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Since early in the COVID-19 pandemic, demonstrated that household transmission was a key driver in the spread of SARS-CoV-2. However, nearly 5 years later, less is known about more nuanced dynamics of SARS-CoV-2 viral transmission in households, particularly in the setting of evolving variants and pre-existing immunity.

Two studies presented at this year's IDWeek annual meeting focused on leveraging genomic analysis to gain a better understanding of whether COVID infections in households represent transmission between household members or the introduction of new infections.

Household density, vaccine status of exposed household members, and individual immunologic factors are all associated with the spread of SARS-CoV-2 among households, said Kathryn Stephenson, MD, MPH, of Harvard Medical School in Boston, during a presentation.

During the Alpha wave, the secondary attack rate for household contacts -- the risk of transmission of SARS-CoV-2 stemming from a household index infection -- was about 36%, dropping to approximately 30% during the Delta wave, but then rising to about 43% during the early Omicron period, as reported in a .

Stephenson and her colleagues conducted a small prospective study in the greater Boston area during 2022, enrolling 38 households with a positive SARS-CoV-2 index case, identified on either rapid diagnostic testing or PCR testing.

Over the next 2 weeks after enrollment, 58% of 38 households had a second case develop among household contacts, and, overall, 39% of 66 household contacts tested positive over that time period.

However, in a strict subanalysis in which researchers excluded index cases that were PCR-negative on day 1 or contacts that tested positive on day 1, the secondary attack rate fell to 22.5%. Furthermore, after performing genomic analysis to identify SARS-CoV-2 lineages with different genetic makeup that could not be caused by infection from the index case, that percentage dropped to 18.4%.

The finding indicated that although the most common source of infection was the index case, there were multiple sources of infection in over half of individual households, Stephenson said. Other sources for those infections likely included new cases introduced from outside the household, shared initial exposures with the index case, and other already infected contacts within the home.

In the larger, ongoing prospective , Amanda Casto, MD, PhD, of the University of Washington in Seattle, and colleagues also used genomic analysis to look at the frequency of simultaneous circulation of multiple viral strains in households.

The investigators analyzed genetic data from weekly nasal swabs from children and adults ages 6 to 49 years in households in Seattle and Portland, Oregon between 2022 and 2024.

Of 1,103 households, 67% had at least one SARS-CoV-2 case, and 24% of households had at least one episode in which two or more participants tested positive. Of these, 7% of episodes involved more than one viral lineage, suggesting multiple introduction events.

"Before the widespread availability of viral genome sequence data, chronologically clustered cases in households were often presumed to be all secondary to intra-household transmission," Casto commented. "Now that genetic data are being integrated into a number of epidemiologic studies, there have been a number of observations of the simultaneous circulation of multiple viral lineages in households within a short time."

Of note, Casto and her colleagues found no association with certain household characteristics, including number of children or people in the household, or household income, which were associated with intra-household transmission.

When more than one viral lineage was present, the investigators also found no association with the presence of any children or children under 5 years of age in the household or with children attending daycare. Nor was there any association with location (i.e., households in Oregon vs Washington), any previous household illness episodes, or households living in apartments versus single-family homes.

What can we take away from these two studies? Household contacts of a person newly diagnosed with COVID-19 are certainly at high risk for SARS-CoV-2 infection in the following weeks. However, this may not only be due to heightened risk of transmission between household members, but also -- perhaps not so surprisingly -- to an overall increase in SARS-CoV-2 transmission in the community where different strains may be circulating.

In other words, "infection within the household can serve as a proxy for broader risk for community transmission, emphasizing importance of public health measures inside and outside the home," Stephenson said.

Casto noted that "in the long run, [genomic data] will really help us refine our understanding of the conditions under which intra-household transmission of respiratory viruses occurs and does not occur."

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