# Overestimation of Severe Acute Respiratory Syndrome Coronavirus 2 Household Transmission in Settings of High Community Transmission: Insights From an Informal Settlement Community in Salvador, Brazil

## TL;DR

This study shows that household transmission of SARS-CoV-2 Omicron may be overestimated due to high community transmission in a Brazilian informal settlement.

## Contribution

The study provides new insights into the relative roles of household versus community transmission of SARS-CoV-2 in high-transmission settings.

## Key findings

- The Omicron BA.1* variant was responsible for 97.1% of sequenced SARS-CoV-2 cases.
- After excluding community-acquired cases, the household secondary attack rate was 24.2%.
- Phylogenetic analysis revealed multiple virus introductions and high incidence rates in the community.

## Abstract

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant has spread globally. However, the contribution of community versus household transmission to the overall risk of infection remains unclear.

Between November 2021 and March 2022, we conducted an active case-finding study in an urban informal settlement with biweekly visits across 1174 households with 3364 residents. Individuals displaying coronavirus disease 2019 (COVID-19)–related symptoms were identified, interviewed along with household contacts, and defined as index and secondary cases based on reverse-transcription polymerase chain reaction (RT-PCR) and symptom onset.

In 61 households, we detected a total of 94 RT-PCR–positive cases. Of 69 sequenced samples, 67 cases (97.1%) were attributed to the Omicron BA.1* variant. Among 35 of their households, the secondary attack rate was 50.0% (95% confidence interval [CI], 37.0%–63.0%). Women (relative risk [RR], 1.6 [95% CI, .9–2.7]), older individuals (median difference, 15 [95% CI, 2–21] years), and those reporting symptoms (RR, 1.73 [95% CI, 1.0–3.0]) had a significantly increased risk for SARS-CoV-2 secondary infection. Genomic analysis revealed substantial acquisition of viruses from the community even among households with other SARS-CoV-2 infections. After excluding community acquisition, we estimated a household secondary attack rate of 24.2% (95% CI, 11.9%–40.9%).

These findings underscore the ongoing risk of community acquisition of SARS-CoV-2 among households with current infections. The observed high attack rate necessitates swift booster vaccination, rapid testing availability, and therapeutic options to mitigate the severe outcomes of COVID-19.

We found a high level of transmissibility of the SARS-CoV-2 BA.1* Omicron variant both within the community and households. Phylogenetic analysis suggests a diverse set of viruses were transmitted within the community and households, consistent with multiple introductions and high rates of incidence.