglas Black, Jackie Martinez, Jamie Naso, Joshua Schwab, Maya Petersen, Diane Havlir, Joseph DeRisi, IDseq Team

Clinical Infectious Diseases
March 31, 2021

This study analysed the relative transmissibility (secondary household attack rate) of new “California” or “West Coast” variants.


Background: Sequencing of the SARS-CoV-2 viral genome from patient samples is an important epidemiological tool for monitoring and responding to the pandemic, including the emergence of new mutations in specific communities.

Methods: SARS-CoV-2 genomic sequences were generated from positive samples collected, along with epidemiological metadata, at a walk-up, rapid testing site in the Mission District of San Francisco, California during November 22-December 1, 2020 and January 10-29, 2021. Secondary household attack rates and mean sample viral load were estimated and compared across observed variants.

Results: A total of 12,124 tests were performed yielding 1,099 positives. From these, 928 high quality genomes were generated. Certain viral lineages bearing spike mutations, defined in part by L452R, S13I, and W152C, comprised 54.4% of the total sequences from January, compared to 15.7% in November. Household contacts exposed to the "California" or "West Coast" variants (B.1.427 and B.1.429) were at higher risk of infection compared to household contacts exposed to lineages lacking these variants (0.36 vs 0.29, RR=1.28; 95% CI:1.00-1.64). The reproductive number was estimated to be modestly higher than other lineages spreading in California during the second half of 2020. Viral loads were similar among persons infected with West Coast versus non-West Coast strains, as was the proportion of individuals with symptoms (60.9% vs 64.3%).

Conclusions: The increase in prevalence, relative household attack rates, and reproductive number are consistent with a modest transmissibility increase of the West Coast variants.

Featured Fellows

Maya Petersen

School of Public Health, UC Berkeley
Faculty Affiliate