It occurred to me, learning that 80% of Covid infected patients lose their sense of smell (Anosmia) that it should be possible to build the cheapest and most effective Covid screener (for use at entrances to schools/airports/buildings/restaurants) with a simple "scratch-n-sniff" card. These cards, which cost pennies, would come with a set of scratch squares, and under each would be boxes with the names of possible scents. A QR code would (encrypted) have the answer. You sniff, check the boxes and then a phone or other device scans your answer. Much more effective than a thermometer, since probably under 20% of Covid patients have a fever. No close contact with anybody needed. Takes longer but faster to process so no waiting in line. Could even be automated but somebody has to be able to tell the people who fail they can't come in.

A bit of research would discover how many scratches you need for a reliable number. An alternate trick would be to have two rows of the same scents in different orders, draw lines between the matches. Something would work. However:

• This is a limited utility home test, since 20% false negative -- but then some of the rapid tests are not a lot better.
• Once you get it, anosmia persists long after the infection, so you need a different test to tell you are clean. You would need to get a card to bypass any anosmia checkpoint.
• A few people already have anosmia, though it's pretty rare. A more expensive rapid test can be used for them.
• At home, it's easy to test yourself with anything around the house with a scent, so you don't need the card, though many people who have it don't realize they have it -- only about 50% of Covid patients report it.
• You are infectious for 1-2 days before you lose your smell, just as is the case with fever.

So mainly this is a better alternative to fever checks. While it uses a consumable it's one that costs a few pennies. Fever checks need a person who comes up close to you. This takes a little longer but many people can do it in parallel, the scan of your card can be done fast by a phone or machine. It could be suitable for high-volume locations, put your card in slot, walk through when green.

There are some other cheap and effective tests, including an AI tool that can analyse coughs to detect Covid. A triple test could measure fever, cough and smell to be even more accurate. Of course, people with a cough should not be trying to enter buildings unless they know it's not Covid.

So, the "scratch and sniff" card industry has been thinking about this, but in a much more limited way, but it does not seem to have taken off.