Explainer: What is herd immunity and how do we achieve it?

The percentage of people who need to be immune in order to achieve herd immunity varies with each disease, according to the World Health Organization (WHO).

The Covid-19 pandemic has been defined by volatility and unpredictability. 

Things have changed once again, predominantly as a result of the Delta variant, which is having an impact on our hope of exiting the pandemic through herd immunity.

This form of immunity, also known as population immunity, is a form of indirect protection from infectious disease that can occur when a sufficient percentage of a population has become immune to an infection.

It can be achieved through either vaccination or previous infections, and it thereby reduces the likelihood of infection for individuals who lack immunity.

The percentage of people who need to be immune in order to achieve herd immunity varies with each disease, according to the World Health Organization (WHO).

Last summer, the WHO estimated that to achieve this immunity for Covid-19, approximately 60% of the population would need to be vaccinated.

However, the emergence of variants has changed the situation once again.

The Delta variant, which is now responsible for more than 90% of new cases in the State, is estimated to be between 40 and 60% more transmissible than the Alpha variant, which already had a 50% higher transmissibility than the original Wuhan strain of the virus.

Dr Colm Henry, CCO, HSE at Dr Steevens’ Hospital for the weekly HSE operational update on the response to Covid-19. Photograph: Sam Boal / Photocall Ireland

As a result of this increased transmissibility, it is estimated between 85% and 90% will need to be vaccinated in order to achieve this status of immunity, according to Chief Clinical Officer of the HSE Dr Colm Henry.

Thus, to achieve this level of blanket immunisation, children would need to be vaccinated, he added.

But this causes a dilemma for health officials, who will be required to weigh up the benefits and risks of vaccinations for children, for whom the likelihood of contracting severe covid is quite low.

In May, the European Medicine's Agency (EMA) approved the use of the Pfizer vaccine in children aged between 12 and 15. The vaccine was already approved for use in over 16s.

The National Immunisation Advisory Committee (Niac) is currently examining the data and is expected to make recommendations to the Chief Medical Officer on the immunisation of young teenagers shortly.

However, according to Professor Philip Nolan, chairman of the Irish Epidemiological Modelling Advisory Group, it’s not as straightforward as that.

“It’s [herd immunity] an abstract concept that applies to a whole population as if everybody in the population is the same. It’s plausible that children under 12 to 15 years of age are somewhat different in how they contract and transmit this virus than children older than 12 to 15 years of age,” Prof Nolan told RTÉ’s Radio One.

Professor Philip Nolan, Chair of the NPHET Irish Epidemiological Modelling Advisory Group said the calculation for herd immunity is quite complex. Photograph: Eamon Farrell / RollingNews.ie

“The calculation for herd immunity becomes quite complex and I honestly don’t think there’s a magic figure where if we can reach that level of vaccine penetration, that everything returns completely to normal.” 

He added that the priority should be reaching the highest level of vaccine protection in the adult population first, before thinking about teenagers and then, beyond that, thinking about the vaccination of children.

“We need to think about it for each of those three cohorts, what are we protecting them from, and then what are we protecting everybody from," he added.