HERD IMMUNITY: CAN IT SAVE US FROM CORONAVIRUS?

How does herd immunity work, and is it a useful way to keep infection rates down?

In mid-March, as coronavirus spread across the UK, Sir Patrick Vallance – the UK government’s chief scientific advisor – said that, to control the impact of the virus, the country would need to build up “some degree of herd immunity” and that about 60 per cent of the population would need to become infected for this to happen. His comments soon came under fire. Over 500 scientists from UK universities wrote to the government, saying that going for herd immunity would risk more lives than necessary.

COVID-19 has an estimated fatality rate of around 1 per cent. If 60 per cent of the UK population became infected – some 40 million people – that could equate to hundreds of thousands of deaths. The Department of Health and Social Care has since clarified that herd immunity is not part of the

UK’s action plan to tackle coronavirus, but is “a natural by-product of an epidemic”.

So where did the 60 per cent figure come from? And will we eventually experience the positive effects of herd immunity?

The ease with which a disease spreads is measured using the ‘reproduction number’, R0, which is the average number of people expected to catch the disease from a single infected person. For COVID-19, R0 is estimated to be around 2.5. Flu has an R0 of 1.3, while measles has an R0 as high as 18. The proportion of the population needed to become immune to achieve herd immunity (the ‘herd immunity threshold’) is calculated by 1 - (1/R0). For coronavirus, this gives the threshold of 60 per cent that was quoted by Vallance.

Crucially, a disease’s infectiousness isn’t set in stone. Measures can be taken to make it harder for a disease to spread, reducing the threshold. A few days after Vallance’s comments, Boris Johnson introduced new guidelines that help to do this, including stopping non-essential contact and travel.

“All the measures that we have put in place will reduce R0,” says Prof John Edmunds at the London School of Hygiene and Tropical Medicine. Edmunds is a member of the Scientific Pandemic Influenza

Group on Modelling, which is providing advice to the government. “Exactly by how much is hard to say at this time, as we have never tried these measures before. But overall, we would expect them to reduce R0 to a low level – even to below 1.”

If this number is kept low for long enough, then fewer people will need to become immune to reach the herd immunity threshold, and the epidemic will slow. But how long it would take to reach this point is tough to predict. Edmunds notes that, even once herd immunity is reached, the epidemic won’t suddenly stop, as there will still be infectious people generating new cases. “One other important strategy is to lower the number of infectious individuals so it is low at the point when the epidemic peaks,” he says.

There are currently three big unknowns when it comes to the coronavirus, says Dr Jeremy Rossman, a virologist at the University of Kent. Each of these will affect the disease’s future infectiousness. First, we don’t know whether the coronavirus will track our seasons. If the coronavirus prefers colder weather, like flu, then the epidemic could gain a second wind during the winter. Second, we don’t know how much this coronavirus will mutate. “Changes to the virus could make it better at evading our immune systems, or more easily transmitted,” says Rossman. And third, we don’t know how long those who are infected are subsequently immune for. Some other viruses in the coronavirus family, such as those that cause the common cold, only provide immunity for around three months. There’s evidence for short-term immunity with COVID-19, says Rossman, “but we don’t have the data yet to know whether it provides long-term immunity.” Rossman estimates that we’d need to be immune for at least a year, possibly two, in order to build up herd immunity. If this isn’t the case, he says, our best hope for herd immunity might be a vaccine, although this is still believed to be at least a year away. For now, he says that the best actions that governments can take are to keep the public informed, keep testing people, and reduce the spread by clamping down on social contact.

In fact, Rossman still thinks there’s a chance that we could control the epidemic before we need the protection of herd immunity. “China’s a great example: the virus didn’t create herd immunity, but they’ve managed to contain the outbreak and stop any new cases from spreading,” he says. “If we can do it there, we can do it around the world. It’ll be difficult, but I think it’s possible.”