Household Transmission of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Alpha Variant-United States, 2021

Abstract

Background: In Spring 2021, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) B.1.1.7 (Alpha) became the predominant variant in the United States. Research suggests that Alpha has increased transmissibility compared with non-Alpha lineages. We estimated household secondary infection risk (SIR), assessed characteristics associated with transmission, and compared symptoms of persons with Alpha and non-Alpha infections.

Methods: We followed households with SARS-CoV-2 infection for 2 weeks in San Diego County and metropolitan Denver, January to April 2021. We collected epidemiologic information and biospecimens for serology, reverse transcription-polymerase chain reaction (RT-PCR), and whole-genome sequencing. We stratified SIR and symptoms by lineage and identified characteristics associated with transmission using generalized estimating equations.

Results: We investigated 127 households with 322 household contacts; 72 households (56.7%) had member(s) with secondary infections. SIRs were not significantly higher for Alpha (61.0% [95% confidence interval, 52.4-69.0%]) than non-Alpha (55.6% [44.7-65.9%], P = .49). In households with Alpha, persons who identified as Asian or Hispanic/Latino had significantly higher SIRs than those who identified as White (P = .01 and .03, respectively). Close contact (eg, kissing, hugging) with primary cases was associated with increased transmission for all lineages. Persons with Alpha infection were more likely to report constitutional symptoms than persons with non-Alpha (86.9% vs 76.8%, P = .05).

Conclusions: Household SIRs were similar for Alpha and non-Alpha. Comparable SIRs may be due to saturation of transmission risk in households due to extensive close contact, or true lack of difference in transmission rates. Avoiding close contact within households may reduce SARS-CoV-2 transmission for all lineages among household members.

Keywords: Alpha; COVID-19; SARS-CoV-2; household; transmission.

Figure 1.

Household recruitment, enrollment, and exclusion. Enrollment of households for the investigation began with individuals being reported to public health with a positive SARS-CoV-2 RT-PCR test. Initial eligibility criteria for households included a positive SARS-CoV-2 test ≤10 days prior to contact for the first reported case in a household (index case). After enrollment was complete, 151 households agreed to the investigation. Following completion of the investigation, 24 households were excluded from analyses. Of excluded households, 12 households had co–primary cases, 10 households had a primary case with an illness onset date >10 days, and 2 households were lost to follow-up. Abbreviations: RT-PCR, reverse transcription–polymerase chain reaction; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Figure 2.

A, SIRs and 95% CIs in unvaccinated household contacts without history of previous SARS-CoV-2 infection, stratified by household lineage group. The overall group is a combination of households with Alpha infections, non-Alpha infections, and infections of unknown lineage. There was no significant difference found between unstratified SIRs in households with only Alpha infections and households with only non-Alpha infections (P = .49). B, SIRs and 95% CIs for subgroups of household contacts stratified by characteristics of household primary cases. There were no significant differences in SIRs between Alpha and non-Alpha household lineage groups within adult (P  = .55) or child (P = .85) categories and there were no significant differences in SIRs for child versus adult primary cases within the Alpha lineage group (P = .34) and the non-Alpha lineage group (P = .53). C, SIRs and 95% CIs for subgroups of household contacts stratified by the relationship of household contacts to the primary case. There were no significant differences in SIRs between Alpha and non-Alpha household lineage groups within any relationship category (see Supplementary Figure 4) and there were no significant differences between relationship categories within lineage groups (see Supplementary Figure 4). The other category for relationship to primary case includes extended family, work colleagues, and significant others. D, SIRs and 95% CIs for subgroups of household contacts stratified by demographic characteristics of household contacts. When comparing SIRs within the same lineage group across demographic characteristics, the Hispanic/Latino and Asian race/ethnicity categories both had significantly higher SIRs than the White race/ethnicity categories for Alpha infections (Hispanic/Latino P = .03, Asian P = .01). Corresponding values for Figure 2 can be found in Supplementary Table 7. Abbreviations: CI, confidence interval; SIR, secondary infection risk.