Free path for coronavirus: herd immunity dream

Just a few weeks after the first infection in March, it should have passed before the gravedigger could no longer continue digging new pits for the victims. The number of people who contracted the new virus grew unabated, in May up to 80 people in the Brazilian metropolis of Manaus, with a population of 1.8 million, succumbed to their Covid 19 disease, the health system groaned – and not a few medical professionals feared collapse because little action was taken.

But then came the almost miraculous redemption of the city of Manaus. While the virus continued to rage across much of Brazil and is still rampant today, the number of infections in the capital of the Amazon region has dropped since the peak of the outbreak in May. And that’s without rigorous measures being taken. On the contrary, the few rules that existed were soon relaxed. So what happened?

It’s possible that the metropolis simply went through the Sars-CoV-2 wave and developed what some have been dreaming about since the start of the pandemic: herd immunity. At least that’s what the study by an international team of health experts, which was published a few days ago on the preprint server medRxiv.org, should suggest. Virology, epidemiology and medicine experts used two different antibody tests to examine several thousand blood donations from different phases of the outbreak in Manaus and São Paulo. Such tests can show if someone has already been infected with the new coronavirus. As the authors now explain, according to their analysis, two-thirds of the population of Manaus have already been infected with Sars-CoV-2 and have therefore at least temporarily become immune.

The price was brutally high: many dead and countless seriously ill

The study is preliminary and has not yet been peer-reviewed. Still, the results of the Brazilian, American and British researchers are remarkable. The rate of an infected population of 66 percent is surprisingly in line with some experts’ predictions for so-called herd immunity. If such herd immunity is achieved with measles, for example, the virus will no longer find enough new victims for a chain of transmission. So it could have been in Manaus with Sars-CoV-2.

The second finding seems even more important: If the scientists are right, the price for this victory over the virus has been brutally high. An estimated 4,200 deaths and an unquantifiable number of seriously ill people, some of whom will have to fight the consequences of Covid-19 for life; this would be the result of the research team in this city. A city that has an exceptionally young age structure, even by Brazilian standards. Most of the population is 30 years old or younger. On the one hand, this could explain why the virus spreads so quickly; after all, younger people are more mobile and often more sociable. On the other hand, the outbreak would most likely have claimed significantly more human lives if the people of Manaus were older on average.

But while the study gives an impression of what happens if you dispense with measurements and the virus runs freely, it probably contains too many imponderables for clear projections. “The number of test subjects examined is very small compared to the population, and it is not a representative sample,” says Gérard Krause of the Helmholtz Center for Infection Research in Braunschweig. The epidemiologist cautions against transferring the results too quickly, even if the order of magnitude of the frequency of detected antibodies may fit the image.

A second coronavirus infection is likely to be mild

What that will mean for the future of the Manaus population in the pandemic is also unclear. Studies have already shown several times that antibodies disappear from the blood of those who have recovered, as the current study shows. There is also evidence that second infections are possible, although they can be mild. “We assume that people who have recovered from Covid-19 only very rarely can become seriously ill with Covid-19 again,” says Krause. The fact that herd immunity through a triggered virus must have as its objective, much less a solution to the problem of the pandemic, certainly cannot be deduced from the Manaus example. “Apart from the question of whether this effect is sustainable, it is important to evaluate the social and health price at which it was purchased,” says the epidemiologist.

Last week should underline this. Shortly after the early publication of the study, the number of infections in Manaus increased again. Bars and restaurants have to close, shops can only open to a limited extent. The second wave is coming, experts say. And it shows that herd immunity in the case of Covid-19 may remain a pipe dream.

Posted today at 9:30 pm