The term ‘herd immunity’ keeps being raised during the Covid-19 pandemic. What does it mean? Herd immunity has been used as a vaccine strategy for diseases such as measles, mumps and rubella and means vaccinating the bulk of the population, thus preventing the ongoing transmission of these infectious diseases, and protecting the vulnerable (eg immuno-compromised) who cannot have the vaccine.
In the absence of a vaccine, it means having a certain percentage of the population contract the virus (50-80 per cent), develop antibodies or a T-cell response, and then have lasting protection from Sars-CoV-2 and not transmit it to others. A herd-immunity strategy without a vaccine has been advocated as segmenting the population by age, and asking vulnerable/elderly groups to ‘shield’ while the virus runs through the young and healthy members of the group (‘the herd’).
Why is this a dangerous approach? First, complete shielding on its own has not worked in any country that has tried it given the difficulties in separating the healthy from the weak, for example, those who work in care homes, who live in multi-generational housing, or who teach in schools and work in essential services. Would this mean removing all children from schools whose parents or household members are in a vulnerable category, or work with vulnerable groups? East Asian countries did not even try this approach: it was one taken by the UK, Sweden and the Netherlands.
Second, it is unethical to lock vulnerable and elderly people away at home for an indefinite amount of time. Seroprevalence studies (which show the extent of previous infection – antibody prevalence – in the population) fall under 10 per cent in Scotland, indicating that it would take over a year, if not longer, to reach some kind of population-immunity.
Third, immunity is still a large unknown, with several cases of re-infection already documented. Having the virus once does not guarantee immunity for life, and other coronaviruses do not provide lasting immunity. A big question is whether re-infections also make individuals infectious to others, or not.
Fourth, there are increased cases of Long Covid, a chronic post-Covid syndrome, affecting previously healthy, young people (largely aged 30-59) who are suffering for months from fatigue and heart, lung or kidney problems. Covid-19 is a multi-system disease and recommending young people get infected is like playing Russian Roulette with young people’s health given these complications. It is not a harmless virus in all young people. In the US, 935 children have been hospitalised with Multisystem Inflammatory Syndrome (MIS-C), which is a rare condition, but does occur if prevalence increases.
As the four-nation agreement states, to get the maximum normality requires suppressing the virus overall with the minimum restrictions and minimal impact on economy and society. This is the balance that every country is trying to find and the trade-offs they involve.
Even New Zealand, with life largely back to normal on the islands, has made the trade-off of bubbling itself off from the rest of the world. Restrictions might mean 90 per cent normality in daily life, or 60 per cent normality, but lockdown/release is not the best way of seeing the coming year, but rather a gradient of moving towards more relaxation or more stringency depending on control of the virus.
And finally, beware of theories and those offering comforting lies over policy arising from public health practice and history. Even after decades and centuries, we do not have herd immunity to cholera, yellow fever, polio, measles, TB, malaria or plague. Public health measures have been used to control their spread until vaccines or elimination strategies were developed.
Professor Devi Sridhar is chair of global public health at Edinburgh University and advises the Scottish Government on Covid-19
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