Masking and Isolating Reduced Omicron Spread in Homes, C.D.C. Finds

The omicron variant of the coronavirus was so contagious that it seemed a foregone conclusion that if one person in a household got sick, other people living there would also become infected with the virus.

But this turns out to be less specific: a small household survey by the Centers for Disease Control and Prevention published on Friday found that when the first infected person wore a mask and stayed in a separate room at least part of the time, the risk of infecting other family members with the virus became noticeably lower.

Vaccinated people who became infected were also significantly less likely than unvaccinated people to transmit the virus to other family members.

However, the study showed how aggressively the Omicron variant spread in the home, especially among people living with children under the age of 5 who tested positive. The CDC said that those children who are not yet eligible for vaccination and often need closer contact with their parents or relatives are shedding the virus among 72 percent of household contacts found in the study, the highest rate of any age group.

“These results further highlight the potential contribution of young children to transmission in the home,” the CDC scientists wrote in the report.

Federal regulators are waiting for data on how well three doses of Pfizer-BioNTech’s coronavirus vaccine work in children under five years of age before deciding whether to approve the vaccine for that age group.

The CDC study was based on 183 households in four states where someone became infected with the Omicron variant from November to early February. After asking household members about their vaccination and infection histories, any home precautions, and whether they tested positive or got sick, CDC scientists determined that the variant had spread in about two-thirds of the households they identified. But when the first infected person was fully vaccinated, only about 44 percent of family members developed Covid, compared with 64 percent when the infected person was not vaccinated, the study said.

And when the original infected family member was alone in the room for at least some time, only 41 percent of other family members became infected, compared with 68 percent in situations without isolation. Disguising as an infected person also helped, reducing the chance of transmission from 69 percent to 40 percent.

The study’s authors note that it was difficult to determine the exact risk of spreading the virus in the home. They ruled out situations where it was not clear who first contracted Covid and did not perform the genetic sequencing needed to know for sure that people contracted the virus from an infected person at home and not at other gatherings.

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