(NEXSTAR) — A leading British immunologist told U.K. lawmakers Tuesday that the idea of reaching herd immunity in a world with the delta variant of COVID-19 was “mythical.”

Sir Andrew Pollard, a professor of pediatric infection and immunity and the director of the Oxford Vaccine Group, said the dangerously efficient transmission of the delta variant makes it impossible for the U.K. to effectively halt the virus through a combination of vaccinations and antibodies from catching the coronavirus.

“The problem with this virus is [it is] not measles. If 95% of people were vaccinated against measles, the virus cannot transmit in the population,” Pollard said, according to Wales Online. “The Delta variant will still infect people who have been vaccinated. And that does mean that anyone who’s still unvaccinated at some point will meet the virus … and we don’t have anything that will [completely] stop that transmission.”

As Britain enjoyed a summertime lull in COVID-19 cases, the nation’s attention turned to the end of pandemic-related restrictions and holidays in the sun.

But scientists are warning the public not to be complacent, saying high levels of infection in the community are likely to lead to another spike in cases this fall.

The reason for their pessimism is the delta variant of COVID-19, now dominant throughout the U.K. Vaccines are less effective against this more transmissible variant, meaning Britain needs to achieve a much higher level of vaccination if it hopes to control the disease. About 60% of the U.K. population has been fully vaccinated.

“I suspect that what the virus will throw up next is a variant which is, perhaps, even better at transmitting in vaccinated populations,” Pollard said.

For those who are unvaccinated, the level of risk when it comes to contracting COVID-19 is far greater thanks to delta, according to Dr. Peter Chin-Hong, an infectious disease specialist at the University of California San Francisco.

“Just the overwhelming amount of virus that delta produces, so you might have escaped this before as an unvaccinated person in 2020, but you may not be able to dodge the bullet in 2021 in that elevator, or in that mall chance encounter,” said Dr. Chin-Hong.

In the United States, cases have continued to mount during the summer, overwhelming hospitals and exhausting the supply of ICU beds.

As the virus continues to rip across the U.S. and other parts of the world, the repeated transmission carries the chance of further mutation.

“This goes with many viruses too, they reproduce. It wants to make more of itself, but that photocopy machine is kind of a cheap model, so it makes a lot of errors. But somehow by chance these errors happen to give the virus’ progeny a survival advantage,” Chin-Hong said.

When it comes to the current known variants, Chin-Hong said that despite delta’s current dominance there is another he continues to monitor.

“I think we’re all watching delta plus because the backbone of delta plus is delta … [and] it has a superpower from South Africa which is that beta variant, and the reason why that’s a little scary … is that it could be a little more vaccine-evasive than delta is.”

Chin-Hong added that, with the speed of mutation following the alpha variant of coronavirus, it may not be too long before we’re talking about a new variant.

For now, the best thing people can do is to get vaccinated, Chin-Hong said, emphasizing that despite the current variants and breakthrough cases, the COVID-19 vaccines are effective at preventing hospitalization and death among people who don’t already have compromised immune systems.

“The vaccine is still really good at that,” Chin-Hong said. “It’s amazing, it continues to be amazing. That’s like the distinction between just infection and being a nuisance, and being seriously ill, requiring a breathing tube and dying.”

The Associated Press contributed to this report.