It could be, for example, that the signature that the dogs are detecting might be a general one associated with respiratory viruses like influenza, rather than a specific one associated with SARS-CoV-2.

Mick Bailey

Infection of dogs with SARS-CoV-2 has been reported, both asymptomatic and associated with variable levels of disease as in humans, so using dogs deliberately to sniff breath and clothes from potentially infected people seems a bit of a risk to the dogs, let alone their handlers.

You'd need to have some way round that, like vaccinating the sniffer dogs. Currently, no SARS-CoV-2 vaccine is licensed for use in dogs although there's no reason to think that the ones approved for humans wouldn't work, but the low reported incidence of infections in dogs would make a phase 3 trial difficult.