The COVID-19 omicron variant set off a tidal wave of new infections in the U.S. that is still receding.

New research by the Centers for Disease Control and Prevention and partners in Chicago, Milwaukee, Connecticut and Utah shows substantial household spread during the surge.

The paper finds omicron transmission occurred in more than two-thirds of 183 surveyed households that had a COVID-19-positive member.

Spread risk was roughly 20% to 30% higher when the COVID-19 patient was not vaccinated or boosted, or did not practice prevention measures such as isolation or mask wearing.

Risk held steady across age groups, including children 4 and younger, who are ineligible for vaccination but might contribute to transmission.

The findings underscore the need for multiple prevention strategies in households with infected members.