How to follow up on the status of the official offer letter
You can probably safely wait a bit longer for the reasons you suggest. The staff that normally handles such things may not have complete access to their office systems and may be poorly supported for distance work.
There is some turmoil generally in hiring now and if the university is part of a State system they will be bound by directives from the legislature. But there is nothing you can do to affect that in any case.
So don't wait more than another week and ask for an update. Just note that you don't have the official notice yet and would like an update.
If you have other offers pending with deadlines don't wait too close to those deadlines to ask for the update. I don't expect bad things to happen, but it is hard to make predictions in times of chaos.
And congratulations.