Star Trek: Everything We NOW Know About The Burn

Trill, the homeworld of one of the galaxy's most prominent joined species, suffered terribly in the aftermath of the Burn. As far back as the 24th century, it was known that a very small number of symbionts were produced annually with the ability to join, at around roughly 500. 

The actual cause of the reduction in joinings was not made clear before the cause of the Burn was discovered. When the Discovery, Burnham, and Adira Tal travelled to Trill in 3188, the planet did not seem to have suffered the same physical damage as Qo'Nos, though there could have been a secondary, radiological reason for the issue. 

Adira, who is a human host, was the first successful blend of a Trill symbiont and an alien host in more than 2000 years. Though Commander Riker briefly carried the Odan symbiont some 800 years prior, that was only temporary. This discovery may have begun a new period of joining for the Trill as a species, even with the knowledge that roughly half of the population were capable of being hosts before this. 

Despite the joining issue, the Trill seemed to have largely done well in their isolation. The planet was in good condition, and the number of symbionts began to increase. They also signalled their interest in joining the Federation in the future, something they had seemingly done by the following year.