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Berlin (dpa) - That doesn't sound good: A metropolis in Brazil, in which the vast majority of the population is said to have already been infected with the corona virus, is currently experiencing a second collapse of the health system.
The trigger is - after months of relative calm - again Sars-CoV-2.
The assumption that herd immunity has been achieved: refuted?
How do virus mutations contribute to the situation?
And is something like that threatened in this country too?
These questions arise on the basis of study results and media reports on the situation in Manaus.
The news from the capital of the state of Amazonas with more than two million inhabitants makes you sit up and take notice: there is a lack of hospital beds and oxygen, and patients are being flown to other states.
The World Health Organization (WHO) organized aid deliveries: oxygen, thermometers, rapid tests.
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What is particularly surprising about the development is that researchers there believed that the theoretical threshold for herd immunity had been exceeded: In January, Brazilian experts in the journal Science estimated the proportion of Manaus residents who had become infected by October at more than 70 percent.
They had examined samples from blood donors for corona antibodies.
Herd immunity means protection by the community: This benefits people who cannot be vaccinated for medical reasons.
If a sufficient number of the population is vaccinated or immune after suffering from illness, the pathogen hardly spreads - and less often it reaches susceptible people.
But there is no uniform threshold.
"How many immune systems are actually necessary for this to work depends on how contagious the disease is and how well the vaccination works and how long the vaccination protection lasts," explains the Robert Koch Institute (RKI) in an information sheet.
With Sars-CoV-2, it is also unclear whether people who have been vaccinated will still transmit the virus and how long an immunity will last.
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Nevertheless, the message stuck with many people that the pandemic has virtually stopped when enough people have become infected or have become immune through vaccinations.
Since spring, experts have put the proportion necessary for the hoped-for effect at two thirds of the population, around 67 percent.
The figure is based on the assumption that an infected person would infect an average of three people if no measures are in place and no one is immune - the base reproduction number (base R-value).
But what happened then in Manaus?
In the journal "The Lancet" researchers cite several explanations that are not mutually exclusive.
Accordingly, the estimates of the infections could have been too high.
The immunity obtained in the first wave could possibly have waned again in December, according to the authors.
In addition, there is evidence of corona variants in Manaus, which escape the immune system of those who have recovered and cause infections again.
And which are obviously more contagious than previous forms.
A more contagious virus also has a higher base reproduction number: The spring calculation that an infected person infects three people on average would have to be adjusted if variants were to spread.
This means that herd immunity would not be achieved at 67 percent, but only at a higher percentage.
Science journalist Mai Thi Nguyen-Kim calculated on her Youtube channel MaiLab that the threshold would be around 80 percent with a base R value of five.
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However, these are theoretical considerations.
In practice, according to virologist Christian Drosten, many other factors such as contact networks and frequency contribute.
He recently said that in the “Coronavirus Update” at NDR-Info.
In combination with mild corona measures, protective effects are likely already formed when less than two thirds of the population are vaccinated.
Because the virus spreads mainly through outbreaks.
If one imagines networks of transmissions, important connections between outbreaks could no longer be closed after a certain threshold.
About Manaus, the Charité researcher said that another wave of severe courses was not really to be expected in an already infected population.
Other experts also expect that patients would get milder symptoms in the event of a second infection.
Drosten therefore doubts the assumption that herd immunity will already have developed in Manaus in 2020.
When asked by the dpa, epidemiologist Rafael Mikolajczyk from the Halle University Hospital emphasizes that 30 percent of susceptible citizens are also a lot if a new virus variant is so much more infectious that the epidemic could spread again.
The proportion is large enough to overload a health system.
In Germany, less than 10 percent of the population has probably had Corona so far - and even this small number has almost caused an overload, he explains.
Only around 2.2 million cases are officially recorded in Germany - around 1.8 percent of the population.
A study at the RKI is still ongoing on the question of the number of unreported cases.
“We are still very far from herd immunity, which comes about as a result of past infections.
It is not a realistic scenario, especially with the new variants, and we are only just beginning with the vaccination, ”says Mikolajczyk.
He believes the continuation and possibly tightening of the current measures are necessary: so that there is no exponential growth in infections with the new variants before a sufficient number of people have been vaccinated to keep the number of deaths low.
In Germany, variant B.1.1.7 found in Great Britain is currently causing concern.
The President of the Society for Virology, Ralf Bartenschlager, recently told the dpa that he considers herd immunity through consistent vaccinations, despite the variants, to be the decisive means to counter the pandemic.
"How far we will get with this - whether we will achieve complete control in the sense of avoiding infections - cannot be conclusively stated at the moment."
In any case, herd immunity will not be achieved suddenly, in Drosten's view.
He referred to the first data from Israel, which are encouraging: According to this, the rate of hospital admissions has already fallen in age groups in which roughly every second person was vaccinated.
However, scientists emphasize that even in a population that is on average well vaccinated, there are always groups with not that many vaccinated.
Even a natural epidemic does not leave a randomly distributed immunity, says Mikolajczyk.
There remain areas in which outbreaks can occur as soon as the virus is introduced.
The highly contagious measles has been an example of this for years.
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© dpa-infocom, dpa: 210211-99-392469 / 2
RKI info sheet - fact boxes for mRNA vaccination against COVID-19
"Science" on location in Manaus
"The Lancet" on location in Manaus
YouTube video from MaiLab - This is how Corona ends
RKI - Study Corona Monitoring Local
RKI FAQ on vaccination
"Herd Immunity": A Rough Guide
Coronavirus update script - episode 74