2080. Reduced risk of SARS-CoV-2 infection among household contacts with hybrid immunity from recent COVID-19 vaccination and SARS-CoV-2 infection
Authors: Rolfes Melissa A , Talbot H Keipp , Grace Morrissey Kerry , Stockwell Melissa , Maldonado Yvonne A , McLean Huong , Lutrick Karen , Bowman Natalie M , Rao Suchitra , Salvatore Phillip P , Izurieta Hector , Schmitz Jonathan , Zhu Yuwei , Hart Kimberly W , Battan-Wraith Steph , Merrill Lori S , McLaren Son H , Sano Ellen , Sarnquist Clea , Goodman Sara H , Petrie Joshua , Ledezma Karla I , Pryor Kathleen , Bullock Ayla , Yang Amy , Biddle Jessica E , Smith-Jeffcoat Sarah E , Johnson Sheroi , Asturias Edwin J , Lin Jessica T , Ellingson Katherine , Belongia Edward , Govindaranjan Prasanthi , Valdez de Romero Ana , Olivo Vanessa , Mellis Alexandra , Grijalva Carlos G 
Published: November 2023

Abstract

Background
COVID-19 vaccines reduce the risk of symptomatic SARS-CoV-2 infection, but it is unclear the extent to which vaccines or prior infection reduce the risk of infection in high transmission settings like households.

Methods
We screened individuals who tested positive for SARS-CoV-2 (index cases) recruited at 7 sentinel testing sites and through a nationwide effort during Sep 2021–Apr 2023. Index cases and their households (HH) were enrolled ≤6 days after the index case’s illness onset. Household contacts (HHC) had daily self-collected nasal swabs or saliva samples tested by RT-PCR for SARS-CoV-2. We determined COVID-19 vaccination status by plausible self-report (with date) or vaccination records, and prior SARS-CoV-2 infection by self-reported prior positive test (with year) or anti-nucleocapsid antibodies assessed at enrollment. We considered HHC with prior COVID-19 and ≥2 COVID-19 vaccine doses as having “hybrid immunity”, and assessed the effects of ≥2 vaccine doses, prior COVID-19, or hybrid immunity on the risk of PCR-confirmed SARS-CoV-2 infection among HHC by GEE Poisson regression adjusted for age of the HHC, recruitment strategy, household density (ppl/bedroom), and enrollment month.

Results
We included 1,324 HHC (Fig 1); 73% enrolled May–Nov 2022, when Omicron BA.4/5 predominated; 28% were aged &lt; 18 years (Table 1). Most (89%) had some immunity to COVID-19: 54% from vaccination only, 7% from infection only, and 26% from hybrid immunity. Most HHC without immunity to SARS-CoV-2 were children (64%). Of HHCs, 61% became SARS-CoV-2 positive during follow-up. In a model that accounted for all sources of immunity (figure 2), prior vaccination or prior infection alone did not provide significant protection, only HHC with hybrid immunity had significantly reduced risk of infection (adjusted relative risk: 0.80, 95% confidence interval: 0.68, 0.94; Fig 2). The risk of infection was lowest (43%) when the HHC’s last immunizing event (vaccination or infection) occurred ≤6 months before the index case’s illness onset.
Eligibility and inclusion of HH contacts in analysis of COVID-19 transmission
Figure 1
Table 1
Demographic characteristics of HH contacts by COVID-19 vaccination or prior COVID-19
Figure 2
Adjusted relative risk of infection among HH contacts by COVID-19 vaccination or prior COVID-19

Conclusion
The risk of SARS-CoV-2 infection among HHCs continues to be high. While vaccination alone was not effective at preventing SARS-CoV-2 infection, HHCs with hybrid immunity from recent vaccination or prior infection had the lowest risk of infection.

Disclosures
Yvonne A. Maldonado, MD, Pfizer: Grant/Research Support|Pfizer: Site Investigator, DSMB member Huong McLean, PhD, MPH, Seqirus: Grant/Research Support Suchitra Rao, MBBS, MSCS, Sequiris: Advisor/Consultant Joshua Petrie, PhD, CSL Seqirus: Grant/Research Support Edwin J. Asturias, MD, Hillevax: Advisor/Consultant|Moderna: Advisor/Consultant|Pfizer: Grant/Research Support Edward Belongia, MD, Seqirus: Grant/Research Support Carlos G. Grijalva, MD, MPH, AHRQ: Grant/Research Support|CDC: Grant/Research Support|FDA: Grant/Research Support|Merck: Advisor/Consultant|NIH: Grant/Research Support|Syneos Health: Grant/Research Support