Ministers look to have given up on containment in favour of a novel approach some experts are wary of

Herd immunity is a phrase normally used when large numbers of children have been vaccinated against a disease like measles, reducing the chances that others will get it. As a tactic in fighting a pandemic for which there is no vaccine, it is novel – and some say alarming.

It relies on people getting the disease – in this case Covid-19 – and becoming immune as a result. Generally it is thought that those who recover will be immune, at least for now, so they won’t get it twice.

But allowing the population to build up immunity in this way – rather than through widespread testing, tracking down the contacts of every case and isolating them, as many other countries in Asia and Europe have chosen to do – could increase the risk to the most vulnerable: older people with underlying health problems.

To reach herd immunity, about 60% of the population would need to get ill and become immune, according to Sir Patrick Vallance, the government’s chief scientific adviser. Though it could need as much as 70% or more. Even scientists who understand the strategy are anxious. “I do worry that making plans that assume such a large proportion of the population will become infected (and hopefully recovered and immune) may not be the very best that we can do,” said Martin Hibberd, professor of emerging infectious disease at the London School of Hygiene & Tropical Medicine.

“Another strategy might be to try to contain [it] longer and perhaps long enough for a therapy to emerge that might allow some kind of treatment. This seems to be the strategy of countries such as Singapore. While this containment approach is clearly difficult (and may be impossible for many countries), it does seem a worthy goal; and those countries that can should aim to do.”

The government’s “nudge unit” seems to favour this strategy. Dr David Halpern, a psychologist who heads the Behavioural Insights Team, said on BBC News: “There’s going to be a point, assuming the epidemic flows and grows, as we think it probably will do, where you’ll want to cocoon, you’ll want to protect those at-risk groups so that they basically don’t catch the disease and by the time they come out of their cocooning, herd immunity’s been achieved in the rest of the population.”

But Anthony Costello, a paediatrician and former World Health Organization director, said that the UK government was out of kilter with other countries in looking to herd immunity as the answer. It could conflict with WHO policy, he said in a series of Twitter posts, which is to contain the virus by tracking and tracing all cases. He quoted Dr Tedros Adhanom Ghebreyesus, the WHO director general, who said: “The idea that countries should shift from containment to mitigation is wrong and dangerous.”

Herd immunity might not even last, Costello said. “Does coronavirus cause strong herd immunity or is it like flu where new strains emerge each year needing repeat vaccines? We have much to learn about Co-V immune responses.” Vaccines, he said, were a much safer way of bringing it about.