One year after the first patient was diagnosed with covid-19, it seems that we are finally facing the final stretch of the health crisis. Pandemics are over when a significant proportion of all people on the planet have antibodies to the microbe that causes them, a situation called herd or group immunity. The more people are resistant to the disease, the less effectively it can spread and, in the end, the virus or bacteria only has the ability to cause localized outbreaks.
This herd immunity, which for covid-19 would have to be at least 70% or 80% of the world's population, is only achieved in two ways: spontaneously, or through vaccination. The former has been the most common route throughout history, until science advanced enough to make mass immunization possible. But even now some pandemics evolve this way. For example, the 2009 influenza A(H1N1) pandemic was resolved without a vaccine, mainly because many people had antibodies to viruses from the same family that also protected them from the new virus. The most obvious drawback of letting group immunity come naturally is that it requires a large number of people to be infected, and this can lead to a very high number of deaths.
Natural group immunity
In the case of covid-19, this idea was already proposed at the beginning as a possible way to respond: the British government announced that it would choose to let the disease run its course without taking strict precautions. The strategy lasted only a few days because many experts warned of the cost in lives it would have. However, the idea resurfaced a few months later thanks to the Great Barrington Declaration, a proposal by a group of recognised experts who called for protecting only the most vulnerable and allowing the rest of the population to be infected freely. Once again, the response against it was almost unanimous, starting with the WHO, because of the high risks (there are covid-19 fatalities in all age groups), the practical complications of carrying it out, and the lack of scientific data to back this up.
However, some countries chose to implement lenient restrictions that favoured, in part, the path to spontaneous immunity. The best known example is Sweden, which for a time seemed to work. But it soon turned out not to be like that: in summer Sweden already had a mortality rate twelve times higher than Norway, seven times higher than Finland and six times higher than Denmark, its immediate neighbours, without herd immunity being particularly important. It does not even appear that this protected the country's economy, which was one of the main objectives of the proposal.
The Manaus case
The consequences are even more evident if you look at what happened in Manaus, Brazil. An article published in Science magazine this week analyzes the effects of not knowing how to protect the two million inhabitants of this Amazonian city, where socioeconomic factors made it very difficult to apply restrictive measures. The result is that between 44% and 66% of the population was quickly infected. By June there were already so many people with antibodies that the pandemic began to go into remission on its own, rather than due to the success of measures of control, and the number of cases has since then remained low. Analysis of IgG antibody levels shows that seven months after the first case in the city, group immunity of about 75 per cent had been achieved spontaneously, which is believed to be the highest figure on the planet.
The problem is that this has resulted in confirmed covid-19 mortality data of 1,193 people per million inhabitants as of 1 October. This is twice as many as the United Kingdom or the United States at that time, two of the countries with the worst statistics. And it would have been more serious if it were not for the fact that Manaus' population is very young: only 6% are over 60 (in Spain and other developed countries, this percentage is at least three times higher). The example of Manaus, then, confirms that it is not feasible to rely on spontaneous immunity when it comes to controlling a disease as infectious as covid-19: the only acceptable option is mass vaccination.
It is important to remember that herd immunity does not automatically eliminate the microbe from the planet. It can only be achieved when virtually the entire population has antibodies for a long time, which is difficult. That is why in the whole of history we have only managed to eradicate one pathogen and the disease it causes: the smallpox virus and the rinderpest virus. Other microbes we have mastered thanks to vaccines (polio virus or measles virus, diphtheria bacteria, etc.) continue to circulate and cause outbreaks, especially in areas where vaccination is deficient.
Although these diseases are almost non-existent in developed countries, they are still a health problem in many places, which shows how important vaccines are. If we were to not get vaccinated, these diseases could become pandemic again.