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. 2022 Aug 5;40(33):4845-4855.
doi: 10.1016/j.vaccine.2022.06.066. Epub 2022 Jul 4.

SARS-CoV-2 infection risk among vaccinated and unvaccinated household members during the Alpha variant surge - Denver, Colorado, and San Diego, California, January-April 2021

David W McCormick  1 , Stacey L Konkle  2 , Reed Magleby  2 , Ayan K Chakrabarti  3 , Blake Cherney  3 , Kristine Lindell  3 , Apophia Namageyo-Funa  3 , Susanna Visser  3 , Raymond A Soto  2 , Marisa A P Donnelly  2 , Ginger Stringer  4 , Brett Austin  5 , Mark E Beatty  5 , Sarah Stous  5 , Bernadette A Albanese  6 , Victoria T Chu  2 , Meagan Chuey  7 , Elizabeth A Dietrich  3 , Jan Drobeniuc  3 , Jennifer M Folster  3 , Marie E Killerby  3 , Jennifer A Lehman  3 , Eric C McDonald  5 , Jasmine Ruffin  3 , Noah G Schwartz  2 , Sarah W Sheldon  3 , Sadia Sleweon  3 , Natalie J Thornburg  3 , Laura J Hughes  3 , Marla Petway  3 , Suxiang Tong  3 , Melissa J Whaley  3 , Hannah L Kirking  3 , Jacqueline E Tate  3 , Christopher H Hsu  3 , Almea Matanock  3 ; COVID-19 Household Transmission Team
Affiliations

SARS-CoV-2 infection risk among vaccinated and unvaccinated household members during the Alpha variant surge - Denver, Colorado, and San Diego, California, January-April 2021

David W McCormick et al. Vaccine. .

Abstract

Background: COVID-19 vaccination reduces SARS-CoV-2 infection and transmission. However, evidence is emerging on the degree of protection across variants and in high-transmission settings. To better understand the protection afforded by vaccination specifically in a high-transmission setting, we examined household transmission of SARS-CoV-2 during a period of high community incidence with predominant SARS-CoV-2 B.1.1.7 (Alpha) variant, among vaccinated and unvaccinated contacts.

Methods: We conducted a household transmission investigation in San Diego County, California, and Denver, Colorado, during January-April 2021. Households were enrolled if they had at least one person with documented SARS-CoV-2 infection. We collected nasopharyngeal swabs, blood, demographic information, and vaccination history from all consenting household members. We compared infection risks (IRs), RT-PCR cycle threshold values, SARS-CoV-2 culture results, and antibody statuses among vaccinated and unvaccinated household contacts.

Results: We enrolled 493 individuals from 138 households. The SARS-CoV-2 variant was identified from 121/138 households (88%). The most common variants were Alpha (75/121, 62%) and Epsilon (19/121, 16%). There were no households with discordant lineages among household members. One fully vaccinated secondary case was symptomatic (13%); the other 5 were asymptomatic (87%). Among unvaccinated secondary cases, 105/108 (97%) were symptomatic. Among 127 households with a single primary case, the IR for household contacts was 45% (146/322; 95% Confidence Interval [CI] 40-51%). The observed IR was higher in unvaccinated (130/257, 49%, 95% CI 45-57%) than fully vaccinated contacts (6/26, 23%, 95% CI 11-42%). A lower proportion of households with a fully vaccinated primary case had secondary cases (1/5, 20%) than households with an unvaccinated primary case (66/108, 62%).

Conclusions: Although SARS-CoV-2 infections in vaccinated household contacts were reported in this high transmission setting, full vaccination protected against SARS-CoV-2 infection. These findings further support the protective effect of COVID-19 vaccination and highlight the need for ongoing vaccination among eligible persons.

Keywords: COVID-19; Household transmission; SARS-CoV-2; Vaccination; Vaccine effectiveness.

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Conflict of interest statement

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Fig. 1
Fig. 1
Investigation flow diagram showing convenience sample enrollment and exclusion criteria, California and Colorado, January-April 2021.
Fig. 2
Fig. 2
Diagram showing classification of persons as recently, partially, or fully vaccinated by vaccine type.
Fig. 3
Fig. 3
Vaccination status for California residents and persons enrolled from California (Panel A) and Colorado (Panel B) as a proportion of the general population (upper chart) and number of persons in the investigation population (bottom chart).
Fig. 4
Fig. 4
Variant status for enrolled households over time stratified by location. The X-axis shows the week of enrollment.
See this image and copyright information in PMC

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