I don’t think there’s any way to know. Three of these people work three days a week at the office (it’s only open three days a week!), and I work two long days. It seems likely that at least some of the cases are related.
Everyone was back at work within a week. The older two of us both took Paxlovid. The 30-something person had the most symptoms (and I believe is the most careful). But who knows.