With the BinaxNow test, you swab the front of your nose, insert the swab into one side of a small card, add saline to the other side, close the card, and see if the reader on the front lights up green or red. A phone app records a negative result for use as a digital passport.

Asking those presumed to be infectious to stay home would cut transmission chains, ending Covid outbreaks within weeks. Each transmission stopped may prevent hundreds more. This isn’t herd immunity, but it has the same effect. Like vaccines, the tests don’t have to be perfect. It’s enough to drop the virus’s reproductive number (the average number of people each infected person infects) below 1.

Cornell University’s quick defeat of its Covid cluster shows the power of frequent testing. Cornell tests all undergraduates twice a week and quarantines those who are positive. After an unauthorized party, Cornell had 60 positive cases a week before starting surveillance testing. It now has about three a week.