I wouldn't be surprised if dogs could detect COVID-19, as they can help detect hypoglycaemia in diabetics, warn people who are about to have an epileptic seizure and have been used to sniff out some cancers.
This article on Nature's website (23 November 2020) has the following
Canines seem to detect coronavirus infections with remarkable
accuracy, but ...
According to the summary of one study:
Of 2,581 COVID-19 patients studied, 95 percent of patients regained their sense of smell within six months, according to the study in the Journal of Internal Medicine.
The study appears to be this one (on patients from continental Europe) and it has more details even its abstract:
A total of 328 patients (24.1%) did ...