A 13-year-old boy in California has died after experiencing symptoms of the novel coronavirus despite initially testing negative for COVID-19, his family claims.

Maxx Cheng, 13, of Claremont, began isolating in his room after showing signs of COVID-19 — namely nausea, vomiting and chest pains — on July 4. He was tested for the deadly virus a few days later, on July 9, but received a negative result, Charlotte Cheng, the teen’s sister, told Los Angeles outlet KCBS-TV.

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“His symptoms matched, but then the test came out negative,” she added. “So we were a little bit confused.”

Maxx returned home following the test and continued to self-isolate in his room despite the negative test result. During isolation, he also developed a fever and cough.

He was seemingly improving until his family found him unresponsive in his room on Thursday night, his sister claims.

“We went to go check up on him like we normally do,” Charlotte Cheng, 21, said. “He wasn’t answering. We found him passed out in the room.”

“There was almost no cough,” she added. “The fever had gone down three days before he passed.”

A GoFundMe page created in the teen’s name described him as “athletic, intelligent, funny, and mature.” Maxx, who played viola and swam competitively, also leaves behind a twin brother.

The family is now awaiting the results of an autopsy to determine his cause of death and to find out whether or not he had COVID-19, according to KCBS-TV. It’s not currently clear if Maxx suffered from any underlying conditions.

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Charlotte Cheng said the family adhered to expert-recommended safety precautions so they are unsure where Maxx contracted the novel virus if he did.

“Somebody who is athletic, somebody who is on the swimming team who is so energetic and active dies from it, it changes the conversation,” Nichole Weinstein, a family friend and the organizer of the GoFundMe page on the behalf of the Cheng family, told the outlet.

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