During an interview with BBC Radio 4, the government’s chief science advisor expressed his take on how to reduce transmission using the term ‘herd immunity’. Two days later, the country's secretary of state for health and social care had to stress to the worried public that ‘herd immunity’ is not the official policy. So what exactly is herd immunity and why is causing so much controversy?
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In epidemiology, the basic reproduction number of a disease means the expected number of infections that can be generated by one case. In other words, the number of people one sick person is likely to infect on average (represented as R0 ). One way to decrease the basic reproduction number and thus stop an epidemic is by vaccination. If a person is likely to infect two other people (R0 = 2), then vaccinating 50% of the population and making them immune would be enough to prevent an epidemic. If a person who is not vaccinated for measles, for example, is surrounded by people that are, he is protected from the disease despite not having gotten the vaccination himself.
Unfortunately, there is no vaccination against COVID-19 yet. In a good scenario where herd immunity against coronavirus occurs naturally, the majority of people would get infected, recover, and from then on be immune. This is opposed to the social distancing method, which aims to prevent infection as much as possible. The logic behind the natural herd immunity strategy is to minimize the risk of a second wave of coronavirus, similar to the second resurgence of the Spanish Flu in 1918.
To achieve herd immunity, over 60% of a country ‘needs’ to get infected. If we use the UK as an example, that is a population of 66 million people. That means over 40 million infected individuals. Mortality rates are currently estimated to be 1% globally, depending on access to health care and other variables. In the worst scenario, one million people could die of the disease before herd immunity is achieved. Furthermore, the National Healthcare System is at risk of being overwhelmed and unprepared for the sheer number of patients needing intensive care.
In light of those risks, a group of more than 200 scientists and health professionals have expressed their concern in an open letter to the government. They stressed the dangers of such an approach and called for more strict social distancing measures to be taken. Other experts, like Tony Costello, a former director in the World Health Organization pointed out that it is not yet certain that those who contract Covid-19 even become immune to it.
As a result of the severe criticism, the Department of Health and Social Care tried to clarify that the BBC interview was misinterpreted. For the time being, it seems the British government is backing away from herd immunity and is leaning closer towards social distancing, although still not in the same seriousness as the rest of Europe.
Bonus Video: Herd Immunity, Explained
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