• The Covid delta variant that was originally discovered in India is now spreading around the world.
• Delta has become the dominant strain in some countries, such as the U.K., and likely to become so in others, like the U.S.
• On Wednesday, the World Health Organization said the variant had been detected in more than 80 countries and it continues to mutate as it spreads.

The Covid-19 delta variant originally discovered in India is now spreading around the world, becoming the dominant strain in some countries, such as the U.K., and likely to become so in others, like the U.S.

On Wednesday, the World Health Organization said the variant had been detected in more than 80 countries and it continues to mutate as it spreads.

The variant now makes up 10% of all new cases in the United States, up from 6% last week. Studies have shown the variant is even more transmissible than other variants.

Scientists have warned that the data suggests the delta variant is around 60% more transmissible than the "alpha" variant (previously known as the U.K. or Kent variant which was itself a much more transmissible than the original version of the virus) and is more likely to lead to hospitalizations, as has been seen in countries like the U.K.

WHO officials said Wednesday there were reports that the delta variant also causes more severe symptoms, but that more research is needed to confirm those conclusions.

Still, there are signs that the delta variant could provoke different symptoms than the ones we've been advised to look out for when it comes to Covid-19.

What to watch out for?

Throughout the pandemic, governments around the world have warned that the main symptoms of Covid-19 are a fever, persistent cough and loss of taste or smell with some domestic variations and additions as we've learned more about the virus.

The CDC's updated list of symptoms, for example, includes fatigue, muscle or body aches, headache, a sore throat, congestion or runny nose, nausea or vomiting and diarrhea as possible symptoms of infection. There there are of course the millions of people who have had Covid-19 with no symptoms at all with the extent of asymptomatic transmission still being investigated by scientists.

But the delta variant appears to be provoking a different range of symptoms, according to experts.

Tim Spector, a professor of genetic epidemiology at King's College London, runs the Zoe Covid Symptom study, an ongoing U.K.-based study which enables the public to enter their Covid symptoms on an app when enables scientists to then analyze the data.

"Covid is also acting differently now," Spector noted in a YouTube briefing last week. "It's more like a bad cold in this younger population and people don't realize that and that hasn't come across in any of the government information."

"Since the start of May, we have been looking at the top symptoms in the app users and they are not the same as they were," he said. "The number one symptom is headache, then followed by sore throat, runny nose and fever." More "traditional" Covid symptoms such as a cough and loss of smell were much rarer now he said, with younger people experiencing much more of a bad cold or "funny off feeling."

The alpha variant first discovered in the U.K. highlighted the emergence of a wider set of symptoms.

A study of over a million people in England within the REACT study (which tracks community transmission of the virus in England) that was carried out between June 2020 and January 2021 — and hence over a period of time in which the alpha variant spread and became dominant — revealed additional symptoms that were linked with having the coronavirus including chills, loss of appetite, headache and muscle aches, in addition to the 'classic" symptoms.

Variant of concern

This week the delta variant was re-classified as a "variant of concern" by the Centers for Disease Control and Prevention "based on mounting evidence that the delta variant spreads more easily and causes more severe cases when compared to other variants, including B.1.1.7 (Alpha)," it said in a statement to NBC News.

Dr. Scott Gottlieb, a former Food and Drug Administration commissioner, said the delta variant will likely become the dominant strain in the U.S. and could "spike a new epidemic heading into fall," during an interview with CBS' "Face the Nation" Sunday.

In the U.K., where the delta variant is now responsible for the bulk of new infections, cases have spiked among young people and the unvaccinated, leading to a rise in hospitalizations in those cohorts. The spread of the variant has also prompted the U.K. to delay further loosening of Covid-19 restrictions.

It's hoped that Covid-19 vaccination programs can stop the wild spread of the delta variant and so the race is on to protect younger people who might not be fully vaccinated. Analysis from Public Health England released on Monday showed that two doses of the Pfizer-BioNTech or the Oxford-AstraZeneca Covid-19 vaccines are highly effective against hospitalization from the delta variant.

The U.K.'s situation shows how quickly the delta variant can quickly become dominant and the U.S. is certainly watching on with concern.

Remarking on how rapidly the delta variant has become dominant in the U.K., Dr. Anthony Fauci, chief medical advisor to the president, noted last week that "we cannot let that happen in the United States," as he pushed to get more people vaccinated, especially young adults.

The latest study on the spread of the virus in England alone does nothing to allay the concerns of experts. The REACT study's latest findings, published on Thursday, warned that cases were rising "exponentially" and said the "resurgence" of Covid-19 infections in England was "associated with increased frequency of Delta variant."  

The study estimated that roughly 1 in 670 have the virus, a sharp increase compared to the study's previous findings, when 1 in 1,000 people had the virus as of May 3. The findings were published Thursday and are based on almost 110,000 home swab tests taken between May 20 and June 7.

Led by Imperial College London, the scientists estimate that the reproduction number is now 1.44 in England, meaning 10 infected people would pass the virus on to 14 others on average, "resulting in fast growth of the epidemic."

Professor Paul Elliott, director of the REACT program from Imperial's School of Public Health, said "we found strong evidence for exponential growth in infection from late May to early June … These data coincide with the Delta variant becoming dominant and show the importance of continuing to monitor infection rates and variants of concern in the community."

Most infections are happening in children and young adults, but they are rising in older people too, the study found.

While it had found that the link between infections, hospitalizations and deaths had been weakening since February, suggesting infections were leading to fewer hospital admissions and deaths due to the vaccination programme, since late April, the trend has been reversing for hospitalizations.

- CNBC's Rich Mendez contributed reporting to this story.