A provisional study claims that the capital of the Amazon region has reached such an infection rate that the coronavirus is hardly circulating. But researchers fear this immunity will be short-lived.
Do the 2.2 million Brazilians in the city of Manaus, capital of the Brazilian Amazon, now benefit from a collective immunity that almost protects them from contagion? The information, which circulated widely on Thursday internationally, should be taken with caution. The source is a preliminary medical study published on Wednesday, according to which 66% of the inhabitants of the city, very strongly affected by the pandemic in the spring, have antibodies against the coronavirus. A rate high enough that the disease could no longer spread effectively. This is called collective, or herd, immunity.
The text is signed by a group of 34 Brazilian and foreign researchers (working notably at Yale University in the United States or at the faculty of zoology in Oxford in England). MedRxiv, the site where he is available (in English), states that the report is “Preliminary work that has not been peer reviewed”, that is to say recognized specialists. “It should not be used as a basis for clinical practices, nor be presented by the media as proven information”, warns MedRxiv.
Located in the Amazon rainforest, Manaus was the scene of images of overwhelmed hospitals, corpses piled into refrigerated trucks and mass graves when the epidemic was at its peak there in May. The need for coffins was four to five times greater than a year earlier. The city has recorded 2,462 deaths from Covid-19. If it were a country, it would have the second highest death rate in the world, with 100.7 deaths per 100,000 population. Brazil’s average is 66.1 deaths per 100,000 inhabitants. But the death toll has dropped dramatically in recent weeks, to a daily average of 3.6 in the past two weeks. This situation allowed a rapid deconfinement of shops, public places and leisure areas, such as its famous opera house built in 1884, when Manaus was the opulent capital of rubber in the world.
“It appears that exposure to the virus has led to a drop in the number of new cases and deaths in Manaus”, said study coordinator, University of São Paulo medical professor Ester Sabino, at the São Paulo State Research Support Foundation (FAPESP) which helped fund the study. However, for health experts, seeking to achieve herd immunity would be a dangerous path for policy makers.
“Collective immunity by natural infection is not a strategy, it is a sign that a government has failed to control an epidemic and that it is paying the price in lives lost”, wrote on Twitter Florian Krammer, professor of microbiology at the Icahn School of Medicine at Mount Sinai Hospital in New York. Applying the Manaus model, the MIT Technology Review calculated that the United States would achieve herd immunity only after 500,000 deaths (compared to 200,000 currently). The United States is the most bereaved country in the world by the pandemic, ahead of Brazil, with more than 138,000 deaths.
Moreover, the researchers do not guarantee that the observed immunity will be of long duration. This is the case of infectious disease specialist Júlio Croda, who visited Manaus last week as an expert for the Pan American Health Organization (PAHO). According to him, respect for physical distance and wearing a mask must remain the rule, in the capital of the Amazon as elsewhere. The 34 researchers end their study with a pessimistic note: “Manaus could serve as a sentinel to determine the duration of the population’s immunity and the occurrence of reinfection.”