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Children infected with the Omicron variant of SARS-CoV-2 appear to be infectious for about 3 days after a positive test, researchers found.

In a small study of 76 kids ages 7 to 18, the median duration of infectivity was 3 days for both vaccinated and unvaccinated children, Neeraj Sood, PhD, of the University of Southern California, and colleagues reported online in a research letter.

"The vast majority of children who get COVID are symptomatic for 1 to 3 days," co-author Eran Bendavid, MD, MS, of Stanford University, told MedPage Today. "Basically that correlates with how long the virus is causing disease in their body."

The authors noted the results are consistent with a study in adults with the Omicron variant, which found no association between vaccination status and infectivity duration.

In that study, which was published as correspondence in the New England Journal of Medicine, adults had a slightly longer median duration of infectivity at , the authors noted.

Sood and colleagues wrote that they conducted the study because the duration of infectiousness in children with newer variants of COVID-19 is generally unknown. One pre-Omicron study from 2021, , found that nasopharyngeal swabs from infected children were about half as likely to contain culturable virus compared with adults.

For their study, conducted between April and September 2022, Sood and colleagues took pharyngeal swabs from the children during 5 home visits over 10 days, with day 0 being the date of their positive PCR test. The majority of kids, about 68%, had been vaccinated, and exactly half were male.

The primary outcome was cytopathic effect (CPE) assessed by bright field microscopy and checked for growth in culture.

Overall, Sood and colleagues reported a median duration of infectivity of 3 days, with 18.4% of participants still being infectious on day 5. Only about 4% were still infectious on day 10, they found.

There was no difference in duration of infectivity by vaccination status, at 3 days for both vaccinated and unvaccinated children, they reported. That relationship remained robust even after controlling for demographics, they said.

Also, among vaccinated kids, the duration of infectivity was similar for those who'd had a booster and those who didn't, they noted.

The study was limited by a small sample size, and by the potential for non-response bias. In addition, cytopathic effect is a proxy for true infectivity, but is considered the gold-standard, the authors wrote.

Their findings align with current recommendations for isolation, they wrote: "Our findings suggest that current policies requiring isolation for 5 days after a positive test might be appropriate, as the majority of children were not infectious by day 5. Additionally, return-to-school policies may not need to discriminate by vaccine or booster status."

Bendavid noted that "by 3 days, the majority of children are going to be non-infectious. By 5 days, it's going to be a relatively small portion [that are still infectious]. The guidelines should at least take that information into account when deciding how long to keep kids out of school."

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