Dr Susan Hopkins, strategic response director for Covid-19 at PHE, told MPs that if the so-called Indian variant was “unmitigated”, left to spread without any lockdown measures, the R value could be “greater than five and maybe up to seven”.
She also warned “we are living in a world of variants now, so everything we see is a variation of the original”.
Dr Hopkins told the science and technology committee: “We’re seeing it [Delta] as much greater transmissibility than Alpha, which had greater transmissibility than the viruses that had gone before unmitigated - so if we were in the real world where we had none of the measures that we were seeing right now - we would estimate R greater than five and maybe up to seven.
“That’s why we need people to have vaccination because that’s a clear mitigation measure.”
She said 25 variants were currently “under monitoring” and eight “under investigation”.
“All of them have mutations that we’re concerned about, but mutations alone is not enough to predict whether it’s really going to impact on our journey through vaccines and impact on the public health risk of hospitalisation,” the expert added.
“That component takes time, in being able to deliver that science accurately and allow us to develop accurate risk assessments.”
Dr Hopkins explained how each new variant that continues to live on without rapidly becoming extinct will either have “a transmissibility advantage or an immune evading advantage”.
She expects there to be more emphasis on personal risk and responsibility after the scheduled easing of remaining lockdown restrictions on 19 July.
For example, in places like shops people may use personal judgment to wear masks, but in more tightly confined spaces, like the Tube, people may still be required to wear face coverings.
“I think we will all need to make decisions for ourselves, particularly on wearing masks, using better ventilation, hand hygiene,” she said.
“So we may find that some people, not all, will change their behaviours, and particularly those that are more concerned about their health or the health of people they live with.
“It will be for governments to decide what rules and regulations will need to be in place and what legislation will need to continue after 19 July.”
Additional reporting by PA