Most reinfections are milder the second time. The early Nevada case involved a relatively younger person who caught it once. Then they caught it again while c

Most reinfections are milder the second time. The early Nevada case involved a relatively younger person who caught it once. Then they caught it again while caring for a parent who also had it and was hospitalized. The case was confirmed by PCR analysis of the samples that were taken combined with validating that the samples had the subjects DNA in them. But they likely had a very mild initial infection and being young they fought it off with mostly an innate immune system response and didn't generate a lot of antibodies or adaptive response. Then when their parent got sick (in the same household) I would bet money the cared for that parent taking little to no precautions believing that their prior infection granted them survivor-island-style invincible immunity to disease (which is how most people think the immune system works). They likely got a whopping large viral dose which (probably combined with a bit of bad luck) is what caused the second infection to be severe and require hospitalization. And due to selection effects that unusual case bubbled up to the top early because it was someone in the hospital with covid who had positive evidence that they'd caught it 3 months earlier. A lot of people with mild covid both times would never have landed into a study and the clinicians wouldn't have bothered with genetic sequencing.