Dad shares warning after kids allegedly contract COVID-19 from unvaccinated relative

A Philadelphia father of two is speaking out about the ongoing risk of COVID-19 for children after his son and daughter tested positive for the virus.

Adam Joseph, a meteorologist for ABC station WPVI, said his 6-year-old son Jacob and 5-year-old daughter Hannah tested positive for COVID-19 after having contact with a relative who was not vaccinated against the virus.

Joseph said the relative tested negative for COVID-19 prior to the visit with his family, but later tested positive for the virus.

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In addition to his children, their nanny also contracted COVID-19, according to Joseph.

He and his husband, Karl, are both vaccinated and have so far both tested negative for COVID-19.

"We kind of feel like we failed as parents, to a degree, after protecting our children for nearly a year-and-a-half from this, and were doing so well," Joseph said in a Facebook video.

Children under the age of 12 are currently not eligible for a COVID-19 vaccine.

Pfizer's COVID-19 vaccine was authorized for use in children ages 12 to 15 in May by the U.S. Food and Drug Administration. The two other COVID-19 vaccines available in the United States, Moderna and Johnson & Johnson, are currently available only for people 18 years and older in the U.S.

Joseph's warning to parents about protecting young children from people who are not vaccinated comes as top health officials warn that COVID-19 has become a "pandemic of the unvaccinated" in the U.S.

In addition to data showing COVID-19 cases and hospitalizations are predominantly among unvaccinated people, the more transmissible delta variant is driving up COVID-19 cases.

In light of the delta variant, health experts are pushing back on guidance from the Centers for Disease Control and Prevention in May that fully vaccinated Americans can go without masks.

The American Academy of Pediatrics on Monday called for schools to enforce universal masking mandates because so many children won’t be protected by fall and schools have no way of verifying COVID vaccine status yet.

People who are not vaccinated, including children between the ages of 2 and 11, should continue to wear face masks in indoor public places and practice social distancing and hand washing, according to the CDC.

Joseph's children experienced symptoms including a fever, chills and coughing, but are now improving. The children's nanny and relative are experiencing additional COVID-19 complications, according to Joseph.

"Please, if you are not vaccinated, do not go around children who are under 12, they are so vulnerable right now," Joseph said on Facebook. "They do not have a choice to get the vaccine at this point."

"We need to protect our children, and make sure they stay healthy because this delta variant is affecting kids. This new delta variant is affecting vaccinated people as well," he said. "It's everyone's choice, we're all adults, whether we want to get vaccinated or not. I'm not here to push that. I'm just asking you, begging you, if you're not vaccinated, to wear a mask wherever you go in public, even though it's not mandatory."

ABC News' Anne Flaherty contributed to this report.