How do we correctly translate the XKCD comic snippet "Good News: You recently had close contact with someone who has not tested positive for COVID"?
I would translate it as such:

好消息：你最近密切接触到测试新冠肺炎未测出阳性的人


Using 接触到 removes the need to introduce other complements with 与 / 和 / 跟， which reduce fluency of the joke. It also removes the negation from the main verb, just as in the English original.
未测出阳性 renders more literally &quot;not tested positive&quot; instead of 无确诊, which doesn't sound very grammatical to me (better with 未, by the way)
I wouldn't use 确诊 anyway, because it literally means &quot;confirmed (case)&quot;, as in 确认诊断，and the joke is more about the test result, not the diagnosis.
新冠肺炎 in your attempt, is accurate, as it identifies the pulmonary inflammation, i.e. the disease, whereas other frequently encountered terms as 新冠病毒 refer to literally the &quot;new coronavirus&quot;, i.e. the vector.
某个～人：this term 某 as in 某人 / 某个人 does translate to &quot;someone&quot; according to dictionaries. However in my experience, the usage of 某 is slightly different than that. I think the most accurate translation is &quot;a certain person&quot;. So yes, it's an unnamed person, a &quot;someone&quot;, but it implies that you choose to not disclose their identity. An example can be found in newspaper articles, when referring to someone involved in a crime. The perpetrator or the victim can be referred to as &quot;Surname + 某&quot;, e.g. 张某, literally &quot;a certain Zhang&quot;. I'm not fully sure the &quot;someone&quot; in the XKCD cartoon is actually this kind of &quot;someone&quot;. If it is, then 某个人 might be appropriate. Otherwise just 人 also works.


With all that said, I'm afraid this won't sound equally humorous in Chinese... but that's my personal opinion.