The University of Sydney's first vaccine trial in September 2020 produced disappointing results, but the stalwart team is not giving up.
A revised vaccine will be administered to the shire's koalas in March when the team returns to the shire, led by veterinary pathologist Mark Krockenberger.
Professor Krockenberger said the original vaccine had reduced the amount of chlamydia the vaccinated and infected animals had shed, but it had not stopped the transferral of the potentially deadly disease, which renders females infertile.
He has since been working alongside Professor Peter Timms at the University of Sunshine Coast to develop a stronger vaccine.
"We need a dramatic effect, so I had discussions with Peter and we've got a strategy for a different design of vaccine we're pretty sure will have a stronger effect," Professor Krockenberger said.
"On the other side of it, that's the strength of this, we're not just saying, 'We've got a vaccine, let's just vaccinate every animal we can find and see if it works'.
"We're trying to put time in now and say, 'Okay, what's the best way we can do it?'. The first trial hasn't been enough to change things in Gunnedah. Hopefully, the second time will be."
Professor Krockenberger said there was no straightforward solution to the chlamydia crisis.
"It's a complex issue. It's really hard. If it was easy, it would have been done years ago," he said.
"We always knew this was a very challenging problem because there are so many animals with quite severe disease in the Gunnedah population, so we'll try the next vaccine and hope we get a better response and take it step by step ... I'm hopeful it will be a dramatic change, but we won't know until we do it."
Professor Krockenberger said the shire's population was also ageing and they hadn't seen many joeys during their visits.
"We're seeing very few young animals. Infertility is very high at the moment in the population. In the meantime, all the animals are getting older and we are seeing deaths from old age even at the moment, so the population is definitely declining," he said.
Professor Krockenberger said it would be difficult to reverse such a huge drop in population, but he believed it could be done.
"People are pretty aware that the population is declining and I think that's going to keep happening despite everything we do, but I am pretty hopeful that the things we're doing will eventually have an impact, long-term," he said.
"I'm pretty pleased to be involved in these efforts to try and change that. I'm optimistic that we will eventually have change. I'm also optimistic about the state government's approach to this problem now. It's really on people's agenda."
Our journalists work hard to provide local, up-to-date news to the community. This is how you can continue to access our trusted content:
Sign up for our newsletter to stay up to date.