Cemetery in Manaus, Brazil, on July 20, 2020
Photo: MICHAEL DANTAS / AFP
If the information is correct, the virologist Christian Drosten from the Charité in Berlin was right: Without countermeasures, he assumed, the Sars-CoV-2 coronavirus would spread exponentially until 60 to 70 percent of the population were infected, he said Beginning of the year.
Now there is evidence that this could have happened in the Brazilian city of Manaus.
There it was not possible to push back the pathogen.
As a result, the virus was so rampant that the population is already approaching what is known as herd immunity or has already reached it.
Sars-CoV-2 is no longer spreading as strongly in the 1.8 million city of Manaus because up to two thirds of the population have already been infected, researchers report in an evaluation that has not yet been professionally tested.
Ester Sabino's team from the Institute of Tropical Medicine at the University of São Paulo analyzed more than a thousand samples from blood donors that had been given since February 2020.
They discovered antibodies against the new coronavirus in 44 percent of the samples.
As the number of antibodies decreases over time, the researchers estimate that 66 percent of the donors were actually already infected.
However, the study has some weaknesses: Blood donors are not representative of the population.
In addition, the antibody value recorded by the researchers is more than double that from an earlier antibody study in Manaus at the end of May.
Many dead despite the young population
However, if the population were actually the first in the world to achieve herd immunity, the price was high: Based on projections, Sabino and colleagues assume that 0.28 percent of all those actually infected in the city died.
That corresponds to more than one death in 400 people who carried the virus.
Since not every resident of the city was infected, the researchers estimate that on average one in 500 residents had to die of Covid-19 in order to achieve the immunity that has now been determined.
In more conservative calculations they come to one death under 800 inhabitants.
To put it into perspective: In Munich, which is badly affected by the virus by German standards, 223 deaths have so far been recorded in around 1.5 million inhabitants.
That's about 15 out of 100,000 people.
The population of Manaus is significantly younger than in the industrialized countries.
The risk of a severe or even fatal course of Covid-19 increases significantly from the age of about 60 years.
In Manaus, only six percent of people fall into this risk group, in Germany it is around 20 percent.
"Herd immunity through natural infections is not a strategy, but a sign that the government has failed to control an outbreak and paid for it with lost lives," tweeted immunologist Florian Krammer of Mount Sinai Hospital in New York (more on the question of whether herd immunity is a worthwhile goal in the pandemic, read here).
Suddenly there was a great demand for coffins
The first corona case in Manaus occurred in March.
Although people began to keep a greater distance from one another at that time, it was not possible to contain the spread of the virus for months.
The reasons for this have not yet been conclusively clarified.
Experts suspect that cramped living conditions, poor water supply and crowds in local transport have promoted infections.
As of March, the number of deaths in the city rose in a few weeks from 20 to 30 a day to more than a hundred, reported the British "DailyMail".
The demand for coffins multiplied.
Some of the dead were stored in refrigerated containers because the undertakers couldn't keep up.
Images show fields in which a fresh grave is lined up next.
There was talk of mass graves.
However, since May the number of proven corona infections and deaths in Manaus has been falling significantly again, without there being an obvious explanation for this.
Sabino and her team now see the many positive antibody tests as a clue, but do not rule out that changes in behavior also played a role.
It is no coincidence that the value of two thirds of immune people in a population is repeatedly cited as the limit for the exponential spread of the coronavirus.
Without countermeasures, every person infected with corona infects around three people with the virus, who in turn infect three new people.
This exponential spread ends as soon as each infected person only infects one other person.
This is the case when two out of three people who could potentially be infected by an infected person already carried the virus and are immune.
There is no indication that the virus has become more harmless
"The way the pandemic went in Brazil, I immediately believe that herd immunity might be possible in some regions," writes Krammer.
However, further investigations must now show whether the protection actually exists.
Experts hope that this will also provide information on how long once infected people remain immune.
In Germany, too, there is currently discussion about why the proportion of deceased among those who tested positive for the virus
Infected has decreased.
The idea is circulating that the coronavirus may have changed and become more harmless.
The Robert Koch Institute (RKI) considers this unlikely in its situation report on Wednesday.
The low case mortality in Germany comes from the fact that many people with mild symptoms are now being tested more and more and there are many young people among those infected.
"If more and more older people become infected again, more serious cases and deaths will occur," according to the RKI.
It is still important to prevent this.
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