Reaching herd immunity is “not a possibility” with the current Delta variant, the head of the Oxford Vaccine Group has said.

Giving evidence to MPs on Tuesday, Prof Sir Andrew Pollard said the fact that vaccines did not stop the spread of Covid meant reaching the threshold for overall immunity in the population was “mythical”.

“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,” he told the all-party parliamentary group (APPG) on coronavirus.

“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.”

Although the existing vaccines are very effective at preventing serious Covid illness and death, they do not stop a fully vaccinated person from being infected by the virus that causes Covid-19.

The concept of herd or population immunity relies on a large majority of a population gaining immunity – either through vaccination or previous infection – which, in turn, provides indirect protection from an infectious disease for the unvaccinated and those who have never been previously infected.

Data from a recent React study conducted by Imperial College London suggests that fully vaccinated people aged 18 to 64 have about a 49% lower risk of being infected compared with unvaccinated people. The findings also indicated that fully vaccinated people were about three times less likely to test positive after coming into contact with someone who had Covid (3.84%, down from 7.23%).

About 75% of all UK adults have now received both their jabs.

The health secretary, Sajid Javid, said on Tuesday that plans were in place to start offering Covid booster jabs to the most vulnerable groups in the UK from next month. He said that the flu jab would be offered at the same time.

But Pollard – who chairs Joint Committee on Vaccination and Immunisation (JCVI) but is not specifically a member of the Covid JCVI committee – questioned whether boosters would be needed.

“The time we would need to boost is if we see evidence that there was an increase in hospitalisation – or the next stage after that, which would be people dying – amongst those who are vaccinated. And that is not something we are seeing at the moment,” he said.

Even if vaccine-induced antibody levels waned, our immune systems would probably remember the vaccination for decades and offer a degree of protection if exposed to the virus, he said. “So, there isn’t any reason at this moment to panic. We’re not seeing a problem with breakthrough severe disease.”

The question of whether or not to vaccinate under-16s, as countries including the US, Ireland and Israel have done, has also provoked scientific debate in the UK.

The JCVI has recommended only vulnerable children aged 12 to 15 be vaccinated, and those who live with at-risk adults. Some critics have said that rich countries with high adult vaccine coverage, such as the UK, should not be hoarding doses for children but should donate those doses to poor countries, many of which have barely vaccinated any of their most at-risk populations.

But Prof Devi Sridhar, the chair of global public health at the University of Edinburgh, pointed out that the only vaccine authorised for use in children aged 12 to 15 in the UK was the Pfizer/BioNTech vaccine.

“I think actually the real issue is not about vaccinating kids versus the world – it’s what do we do about boosters that is being discussed in rich countries, because those are the doses that could be going abroad,” she said.

“Luckily … we have a very effective vaccine in AstraZeneca that can be used for the world, but it’s not going to solve our children issue here.”