Herd immunity: a misunderstanding

Many people have been led to believe that if 60 percent of the population were infected with Covid-19, the rest would be protected. This is based on a misunderstanding, write Philip Hill and Matthew Parry.

Covid-19 presents an unprecedented challenge to the world.

With approximately 1 percent mortality in those infected overall and much higher mortality in the elderly population, most countries have been driven to lockdowns and other extreme measures to contain the virus.

One approach that was considered early in the pandemic was to build herd immunity to the virus by allowing it to run through the population. Many people have been led to believe that once 60 percent of the population is infected, the rest are protected from Covid-19. However, this belief is based on a misunderstanding.

The concept of herd immunity comes mainly from our understanding of how vaccines protect populations and not just individuals. We can estimate what proportion of the population needs to be vaccinated to maintain elimination of a pathogen, using a simple calculation related to the pathogen’s transmissibility. For example, because measles is highly transmissible, we know that we have to maintain well over 90 percent vaccination status to keep it from causing an epidemic when it next enters the country.

The equivalent ‘threshold’ for Covid-19 is not known exactly, but it has been estimated to be about 60 percent. This is because the virus that causes Covid-19 is much less transmissible than the measles virus.

As described by Carl T. Bergstrom and Natalie Dean in a New York Times op-ed in May, the problem is that you can’t take the principles of vaccine herd immunity and apply them directly to a pandemic. In the case of a vaccine, we protect people before anyone becomes infected, but in a pandemic, infection is rife.

When 60 percent have been infected and become immune, the number of other people who are actively infectious is still around 20 percent of the population. These people will also proceed to infect others with the virus. Thus, the 60 percent threshold doesn’t apply because of the sheer number of actively infectious people at the time that threshold is reached. Instead, modelling shows a herd immunity approach could lead to around 90 percent of the population becoming infected and the virus still not eliminated.

The graph below illustrates the difference between reaching the 60 percent threshold through the virus itself and reaching it through vaccination before introduction of the virus. In a fully susceptible population (Figure A), after Covid-19 engages a population of five million with 10 initial infections, the percentage of the population that becomes infected and immune reaches approximately 90 percent over time.

The percentage that is actively infected and infectious peaks just before 60 percent become immune, driving ongoing infections. In contrast, if the virus enters the population when 60 percent are already immune through vaccination (Figure B), the rest of the population remains susceptible but is protected by herd immunity and the virus is unable to take hold.

We can combine these principles when considering what proportion of the population will need to receive a new vaccine if it becomes available in the midst of a pandemic. If the vaccine arrives in the presence of a large number of active cases, a very high proportion of the remaining susceptible population will need to be vaccinated to avoid more infections. If there are no active cases at the time the vaccine is introduced, approximately 60 percent of the population need to have been infected already or to be vaccinated to achieve herd immunity and protect the remainder of the population from a new introduction of the virus.

As a concrete example, the United Kingdom has already suffered over 40,000 deaths. Assuming a 1 percent Covid-19 case mortality, the 40,000 deaths suggest that four million people have been infected with Covid-19, which is only 6 percent of the population. If the virus is left unchecked and infects 60 percent of the population, there could be up to 400,000 deaths; if it infects 90 percent of the population, there could be up to 600,000 deaths.

Figure: Percentage immune (blue) or actively infected (gold) over time (days) for Covid-19, starting with the introduction of 10 infections into a population of five million, according to whether the population is initially fully susceptible (A) or 60 percent are already immune due to vaccination (B).

In the absence of a vaccine or truly effective anti-viral agents that reduce mortality, the percentage of the population that ends up being infected, and the number of subsequent deaths, can be reduced if there are effective measures to slow down or eliminate the virus. For Covid-19, the options are lockdowns and high-performing rapid case contact management in the context of general social distancing and protection measures.

A strategy of elimination would be ideal; otherwise, public health measures need to be sustained at a relatively high level. Indeed, early elimination (such as has been achieved in New Zealand), is better than delayed elimination because there are fewer deaths.

As described above, the percentage of the population immune in the UK so far is very low and of little advantage. Furthermore, in the presence of effective public health measures, the percentage of the population that is immune will decrease with time because of new births and low numbers of new infections.

A policy of herd immunity has become unacceptable already in many countries, but it becomes even more untenable if one considers the actual percentage of the population that could end up being infected and die with such an approach.

Until a vaccine is available, the effectiveness of the post-lockdown strategy of the UK and other countries - comprising careful opening up the country after building up a very high testing capacity, a large contact tracing workforce and other general measures - will determine the outcome of the pandemic. Fortunately, many Asia-Pacific countries, and some other European countries, have shown that there is every chance of success.

Acknowledgement: We thank Professors Harlene Hayne, Tony Ballantyne, Richard Barker and Rod Jackson, and Associate Professor Brian Cox for interactions related to this article.

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