Last March, group immunity was a topic that was on everyone’s lips. Then came the first preliminary results of the seroprevalence study, according to which only 5% of the Spanish population had antibodies. This took us away from the percentage necessary for the population to stop transmission of the virus.
What percentage is necessary?
The group immunity it is the level of population immunity to a disease that causes its spread slow down and stop, even after relaxing all preventive measures, because there are not enough new people to infect. If this threshold is not exceeded, you can start a second wave of infection as soon as the restrictions are lifted.
The group immunity equation is defined as hc = 1 – 1/R₀, where R₀ is the basic breeding number, that is, the average number of new infections caused by a typical infected individual during the early stage of an outbreak in a population fully susceptible to infection.
If we review the data for Spain, we can also remember that the R₀ value varies with circumstances and with time.
Suppose, according to Figure 1, a R₀ of 2 for SARS-CoV-2. That means that each infected person infects, on average, two others. In that case, applying the formula, the threshold for COVID-19 is 0.5 (50%). In other words, the virus will spread at an accelerated rate until half the population is immune.
At that point the virus will still spread, but at a slower rate until it stops completely. Just as a car doesn’t stop the moment the driver takes his foot off the gas, the virus does not disappear the moment it is reached group immunity. Although the number of infections is starting to decrease, another part of the population is likely to get it while the disease is disappearing elsewhere.
That 50% is also the threshold, beyond which new introductions of the virus transmitted, for example, by an infected passenger disembarking at a city airport with group immunity will quickly run out. It doesn’t mean I can’t start an outbreak, but it won’t spread.
However, things are not so simple. No model that claims to reflect reality will present human populations as homogeneous, because in human societies there are many heterogeneities that will influence the transmission of the virus.
For starters, it all depends on how many people each infected person infects, a number that can vary depending on the circumstances. Although an R₀ = 2 for SARS-CoV-2 may be a reasonable mean for a large population, it is certain to vary considerably at a more local level. The mean will be much higher in some places and lower in others.
As R₀ turns out to be a variable, and not a fixed number, the path through which people acquire immunity also varies, which has important implications for calculating the threshold of group immunity. Differences in social habits make some people more exposed to a disease than others.
An infected person living in a block of flats can infect many more people than if you were in a Rural environment. Thus, the available data indicates that the threshold may be more than double in some urban settings than the overall average for a country.
Nor is the activity carried out by an infected person the same. If, for example, you are dealing with a professional with a lot of social contact (as was the case with unprotected health personnel at the beginning of the pandemic), the threshold will be higher. The percentage It could be low as long as many people wear masks and avoid large gatherings, or much larger if people let their guard down.
Biological, genetic and social determinants
Biological and genetic differences also play a role in how likely people are to become infected and fight the virus. Nor can we forget the importance of Social determinants to explain the health of both populations and individuals.
Epidemiologists refer to these variations as “susceptibility heterogeneity.” That is, the differences that make some people more or less likely to get it. In a pandemic like the current one, that heterogeneity has real implications for calculating group immunity. In some cases the required percentage will greatly increase. This is what has happened in places such as nursing homes, whose inhabitants were more susceptible to coronavirus than the national average.
A larger scale heterogeneity generally lowers threshold for group immunity. According to standard models, around 60% of the Spanish population would need to be vaccinated against COVID-19 or recover from the disease to reduce the speed of spread until it stops. If 60% of the population were immunized, the coronavirus that tried to pass from one person to another would fail in six out of ten attempts and would end up being one of the many infectious agents that the world has known.
Recent research has attempted to quantify heterogeneity. In June, the magazine Science published a study that estimated the threshold for COVID-19 at 43% in large populations. Another study, who takes a different approach to estimate differences in susceptibility, estimates that 20% of the immunized population could suffice.
Whatever that percentage, even in the lowest scenario (20%), the third and last report on seroprevalence published last Monday shows that we are still very far away.
The percentage (5.2%) has hardly changed in the three deliveries of which the study consists and confirms that, despite the great impact of the pandemic in Spain, these figures are not high.
It does not seem possible that we will reach this threshold without effective vaccine and available on a large scale. For that there is more than a year, at best.
Ultimately, the only way to truly escape the COVID-19 pandemic is to achieve group immunity on a large scale, everywhere, not just in a small number of places where infections have been highest.
Meanwhile, the uncertainty about the threshold naturally acquired to prevent the spread of the virus and reduce the value R₀ As much as possible, it only leaves one way to go: distancing, facemasking, testing, and contact tracking everywhere. So calm and patience.
This article was originally published in The Conversation
Authors. Manuel Peinado Lorca. Professor of University. Department of Life Sciences and Researcher at the Franklin Institute for American Studies, University of Alcalá. Luis Monje. Biologist. Professor of scientific photography, University of Alcalá.