The chairman of the Oxford Vaccine Group believes that herd immunity is "not a possibility" because the Delta variant is still spreading rapidly and even infecting fully vaccinated people.

Andrew Pollard cautioned a parliamentary panel on Tuesday that herd immunity is a "mythical" concept that should not be used to build vaccination programs in the U.K. or globally.

"The problem with this virus is [it is] not measles," Pollard told the all-party parliamentary group on coronavirus. "If 95% of people were vaccinated against measles, the virus cannot transmit in the population."

"The Delta variant will still infect people who have been vaccinated," he added.

He went on to say that, while vaccines may marginally decrease the transmission process since vaccinated persons remain contagious for a shorter length of time, "we don't have anything" that will entirely stop the spread of COVID-19.

Herd or population immunity is based on a great majority of a population developing immunity -- either through vaccination or prior illness. This, in turn, gives unvaccinated individuals and those who have never been afflicted with an infectious disease with indirect protection.

The remarks come amid a shifting understanding of the role vaccinations can have in reducing COVID-19 circulation. There had been some hope that, if coverage was high enough, the virus would no longer be able to spread, effectively terminating the pandemic and providing some protection to people who are unable to receive a shot.

These hopes, however, have been crushed in recent weeks. While vaccines have been found to protect against serious disease, hospitalization, and death, mounting evidence suggests that they do not stop transmission.

According to data released last week by Public Health England, there is little reduction in the level of virus present in vaccinated and unvaccinated patients who test positive for COVID-19, implying that the vaccinations do not reduce viral replication as well as predicted. Scientists thought that a lower viral load would inhibit further transmission.

The findings are consistent with data from the U.S., where a recent study of an outbreak in Massachusetts discovered that virus loads were identical among 127 fully vaccinated people and 84 unvaccinated people.

The findings prompted the Centers for Disease Control and Prevention to reverse previous guidelines that claimed fully vaccinated adults did not need to wear a face mask indoors.