Originally Published: 09 JUL 21 19:41 ET

Updated: 12 JUL 21 11:38 ET

By Deidre McPhillips, CNN

(CNN) -- All eyes are on the Delta variant that is now dominant in the United States as new Covid-19 cases rise week-to-week and the variant -- first identified in India and also known as B.1.617.2 -- accounts for a growing share.

But trends from Israel and the United Kingdom -- where the variant became dominant a few weeks sooner than in the US -- present hope for a less deadly and severe surge than others that have come before. And experts say that vaccination progress will be the most critical factor in preventing the worst outcomes.

In Israel, average daily cases are twice what they were in mid-April when the first cases of Delta were identified in the country. At that time, there were an average of five deaths each day in Israel. But despite the rise of the Delta variant -- which now accounts for more than 90% of new cases in the country -- average daily deaths have stayed consistently below that. In fact, Israel has had an average of less than two Covid-19 deaths per day since the last week of May, according to data from Johns Hopkins University.

In the United Kingdom, both cases and deaths are higher than they were when the Delta variant became the dominant strain in the country in mid-May, but cases have climbed exponentially faster than deaths. Average daily deaths in the UK are about twice what they were when the Delta variant became dominant, and cases are about 12 times what they were.

But trends in death due to Covid-19 lag a few weeks behind trends in cases, so the latest data on deaths should be closer in line with data on cases from a few weeks earlier. And even three weeks ago, average daily cases in the UK had multiplied more than the most recent daily deaths.

While both Israel and the UK foreshadow some optimism for Delta's trajectory in the United States, experts say that Israel's outcomes have been more overwhelmingly positive because of their substantial vaccination rate.

"In my mind, vaccines are the single most important factor" in the fight against the Delta variant, Becky Dutch, a virologist and chair of the University of Kentucky's department of molecular and cellular biochemistry, told CNN.

When the first cases of the Delta variant were identified in Israel, about 56% of the population was already fully vaccinated, according to Our World in Data. But in the UK, only 2% of the population was fully vaccinated when the Delta variant was first identified there, only reaching 50% vaccination within the past week.

"There is reason to be moderately hopeful -- with the caveat that the reason deaths and hospitalizations have not gone up as much is that there's pretty high immunity from vaccination and natural infection in individuals most at risk," Justin Lessler, a professor of epidemiology at the University of North Carolina, told CNN.

In a statement released Monday, the Israeli government said that its analysis has shown the Pfizer/BioNTech vaccine to provide 64% protection against infections caused by the Delta variant but 93% effective in preventing severe disease and hospitalizations, compared to research from May that reported it to be 97% effective.

"If we picked a flu vaccine that is effective as the current mRNA vaccines appear to be against Delta, we would be celebrating. They're only slightly less good against Delta than the originally circulating strain," Lessler said.

"The concern is places in the US that have not seen a lot of Covid and vaccination rates among the high-risk population is low."

Overall, vaccination rates in the US fall somewhere between Israel and the UK. About 16% of the population was fully vaccinated when the first cases of Delta were identified in the US and about 48% are fully vaccinated now that the variant has become dominant.

And vaccination rates vary widely across the country. Less than a third of people in Alabama are fully vaccinated, compared to about two-thirds of people in Vermont, according to the latest data from the US Centers for Disease Control and Prevention.

"The US is a patchwork now," Dutch said. "It depends on where you live. If you live in a place with high vaccination rates and you're vaccinated yourself, I'm not overly concerned about you. But if you're sitting in an area of the country with 35% of the population vaccinated and you're not vaccinated, I'm much more concerned."

And while there is some evidence that the virus may evade natural immunity from previous infections and slightly lower the efficacy of the vaccines, experts say these findings are something to pay attention to, they're not something to panic about.

Instead, Lessler says the rise and spread of the Delta and Alpha variants are a "warning that the virus is going to continue to evolve and continue to -- in that evolution -- find ways around existing immunity," but that the hope is that vaccines will "virtually eliminate severe disease" for quite some time.