British and international authorities are closely monitoring a subtype of the Delta variant that is causing a growing number of infections in the United Kingdom.

This descendant of the Delta variant, known as AY.4.2, accounted for an estimated 6% of cases in the week of September 27 -- the last week with complete sequencing data -- and is "on an increasing trajectory," a report by the UK Health Security Agency said.

Little is known about AY.4.2. Some experts have suggested it could be slightly more transmissible than the original Delta variant, though that has not yet been confirmed. While it accounts for a growing number of infections, it is not yet classified in the U.K. as a "variant of concern." It currently remains rare beyond Britain, with a small number of cases being recorded in Denmark and the U.S., expert Francois Balloux told the Science Media Center (SMC) on Tuesday.

"As AY.4.2 is still at fairly low frequency, a 10% increase its transmissibility could have caused only a small number of additional cases. As such it hasn't been driving the recent increase in case numbers in the U.K.," Balloux, Professor of Computational Systems Biology and Director at the UCL Genetics Institute, told the SMC.

While new variants have repeatedly overtaken one another to become the dominant strain globally in the past year, experts say it is too soon to know whether AY.4.2 will become significant. In the U.K., "Delta very rapidly in a matter of weeks" outpaced the Alpha variant by the summer, Deepti Gurdasani, a senior epidemiology lecturer at Queen Mary University of London, told CNN. "That's not what we're seeing here, we're seeing sort of a slow increase in proportion that suggests that it's not hugely more transmissible, it might be slightly more transmissible.

Balloux agreed, telling SMC that "this [is] not a situation comparable to the emergence of Alpha and Delta that were far more transmissible (50% or more) than any strain in circulation at the time. Here we are dealing with a potential small increase in transmissibility that would not have a comparable impact on the pandemic."

AY.4.2 has caught the attention of public health experts across the pond. In a series of tweets Sunday, former U.S. Food and Drug Administration Commissioner Dr. Scott Gottlieb called for "urgent research" into this Delta offshoot and said it was a "reminder that we need robust systems to identify, characterize new variants."

The emergence of AY.4.2 in Britain, however, points to what scientists have warned throughout the pandemic: soaring transmission can create new variants. The U.K. has had the highest rate of daily COVID-19 cases and deaths per million people in Western Europe since most pandemic restrictions were dropped in the summer. On Tuesday, it reported 223 COVID-19 deaths, the highest daily figure since early March, and health leaders are urging the government to reintroduce measures such as mask mandates in enclosed spaces to help ease the pressure on the health system.

The "whole problem with this approach to living with the virus and allowing between 30,000 to 50,000 cases a day -- which has been the UK's case rate since [the summer] -- is the [virus's] evolution will continue ... we need to suppress cases and suppress the virus," Gurdasani said.

YOU ASKED. WE ANSWERED.

Q: When vaccinated people die from COVID-19, how do you explain that vaccines are still worth taking?

A: We need to start with science and what the research shows, according to CNN Medical Analyst Dr. Leana Wen. The COVID-19 vaccines are extraordinarily effective in preventing illness and especially severe disease, she said.

The most recent data from the U.S. Centers for Disease Control and Prevention (CDC) shows that they reduce the likelihood of testing positive for COVID-19 by six-fold and the likelihood of death by 11-fold, she added.

"That means that if you are vaccinated, you are six times less likely to get COVID-19 than someone who's unvaccinated. And you are 11 times less likely to die from COVID-19 compared to an unvaccinated person. That's really excellent," she said.

"However, the COVID-19 vaccines do not protect you 100%. No vaccine does, just likely virtually no medical treatment is 100% effective. That doesn't mean the vaccine doesn't work, or that you shouldn't take it."

READS OF THE WEEK

Why Colin Powell's cancer likely reduced his protection from the COVID-19 vaccine

Colin Powell, the first Black U.S. secretary of state, died Monday from complications of COVID-19. Experts say his death shows how important it is for more people to get vaccinated and stop the spread of the virus, Jen Christensen reports.

Powell was fully vaccinated, but a source close to the matter confirmed to CNN he had multiple myeloma, a type of blood cancer that would have affected his immune response to the vaccine, and made it difficult to fight the virus. Peggy Cifrino, Powell's chief of staff, said Powell, 84, also had Parkinson's disease, a neurodegenerative disorder.

Although the COVID-19 vaccines provide strong protection against severe disease and death in healthy people, multiple myeloma patients are among the immunocompromised groups who may not respond as well, studies have shown. One study published in Nature in July showed that only 45% of multiple myeloma patients developed an adequate response to the vaccine, while 22% had a partial response. One-third had no response.

Despite Powell representing a high-risk population group, some conservative media figures have dishonestly seized on his death to cast doubt on the effectiveness of COVID-19 vaccines, Oliver Darcy writes.

These doctors are spreading misinfo about vaccines

Dr. Christiane Northrup was a frequent guest on The Oprah Winfrey Show -- an Ivy League-educated obstetrician-gynecologist who often spoke about women's health and holistic medicine. She was a media darling, and in 2013 made Reader's Digest's annual list of the 100 most trusted people in America.

But Northrup is also among a small group of doctors who have emerged as a huge source of misinformation -- whether as social-media influencers or family doctors meeting with patients in person -- about COVID-19 vaccines, which have thus far proven the most effective weapon against the deadliest pandemic in 100 years, Rob Kuznia, Scott Bronstein, Curt Devine and Drew Griffin report.

Moscow orders unvaccinated over-60s to stay home for 4 months

Millions of Russians face strict new Covid-19 restrictions from this week, after a slow vaccination drive, an overwhelmed health care system, and widespread mistrust in government combined to plunge the country into its most deadly phase of the pandemic to date, Anna Chernova and Rob Picheta report.

On Tuesday, Moscow's mayor ordered all unvaccinated residents over 60, as well as unvaccinated people "suffering from chronic diseases," to remain home for four months until late February as the city grapples with a growing crisis.

The national government has also proposed introducing a non-working week at the start of November, but senior figures have started openly admitting that the situation is dire as Russians head towards a bleak winter.

Russia has recorded its highest numbers of daily cases and deaths multiple times in recent days, and registered 1,002 official fatalities on Sunday -- the first time the country has crossed the four-digit deaths barrier so far.

Experts point towards a lagging vaccination program and a failure in government messaging as factors behind the surge, which is now threatening to deluge hospitals across the country.

"I think the country is now falling into disaster," Vasily Vlassov, a Russian epidemiologist and former adviser to the World Health Organization (WHO), told CNN.

TOP TIP

The test-to-stay approach in U.S. schools

The CDC is working with select school districts across the nation to evaluate the practice of the test-to-stay approach.

This type of policy would prioritize testing to monitor students who may have been exposed to COVID-19, allowing students to continue to attend classes if they test negative, instead of having to quarantine. But there are many health experts who agree that quarantines are still necessary.

In an email to CNN, CDC said it views test-to-stay as a "promising practice" and that it's "working with multiple jurisdictions implementing test to stay to evaluate the effectiveness of this strategy." But it's not clear when that guidance could be available. Read more about it here.