I would use this as an opportunity to educate them via telephone. Explain the highly communicable nature of this virus, that it's not just the flu. Emphasize that staying home is potentially saving lives. And with that, say "No, thanks. I can't join you."

If it would help, talk to them about the expansion rates in NY and Louisiana where the number of cases has exploded. CA and WA have lots, but they've been slower rates of communication comparatively speaking.