That mass vaccination would be “the beginning of the end of the pandemic” has been said for a long time. However, no one dares to estimate how long that end will be, when we will return to “normality”. The problem is that that (and not another) is the million dollar question.
For a long time, we have lacked data to answer it. In fact, this is still the case in almost every respect. However, the start of vaccination campaigns around the world and the government’s own vaccination plans give us a frame of reference to try to shed some light on the coming months of the pandemic and the challenges that they will demand of us.
What role do vaccines play in curbing the pandemic? Group immunity.
One of the concepts that has been handled the most during this pandemic year is that of “group immunity” (also sometimes called “herd immunity”). Is about an indirect protection against an infectious disease that occurs when a population is immune. In more technical terms, we could say that “the risk of infection among susceptible individuals in a population is reduced by the presence and proximity of immune individuals”.
From the point of view of an infectious pathogen, complex societies are huge labyrinths in which the immune act as walls. Naturally, when an outbreak occurs, infectious agents try to cover the entire population likely to be infected: more immune people, more dead ends; fewer opportunities to find new hosts; more chains of contagions that will dissolve into nothingness.
Perhaps the best way to see how this risk decreases is with this animation that simulates the evolution of an outbreak in scenarios with different percentages of population immunization.
It sounds like sheer common sense, and in fact it was first formulated over a century ago. But until the 1970s, when the eradication of smallpox (the last contagion natural was detected on October 26, 1977 in Somalia) and the cost benefit analysis of vaccination began to be taken seriously, we did not fully understand the health implications of this idea. Today, it is not only one of the key arguments behind mass vaccinationsRather, it has become central to today’s public debate.
After a year of pandemic (and after analyzing different strategies to combat the pandemic) there is a general consensus that, as the WHO emphasizes, group immunity can only be an achievable goal if it is achieved “by vaccination”. Allow “the disease to spread to any segment of the population […] it would lead to unnecessary cases and deaths. ”
However, As COVID vaccination campaigns get underway, there is a fundamental question: When will we achieve herd immunity? In other words, what is “the threshold of immune people after which we should expect a decrease in the incidence of the disease”: how many people do we have to vaccinate and when are we going to achieve these percentages.
Group immunity in numbers
A priori, it might seem that defining what that threshold is is an easy task. However, during these months we have seen analyzes that put that threshold at 60%, 43% or even 10% of the population. We already know that the estimates that spoke of very low thresholds were wrong, but it is that, just a few days ago, the director of the US National Institute of Allergies and Infectious Diseases and one of the most visible experts in the fight against COVID in the country, Anthony Fauci acknowledged in the New York Times that their estimates already raised that figure to 90%Figures very similar to those needed to curb measles.
Fauci is not alone in this. The general trend among experts has been to raise the threshold as we improve our knowledge about the transmissibility of the virus and our understanding of “transmission super-events” advances. In part, because even today we have approximate and partial data. In part, because finding this threshold is difficult even on a theoretical level.
In the “emphasis on simple thresholds” the “heterogeneity of the population” is forgotten, as one of the leading experts on the subject said in 1971). And it is true, Each population group has its own characteristics (social, labor, urban, etc …) and, therefore, a specific threshold. The 60-70% range was initially calculated on data from China and Italy (and has been supported by natural experiments and animal studies); however, the differences between countries can be very large.
However, this apparent contradiction has another explanation: the percentage of the immunized population that we will need to “control” the pandemic and that we will need to declare “COVID-free countries” is not the same. In the absence of better data, everything seems to indicate that indeed, we will need to make immune between 60 and 70% of the population to start controlling the pandemic (and that the waves become slow and manageable), but only with very high numbers of vaccinated citizens (above, in any case, 80%) COVID will continue among us on a regular basis.
When will we achieve group immunity?
Spain, unlike other countries, does not have an exhaustive calendar for the entire vaccination process. However, the Government has maintained on several occasions that its objective would be to vaccinate 2.5 million people in a first stage (until the end of March) and, already in a second stage, reach “May or June” with “between 15 and 20 million vaccinated Spaniards.”
In other words, according to the Spanish Government’s own plans, in the best of cases the country it would reach the summer with 42% of the population vaccinated. Far from 60-70% of the first group immunity threshold. Far even if we add that 9.9% to the accumulated prevalence found in the last wave of the seroprevalence survey of the Ministry of Health (something that, in any case, is not possible because, at the moment, vaccination protocols do not distinguish between people with antibody to everyone regardless of whether they have antibodies or not).
With this in mind, and taking into account that some of the most proactive regional administrations do not expect reach 60% of the population before the end of the yearIt seems clear that those 100% objectives that were being managed in recent months have already been discarded as logistically and healthily unfeasible. The question is no longer when we will achieve group immunity (because speaking a year from now in the midst of a pandemic is virtually impossible), but whether we can achieve the goals that the government itself has set.
How is vaccination progressing? We’ll arrive on time?
Today, only 1.04% of the Spanish population has received the first dose of the vaccine. Obviously, if we compare it with 22.34% in Israel or 12.9% in the United Arab Emirates, it is not enough. Even if we compare it with 2.02% in Denmark, the leading country of the Union, which started vaccinating the same day, Spanish figures are not as good as we would expect. However, we are among the ten countries with the highest number of immunized people. Of these, only three Europeans (Denmark, Slovenia and Italy) surpass us.
It’s more, are acceptable figures taking into account the traditional problems of Spanish health to vaccinate and that, during the first weeks, lack of control and lack of foresight slowed down the process. According to our data, if the pace of last week were to be maintained, we would reach the figure of 2.5 million vaccinated Spaniards at the end of March. Right on time.
However, especially if you improve available vaccines, we have room for improvement. As you can see in the number of lives to save.
What raises more doubts is the target of 17.5 million for the end of June. As can be seen in the graph below, if we want to achieve this objective we must significantly increase our vaccination rate. It is not impossible and, in fact, we know that, when the vaccine begins to be given in health centers to larger population groups, the rate must necessarily increase. However, seeing the problems that we have been carrying for weeks and what seems like a third wave much more pronounced than the second, the challenge is not trivial.
On the contrary, the analyzes show that getting 40% of the population immunized will require a very important effort from the health system. We do not yet have definitive figures for the flu campaign, but (if we look at it with historical perspective) it is a viable goal. However, if more resources are not added, we will be asking primary care to work beyond its real capacity knowing that the waves of the pandemic will continue to arrive, with similar force, until the end of the year.
Image | Hospital Clinic