1943. Effect of Vaccination on Household Transmission of SARS-CoV-2
Authors: Burns Julianne E , Altamirano Jonathan , Salinas Jorge , Sarnquist Clea , Behl Rasika , Thiessen Rosita , Chun Leanne X , Tam Grace K , Lopez Marcela , Stainton Emma , Carrington Yuan J , Maldonado Yvonne A 
Published: December 2022

Abstract

Background
Understanding how COVID-19 vaccination affects transmission of SARS-CoV-2 within households may affect policy and healthcare decisions. We hypothesized that vaccination reduces transmission and viral load in vaccinated household members.

Methods
We prospectively enrolled participants during March 2020 – October 2021. Index cases (IC) were eligible if they tested positive for SARS-CoV-2 within the previous 10 days and did not have household contacts (HC) who had tested positive or had symptoms of COVID-19. Participants self-collected anterior nares swabs daily for SARS-CoV-2 RT-PCR for at least 21 days, or once every member of the household had 7 consecutive negative tests. Baseline data included demographics and self-reported vaccination status. Complete COVID-19 vaccination was defined as receiving 2 doses of Moderna/Pfizer or 1 dose of Johnson & Johnson vaccine, and incomplete vaccination as receiving 1 dose of Moderna/Pfizer vaccine. Household transmission was analyzed via STATA 14.2 using logistic regression with robust standard error clustered by household, and SARS-CoV-2 cycle threshold was graphed by day of study enrollment using lowess smoothing.

Results
There were 60 households with positive ICs and 103 HCs for a total of 163 participants. ICs had median age 41.5 years (range 1–86) with 9 (18.0%) &lt; 18 years. HCs had median age 34 years (range 0–87) with 32 (31.1%) HCs &lt; 18 years. Overall, 33 (20.2%) participants received at least one COVID-19 vaccine dose. A total of 50 households had at least one HC (median 2, max 7). Transmission of SARS-CoV-2 occurred in 45 HCs (43.7%). Odds of SARS-CoV-2 transmission was lower in HCs who were vaccinated prior to study enrollment, though this finding was not statistically significant (Table 1). There were 507 positive SARS-CoV-2 tests collected among 74 participants (Figure 1). Table 1.Household Transmission By COVID-19 Vaccination Status Prior to Study Enrollment (among households with ≥ one household contact)*Using logistic regression with robust standard error, clustered by household. Reference is top group listed in each comparison (includes unvaccinated).CI: Confidence IntervalCompletely vaccinated: Received primary COVID-19 vaccination series (2 doses Pfizer or Moderna vaccine, or 1 dose Johnson & Johnson vaccine) prior to enrollmentIncompletely vaccinated: Received of 1 dose of Pfizer or Moderna vaccine prior to enrollmentUnvaccinated: No COVID-19 vaccines prior to enrollmentFigure 1.SARS-CoV-2 Cycle Threshold Over Time by COVID-19 Vaccination Status
Completely vaccinated: Received primary COVID-19 vaccination series (2 doses Pfizer or Moderna vaccine, or 1 dose Johnson & Johnson vaccine) prior to enrollment

Conclusion
Vaccination of HCs may be protective against household SARS-CoV-2 transmission. However, analyses were limited due to low numbers of vaccinated study participants. Study enrollment is ongoing, and future analyses will include transmission during the 2022 Omicron surge, and daily symptom data which has been collected.

Disclosures
Yvonne A. Maldonado, MD, Pfizer: Grant/Research Support|Pfizer: Member, DSMB, Pfizer Meningococcal Vaccine clinical trial.