Parts of Brazil may be close to reaching herd immunity

“Herd immunity” has been one of these terms that the pandemic brought into our daily lives. In broad terms, it is achieved when the number of survivors with immunity against a given disease increases to a certain level, slowing down — and eventually stopping — the spread of the virus in question. It was believed that this stage would be reached for Covid-19 if 60 percent of a given population developed antibodies.

Some governments have tried to apply the herd immunity strategy, such as in Sweden or a famously short-lived attempt in the United Kingdom. But these moves, highly controversial as they are, have not proven successful. 

A paper published by the Journal of the Royal Society of Medicine states that Sweden’s higher rates of viral infection, hospitalization, and mortality compared with neighboring countries may have serious implications for Scandinavia and beyond. And in the United Kingdom, Prime Minister Boris Johnson eventually U-turned from his initial coronavirus approach toward herd immunity.

Now, mathematical models suggest that coronavirus herd immunity could be reached after just 10 to 20 percent of people develop antibodies, depending on the specific characteristics of a place and its population. But that doesn’t mean herd immunity should be a goal public authorities should seek. At a news briefing last week, World Health Organization officials called pursuing such a strategy “very dangerous.”

Rodrigo Corder, an infectious disease modeling researcher, explained the fundamental principles of his studies to The Brazilian Report. As a Ph.D. candidate at the University of São Paulo’s Institute of Biomedical Sciences, he is part of a group of bio-mathematicians and health researchers trying to break down and understand the Covid-19 spread and establish the herd-immunity threshold for different areas.

Why do you believe that the threshold for “herd immunity,” or collective immunity, may be lower than initially considered?

Mathematical models usually consider populations to be homogeneous. We assume that the populations are heterogeneous, contain different susceptibilities and different exposures to the coronavirus. Some have a higher risk, and others have a lower risk of becoming infected. At the beginning of the infection, those with the highest risk are expected to get infected first, leaving only those at low risk for subsequent infections. With those at...