What is herd immunity? Why do we need it? What will happen when we aren’t able to achieve it?

With considerable jubilance and consternation following last week’s press release regarding the Pfizer Covid-19 mRNA vaccine, this seemed like an appropriate time to discuss the impact of herd immunity.

In the early days of the first lockdown, herd immunity seemed to be the buzzwords on everyone’s lips. It became the by-words for allowing infection, by a virus we knew little about, to spread through the population. Some even deemed it to be the unofficial policy of the British and Swedish governments; to allow infections to rise to the point where there was significant immunity amongst the population. While these two governments have taken vastly differing approaches, whether motivated by a herd immunity strategy or otherwise, it is interesting to see how the term has come to have a changing meaning. 

In its initial parlance, herd immunity was used exclusively in a vaccination context to refer to when a population reached a threshold of vaccination that effectively meant that the population was protected from outbreaks or widespread infection (https://www.who.int/news-room/q-a-detail/herd-immunity-lockdowns-and-covid-19). An illustrative example of what herd immunity is can be seen in the following WHO example:

“The percentage of people who need to have antibodies in order to achieve herd immunity against a particular disease varies with each disease. For example, herd immunity against measles requires about 95% of a population to be vaccinated. The remaining 5% will be protected by the fact that measles will not spread among those who are vaccinated. For polio, the threshold is about 80%” (ibid). 

The use of the term that seems to have spread during this pandemic is a mischaracterisation, in that it became widely understood to mean that once enough people had caught the disease, that there would be immunity in the community. The two meanings are vastly different, in that in the first context herd immunity can only be achieved through proactive protective measures, whereas in the second context, herd immunity can be achieved through inaction (or a lack of affirmative action to stop the spread). If we’ve learnt anything from 2020, it is that words have meaning. The rationale accorded to the second meaning is that vaccination is not required and everything will sort itself out. Sadly, this rationale is flawed. 

The importance of herd immunity achieved through mass vaccination to protect a community is very clearly illustrated in the following six second video, made by Reddit user theotherredmund using 1993 data published in the Epidemiological Reviews.

This demonstrates just what happens when herd immunity is achieved through a consistent and widespread vaccination programme. Even if an infected person comes into the community, the virus soon runs out of people to infect and dies out. It also shows what happens without a vaccine or where vaccination rates are not high enough, allowing an infectious disease to spread rapidly with devastating effect.  

In today’s globally connected world, the speed at which Covid-19 infection spread should not be a surprise. And with over 1.3m deaths globally from the disease, the only route to a return to normality is widespread vaccination. 

What is clear from last week’s news is that when the vaccine(s) for Covid-19 become available it will be of the utmost importance for all those who are able to be vaccinated, to do their part. Without the vast majority of people in a community being vaccinated, the impact of the vaccine(s) themselves will be limited, possibly even negligible, as Covid-19 will still be able to spread and thrive in our communities. 

This is particularly important for those who are older or have underlying health conditions, who are most greatly impacted by the virus. This, together with our increasing knowledge about the length of time immunity lasts (following infection), it can be reasoned that without approximately 70% of the population being vaccinated, the disease will continue to spread including through re-infection. (https://www.wired.co.uk/article/coronavirus-vaccine-hesitancy-pfizer)

While there may be early skepticism about the Pfizer vaccine, as few details have been published to date, it is important to keep an open mind regarding vaccinations generally. However, many people are cautiously optimistic regarding this breakthrough:

“… a global survey of more than 13,000 people across 19 countries reveals widespread ‘vaccine hesitancy’. Overall, 71.5% of people surveyed said they would take a proven safe and effective vaccine – but 14 per cent said they would refuse it outright, and another 14 per cent said they would be hesitant to take it. In the UK, 36% of people have said they are uncertain or unlikely to be vaccinated against Covid, and in the US that figure rises to 51 per cent” (ibid).

If these surveys are representative, it becomes clear that it will be difficult to achieve herd immunity and eradicate Covid-19 without an in-depth education programme and understanding of vaccine offerings. 

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