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Risk of contracting COVID-19 during the Omicron surge was higher when the index case was unvaccinated, though this risk was still high among vaccinated cases, researchers found.

Among index cases who were unvaccinated, the attack rate in household contacts was 64% compared with 44% in contacts of index cases who completed their primary vaccination series within 5 months and 43% among those who were vaccinated and boosted, reported Julia Baker, PhD, of the CDC, and colleagues in the .

Not surprisingly, attack rates were lower when index cases isolated or wore a mask at home. Among household contacts of index patients who isolated, the attack rate was 41% versus 68% for those who did not isolate (P<0.01). These rates were 40% for contacts of index cases who reported "ever" wearing a mask at home "during their potentially infectious period" versus 69% for those who never wore a mask at home (P<0.01), they noted.

These findings emphasize the importance of a multi-pronged strategy to prevent COVID transmission in households, including isolation, mask-wearing, and being up-to-date on vaccination, Baker and team concluded.

Among household contacts with and without a prior COVID infection, attack rates were 41% and 60%, though this difference was non-significant (P=0.08).

Baker and colleagues examined data from telephone surveys in four U.S. jurisdictions: the states of Connecticut and Utah and the cities of Chicago and Milwaukee. Households were eligible if they had at least two people living there, and were not in a congregate setting.

Index cases were the first person within each household to either experience COVID symptoms or receive a positive test. Household contacts were anyone who "spent one or more overnights in the residence with the index patient during their potentially infectious period," the authors noted.

Overall, 183 index cases and 439 household contacts were included. Median age of index cases was 39, and 59% were white.

Transmission occurred in 68% of households, and 227 contacts were classified as having COVID, for an overall attack rate of 53%.

Examining contacts by vaccination status, attack rates were unsurprisingly lowest among vaccinated and boosted contacts (48%), followed by contacts fully vaccinated within the prior 5 months (50%).

Notably, household contact attack rates were lowest when the index case was 5 to 11 years old (48%) and highest when the index case was 4 years old or younger (72%).

"These findings further highlight young children's potential contribution to household transmission of SARS-CoV-2, as well as their ongoing susceptibility to infection when SARS-CoV-2 is introduced in the home," Baker's group wrote.

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