Has the coronavirus reached 'peak fitness'? Experts wonder if the Delta variant is the worst we'll see.

• World Health Organization officials called Delta the "fittest" variant to date on Monday.
• Delta is more transmissible, and potentially deadlier, than any other coronavirus variant so far.
• Scientists now wonder if the virus has reached "peak fitness," though it's likely too soon to tell.

No coronavirus variant spotted so far is more concerning than Delta, the strain first identified in India in February. World Health Organization officials on Monday said Delta is the "fittest" variant to date, since it spreads even more easily than other variants and may lead to more severe cases among unvaccinated people.

"Delta is a superspreader variant, the worst version of the virus we've seen," Eric Topol, director of the Scripps Research Translational Institute, tweeted last week.

But it's possible that Delta is the worst the coronavirus is going to throw at us - that the virus, in other words, has reached what epidemiologists call "peak fitness."

Topol and Italian virologist Roberto Burioni explore that scenario in a letter published in the journal Nature on Monday. The virus, they wrote, is likely to hit a point after which it no longer mutates to become more infectious. In that case, they said, "a "'final' variant will prevail and become the dominant strain, experiencing only occasional, minimal variations."

It's too soon to know whether that's happened, since Delta isn't yet dominant worldwide. But it likely will be soon -Delta has been detected in more than 80 countries so far and is already dominant in India and the UK.

"Delta is absolutely going up the fitness peak - whether it's at the top, I think that's very hard to say until we just don't see any further change," Andrew Read, who studies the evolution of infectious diseases at Pennsylvania State University, told Insider.

"If Delta takes over the world and nothing changes," he added, "then we'll know in a while - a year or two - that it is the most fit."

The fittest variants are the best at spreading

The coronavirus is constantly mutating in relatively harmless ways, but every once in a while, a mutation turns the virus into a more menacing threat. A new variant develops that can evade antibodies generated in response to a vaccine or prior infection, results in more serious illness, or spreads more easily.

Emerging research indicates that Delta checks at least two of those boxes.

Public Health England found that Delta is associated with a 60% increased risk of household coronavirus transmission compared to Alpha, the variant discovered in the UK. Alpha is already around 50% more transmissible than the original strain, according to the Centers for Disease Control and Prevention.

Researchers in Scotland also found that getting infected with Delta doubles the risk of hospital admission relative to Alpha. (Previous studies have suggested that Alpha may be 30% to 70% deadlier than the original strain.)

What's more, emerging research indicates that a single vaccine dose doesn't hold up as well against Delta as it does against other coronavirus strains. Recent Public Health England analyses found that two doses of Pfizer's vaccine were 88% effective at preventing symptomatic COVID-19 from Delta, while a single shot was just 33% effective. That's compared to 95% efficacy against the original strain, with 52% after one shot.

The best way for the coronavirus to achieve peak fitness, Topol and Burioni wrote in their letter, is to become more contagious. If a variant is already spreading quickly, there's no urgent need for it to evade the body's immune response; it can simply jump to another person.

"Increasing rate of transmission from person to person is what we're looking for," Read said.

So far, Delta is by far the most transmissible variant. The US's Delta cases appear to have tripled in just 11 days, from 10% of all cases sequenced in early June to 31% last week, according to a recent estimate from the Financial Times. At that rate, experts predict Delta will become the nation's dominant strain in weeks.

That doesn't necessarily mean the coronavirus has reached maximum transmission, though.

Read said Delta could still acquire combinations of mutations that make it even better at spreading (what he called a "Delta-plus" variant). It's also possible that two separate variants - Delta and Alpha, for instance - could combine mutations to produce an even more infectious strain. Under a third scenario, Read said, an entirely new lineage might replace Delta as the dominant variant.

"The biggest concern at the moment is just the sheer number of people that have the virus and therefore the sheer number of variants that are being generated," Read said. "Some of those might be the jackpot which are even fitter than Delta."

Still, vaccines will likely provide at least some protection against whatever strain represents the coronavirus' peak fitness.

"No human vaccine has ever been undermined by a variant to the point where the vaccine was completely useless," Read said.