How low should we go? The challenge of eradicating an infectious disease

The idea of eradicating AIDS, malaria, or indeed any infectious disease, is hugely appealing. Eradication means no more disease and subsequently, no need for interventions. It removes the need for costly vaccination programmes which are constantly challenged to stretch across hard-to-access populations in war zones or remote locations. We no longer face a threat of resurgence should we ever let our guard down. Eradication is what economists call a global public good: everyone benefits.

So far, we have only eradicated one human disease: smallpox. That was a remarkable accomplishment, as there were ten million cases across forty-three countries when the campaign began in 1967, yet the world was declared smallpox-free (other than some laboratory stocks) just thirteen years later. We are getting close with polio – although pockets of infection remain – and with dracunculiasis – an unpleasant worm infection in the tropics. Both campaigns have been running for over thirty years.

Eradication won’t happen unless key conditions are met. First, a robust system must be in place for detecting cases and monitoring levels of infection. Second, we need a way of interrupting transmission, such as a highly effective and easy-to-administer vaccine. Third, there has to be a consensus as to how to proceed and a shared commitment to seeing the programme through.

In the terminology of public health, elimination is not eradication. The World Health Organisation defines elimination as the reduction to zero incidences of infection in a defined geographical area as a result of deliberate efforts, with minimal risk of re-introduction, and with continued measures to prevent re-establishment of transmission. The US eliminated malaria in the 1950s. A few years ago the UK was considered to have eliminated measles, but we have since lost our measles-free status.

The problem with measles is a steady trickle of imported cases that sometimes spark chains of transmission. Even so, thanks to a highly effective vaccination programme, measles in the UK is nothing like the public health problem it was fifty years ago. We keep close tabs on measles cases – it’s a notifiable disease – and it’s important to keep vaccine coverage high. We will continue to do both whether the UK is technically measles-free or not. Unlike eradication, elimination is not an end point.

A leading science journal recently asked a hundred experts about the prospects of eradicating or eliminating COVID-19. Hardly any of them thought that eradication was achievable in the foreseeable future. A minority thought elimination might be, but not everywhere and not quickly. Most experts have believed from the onset of the current pandemic that COVID-19 is here to stay.

Living with COVID-19 is a much less worrying prospect than it seemed last spring. We are rolling out an effective vaccine developed in under a year – an extraordinary achievement by any standards. As a result, the public health burden is quickly becoming far more manageable. The fewer cases the better but there is no need to aim for zero and little chance of success if we tried.