Incredible new data out of Japan has left experts convinced the nation of 125 million has successfully eradicated the Delta variant of Covid-19.
New research out of Japan's National Institute of Genetics suggests the strain drove itself towards a "natural extinction" there after several mutations led to it being unable to make copies of itself.
The tightly packed Japanese population has been on high alert since the beginning of the pandemic, especially after the highly transmissible Delta variant broke through its borders in 2021.
During the peak of its fifth wave, Japan was recording around 26,000 cases per day as countries around the globe, including New Zealand, reintroduced strong lockdowns to squash the Delta curve.
But in November, the nation has seen an amazing recovery, recording under 200 cases in recent weeks and on Friday registered its first day without a Covid death in 15 months.
According to a "potentially revolutionary" theory put forward by Professor Ituro Inoue, a genetics expert, the Delta variant simply accumulated too many mutations to the virus' error-correcting protein called nsp14.
Prof Inoue says the virus struggled to repair the errors in time and ultimately caused its own "self-destruction".
When the Delta variant first emerged, the US Centres for Disease Control and Prevention declared it to be more than twice as contagious as previous variants, warning it could cause more severe illness in unvaccinated people.
The general assumption was the Delta strain would have a far more vibrant genetic diversity than the original Alpha that took the world by storm in 2020.
However, according to Prof Inoue's research, the opposite was found to be true.
"We were literally shocked to see the findings," Inoue told The Japan Times.
"The Delta variant in Japan was highly transmissible and keeping other variants out. But as the mutations piled up, we believe it eventually became a faulty virus and it was unable to make copies of itself. Considering that the cases haven't been increasing, we think that at some point during such mutations it headed straight toward its natural extinction."
While some experts have attributed the downturn in cases to the country's 76.2 per cent vaccination rate and strong adherence to mask-wearing, Inoue believes new infections would still be on the up if the Delta strain were still "alive and well".
"If the virus were alive and well, cases for sure would increase, as masking and vaccination do not prevent breakthrough infections in some cases," he said.
Professor Takeshi Urano, a researcher at Shimane University's Faculty of Medicine, weighed in on Inoue's findings, claiming the breakthrough discovery could be used in "promising" new medical treatments.
"Studies have shown that a virus with a crippled nsp14 has a significantly reduced ability to replicate, so this can be one factor behind the rapid decline in new cases. The nsp14 is virus-derived, and the chemical agent to curb this protein could become a promising medicine, with development already underway."
Japan declared its state of emergency over in early October, reopening society after a period of heavy restrictions. It now boasts one of the lowest infection rates of any developed nation, but Inoue warns it is not immune to potential new strains.
"There's clearly a threat," he said.
"We have been all right because there was a Delta variant. Other variants snuck in little by little but Japan's Delta was keeping them away. But because there's nothing now to keep them at bay, there's room for new ones to enter as the vaccines alone would not solve the problem.
"In that sense, I think the quarantine measures for immigration control are very important because we never know what comes in from foreign countries."
Inoue's research could also shine a light on the similar disappearance of SARS in Japan in 2003.
Conducting an in vitro experiment, researchers caused mutations in nsp14 in the virus that causes SARS, eventually finding the virus could not replicate itself after it completed several mutations. However, Inoue says it's still just a hypothesis, as no genome data exists.
"No genome data exists, so it's just a hypothesis, but because it has disappeared, it will never see the light of day again," he said.
At the moment, the Japanese expert says it's still too optimistic to believe the Covid-19-causing SARS-CoV-2 virus will experience a similar decline globally.
"The chances are not zero, but that seems too optimistic for now as we're unable to get hold of any such evidence, though we have looked at various data of other countries," he said.