The UK government recently enacted its second phase of response to the COVID-19 pandemic: “delay”. According to ITV journalist Robert Peston, the government’s strategy to minimise the impact of COVID-19 “is to allow the virus to pass through the entire population so that we acquire herd immunity, but at a much delayed speed so that those who suffer the most acute symptoms are able to receive the medical support they need, and such that the health service is not overwhelmed and crushed by the sheer number of cases it has to treat at any one time”. At face value, this seems like a sound strategy, but what exactly is herd immunity and can it be used to combat COVID-19?

Our bodies fight infectious diseases through the actions of our immune systems. When we recover, we often retain an immunological memory of the disease that enables us to fight off that same disease in the future. This is how vaccines work, creating this immune memory without requiring getting sick with the disease.

If you have a new disease, such as COVID-19, that we don’t have a vaccine for and no one in the country has ever been infected with, the disease will spread through the population. But if enough people develop an immune memory, then the disease will stop spreading, even if some of the population is not immune. This is herd immunity, and it is a very effective way to protect the whole of a population against infectious disease.

But herd immunity is typically only viewed as a preventive strategy in vaccination programmes. If we don’t have a vaccine – as we don’t for COVID-19 – achieving herd immunity would require a significant proportion of the population to be infected and recover from COVID-19. So what would this mean for the spread of the disease in the UK?

The percentage of the population that needs to be immune to enable herd immunity depends on how transmissible a disease is. This is measured by the term R0, which is how many new infections each case will generate. For COVID-19, the R0 is estimated to be 3.28, though studies are still ongoing and this number will probably change. This means that for herd immunity, about 70% of the UK population would need to be immune to COVID-19.

Achieving herd immunity would require well over 47 million people to be infected in the UK. Current estimates are that COVID-19 has a 2.3% case-fatality rate and a 19% rate of severe disease. This means that achieving herd immunity to COVID-19 in the UK could result in the deaths of more a million people with a further eight million severe infections requiring critical care.

However, it is not clear how much of this discussion of herd immunity – reportedly proposed by David Halpern, chief executive of the Behavioural Insights Team, and later blogged about by Robert Peston – is actual government policy.

Also, the concept as discussed is not simply to let the disease run its course through the population, but to slow its spread and protect those most vulnerable from severe disease.

Slowing the spread of COVID-19 is a promising strategy, especially when combined with enhanced measures to protect the elderly and those with underlying health conditions. By slowing the spread of the disease, the NHS might have more time to prepare, we might be able to develop treatments or vaccines and we will be closer to the summer when we have lower incidences of other diseases that burden the NHS, such as the flu.

A delay strategy when combined with surveillance and containment, as recommended by the WHO, could be very effective in combating the spread of COVID-19. Yet if we slow the spread of the virus but are relying on herd immunity to protect the most vulnerable people, we would still need 47 million people to be infected.

Even if we manage to protect the most vulnerable people (though no discussion is provided on how this will be done or for how long) the fatality rate for the otherwise healthy portion of the population may still be 0.5% or higher. This means that even in this unlikely “best case” scenario we would still be looking at more than 236,000 deaths.

We can and we must do better than that. China is rapidly controlling the spread of COVID-19 without requiring herd immunity (only 0.0056% of its population has been infected). Waiting for herd immunity to COVID-19 to develop in the UK by letting the virus “pass through the community” is not a good public health strategy.