The World Health Organization (WHO) recently warned Delta might just be a warning shot fired before a possibly deadlier, more contagious COVID-19 variant emerges.
But just how bad could it get? How about 600 times worse?
"We have safe ways of getting a glimpse into the future of what this virus can become," Kirby Institute virologist Stuart Turville told news.com.au. "There are some good studies out there that have looked at a kind of sped-up evolution."
One of those was carried out in Israel. Researchers looked at the SARS-CoV-2 virus' spike protein - the hooky bit that latches onto our cells - and broke it down into its component parts. They worked out all the possible combinations they could be put back together, and came across one that appeared to make the virus 600 times better at grabbing onto our cells than the original strain which emerged from China last year.
Dr Turville's team confirmed its superior ability in an experiment of their own.
"To cut a long story short, yes, it transmits much better than Delta," he said.
But luckily, it doesn't exist in the wild - that we know of. The 'COVID ultimate' variant, as it's been dubbed, requires seven specific mutations to happen in the same copy of SARS-CoV-2.
"Some variants of concern have some of these changes. But we've yet to see all seven locked in," said Dr Turville. "That future virus - if it does turn up - does have the capacity to infect better and also evade immune responses."
SARS-CoV-2 mutates slower than most other viruses, but the sheer number of people getting infected has greatly increased the chances of mutations appearing.
Half-finished vaccine rollouts with large numbers of people not fully immunised might also be a problem. The Delta variant has shown an increased ability to infect people who've been vaccinated, and though they're far less likely to fall seriously ill and die than the unvaccinated, they can still spread the disease.
A study released last week found vaccine-resistant strains of COVID-19 are most likely to emerge when around 60 percent of people have been vaccinated - particularly if mask mandates and social distancing practises are dropped.
"A counterintuitive result of our analysis is that the highest risk of resistant strain establishment occurs when a large fraction of the population has already been vaccinated but the transmission is not controlled" the study, published in Scientific Reports, said. That's because the virus still has plenty of chances to mutate, and gets into an "evolutionary arms race" with the vaccine, which is left playing catch-up.
The best strategy was a fast rollout to reduce transmission of the virus, limiting its opportunities to evolve. That hasn't happened though - much of Africa haven't even had a chance to have their first dose, let alone be fully vaccinated, the WHO warned on Thursday. And worryingly, many countries with comprehensive vaccine rollouts appear to be plateauing around 50-60 percent coverage, including the US and UK.
Dr Turville said if a more infectious SARS-CoV-2 variant emerges, it won't necessarily be deadlier - and vice versa. Changes it makes to the spike protein to evade vaccines could make it less lethal, and if it evolves to be deadlier, it might kill them before it can find new hosts and burn out - like the original SARS-CoV-1 virus did in the early 2000s.
