Herd immunity and the coronavirus

Stephen Leeder

The first preliminary (Phase 1) trial in humans of a coronavirus vaccine in the US is a good moment for reflection on the immunology of this illness.

We need to be cautious. It is not the virus that leads to interstitial lung disease and death. It is our immunological response, the “cytokine storm”, that causes severe illness.

This is comparable to the devastation of the respiratory syncytial virus, persistently responsible for bronchiolitis in the very young, killing tens of thousands worldwide each year.

If the vaccine is both effective and safe, enough people will be immune, and transmission from them to the rest of us will be much diminished. Indeed, if enough people are immune — generally about 60% (“herd immunity”) — either through exposure or vaccination, transmission ceases.

Herd immunity in the UK

Recent intense British debate has considered whether transmission should not be excessively impeded, thereby permitting future herd immunity. Herd immunity would be less likely if stringent measures are used to prevent spreading.

The UK’s chief scientific adviser, Sir Patrick Vallance, outlined the approach taken since January by the experts, epidemiologists, disease modellers and others exploring the dynamics of the current pandemic. They did not entirely discount the benefits of widespread infection in raising herd immunity.

Dr Richard Horton, editor of the Lancet, wrote in The Guardian that Graham Medley, one of the UK government’s expert scientific advisers, explained that the UK’s approach was “to allow a controlled epidemic of large numbers of people, which would generate ‘herd immunity’”. Vallance suggested that the target was to infect 60% of the UK’s population.

The UK has not, until recently, advocated severe social distancing, such as closing schools and discouraging large gatherings, as have other countries. (Vallance asserts that the risk is greater in small gatherings.) Other experts are surprised that tougher action was not taken sooner.

Into the unknown

Herd immunity is built on the assumptions that most people will have only a mild illness, and that our immune response will be like those we mount against diseases for which we currently successfully vaccinate.

However, we know little about the universe of infected people. We can count those who require clinical care or who die. Mild cases are invisible. Screening a probability sample of the population might reveal the proportion of people with mild disease.

In these early days, we also don’t know how effective natural immunity will be, but the recovery of many people with mild cases in China and elsewhere is encouraging. How long that immunity lasts will determine how much coronavirus infection we will experience in future.

Dr William Hanage, a Harvard professor of the evolution and epidemiology of infectious disease, wrote recently in The Guardian that we should “learn from South Korea, Singapore, Hong Kong and Taiwan, all of which have so far done a good job mitigating the worst outcomes despite having reported cases early in the pandemic, and in the case of South Korea, suffering a substantial outbreak”.

For Hanage, the approaches of those countries — which have been energetic in tracing contacts and implementing containment, including vigorous handwashing, school closures, social distancing and self-quarantine ­— are the way to go. However, these countries are geographically small.

The dangers of depending on natural infections to achieve herd immunity were recently spelled out in the Sydney Morning Herald. Professor Raina MacIntyre, head of biosecurity research at the Kirby Institute, a medical research centre concerned with HIV/AIDS, viral hepatitis, sexually transmitted infections and other infectious diseases based at UNSW, wrote that achieving the necessary infection rate for herd immunity would take Australia “into the catastrophic range of epidemic scenarios, worse than what we are seeing in Italy right now and far worse than China … viruses cannot be instructed to only infect healthy young people, so allowing transmission will result in vulnerable people becoming infected and dying”.

The UK government has since announced a sudden U-turn, declaring that new modelling by Imperial College scientists had convinced them to change their initial plans in favour of radical lockdown.

It is likely that we will see continued endeavours toward containment and mitigation and massive efforts to develop a vaccine and effective therapeutic pharmaceuticals. Herd immunity will await a vaccine.

Stephen Leeder is Emeritus Professor of Public Health and Community Medicine in the Menzies Centre for Health Policy at the University of Sydney and Editor-in-Chief of the International Journal of Epidemiology.

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