Herd Immunity

Herd immunity is the indirect protection from a contagious infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection.
This means that even people who aren’t vaccinated, or in whom the vaccine doesn’t trigger immunity, are protected because people around them who are immune can act as buffers between them and an infected person.
Once herd immunity has been established for a while, and the ability of the disease to spread is hindered, the disease can eventually be eliminated.
This is how the world eradicated smallpox, for example.

The more infectious a disease, the greater the population immunity needed to ensure herd immunity.
Example: measles is highly contagious and one person with measles can infect up to 18 other people. This means that around 95% of people need to be immune in order for the wider group to have herd immunity.
Natural herd immunity – achieved through infection rather than vaccination – can be challenging to induce through unchecked infection as there would be a very high rate of serious illness and death, with health systems overwhelmed well beyond their surge capacity, even in high-income countries.
This is why herd immunity is generally pursued through vaccination programmes.
Even when vaccines are available, it is not always possible to achieve herd immunity for very long.
Some viruses, such as seasonal flu, mutate frequently, evading the body’s immune response.
So immunity doesn’t always last forever.

The new coronavirus has a lower infection rate than measles, with each infected person passing it on to two or three new people, on average.
This means that herd immunity should be achieved when around 60% of the population becomes immune to COVID-19.

A new study published in The Lancet has concluded that herd immunity against Covid-19 is difficult to achieve based on estimates of seroprevalence for the entire Spanish population.

It is a large-scale seroepidemiological study, and concluded that just five per cent of the Spanish population has developed antibodies in response to SARS-CoV-2, the virus that causes Covid-19, which implies that an estimated 95 per cent continues to be susceptible to the virus.

While a seroprevalence study largely provides information only about previous exposure to the virus, this study strengthens the line of argument that in the absence of treatment or a vaccine against Covid-19, achieving herd immunity at this stage is not possible.
Herd immunity is difficult to achieve without accepting the collateral damage of many deaths in the susceptible population and overburdening of health systems.
Any proposed approach to achieve herd immunity through natural infection is not only highly unethical but also unachievable.
Beyond Spain, the study sends signals to other countries: that even in countries that have reported high prevalence of Covid-19, to make them cautious about easing of restrictions.

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