According to the health department, this is the latest update:
https://www.zambiaimmigration.gov.zm/wp-content/uploads/2022/03/Revised_Travel_Guidelines_Zambia.pdfYou can track their regulations here -
https://www.zambiaimmigration.gov.zm/covid-19-news/There is a challenge here as they require a 72 hour PCR test. Pretty difficult if you are traveling from the US. I ran into this problem two months ago. But fortunately, my PCR test was in a PDF form. Was a simple matter to correct. If you don't go that route, you will need to get a current PCR test at JNB. Make sure you account for this as you will need to clear Customs/Passport control, take your test, get a negative and then go back through customs. I found correcting a PDF much less of a hassle.
Since you are overnighting, it will not be a big deal but make sure you get your test when you arrive. The rapid PCR kiosks do not open until 8:00am'ish. So attempting the day-of travel can make it tight.
One other interesting quirk is that Zambia requires three blank pages on your passport. As they say, give an African a rubber stamp and you will quickly run out of paper and ink.
