Brazilian City Ravaged by Coronavirus Reached Herd Immunity in Just Months, Study Finds

A city in the Brazilian Amazon reached the herd immunity threshold for coronavirus after it swept through with little to no measures in place to stop infection, a study has found.

The city of Manaus, with a population of over two million, was one of the worst hit cities for COVID-19 cases in Brazil. No lockdown was implemented and no major steps were taken to limit the spread of the virus. The first case was reported there in the middle of March. Within two months, hospitals were overwhelmed and the death rate was far above the average for Brazil.

Now, an international team of researchers has assessed the proportion of the population in Manaus that was infected with coronavirus at its peak, and how cases fell in the aftermath, concluding herd immunity had been achieved. This is where such a large proportion of the population has been infected with a disease that it is no longer able to spread at a significant rate.

The preprint study that appears on the website medrxiv.org has not been peer reviewed or published in a scientific journal, meaning it has not been assessed by a panel of experts to assess the validity of the findings. As such, the results should be taken with caution.

According to a Reuters report from May, authorities in Manaus struggled to keep pace with burials. It said the State of Amazonas, in which Manaus is the capital, had a death rate of 19.4 per 100,000, compared to 4.4 for Brazil as a whole.

It is thought poor testing infrastructure means many cases were missed at the peak of the spread. However, recorded cases and deaths in Manaus started falling significantly from around June, prompting questions over whether the city had developed "herd immunity."

Normally, herd immunity is associated with vaccines—if a certain percentage of a population is inoculated against a virus, then vulnerable people who cannot or have not been vaccinated are protected by the "herd." However, throughout the coronavirus pandemic, herd immunity has been discussed as a potential way for regions to recover.

This approach has been called dangerous by World Health Organization officials. However, Manaus and its extreme level of infection potentially now provides an insight into herd immunity and the level of infection required before it is achieved.

In August, researchers noticed the fall in cases in Manaus and suggested herd immunity may have played a role.

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A nurse walks with a coronavirus patient at a field hospital in Manaus, Brazil, in May. Researchers have said the city reached a point of herd immunity, with around 66 percent of the population infected with the virus. Andre Coelho/Getty Images

Following the release of the Manaus study, Lewis Buss, from the University of São Paulo, said the findings are part of a wider research project looking at how antibody levels in eight different cities in Brazil change over time. "When an antibody test is positive this is evidence that a person has been previously infected with the SARS-CoV-2 virus," he told Newsweek in an email. "Based on the proportion of blood donors that we found to have antibodies, we estimated that over the course of the epidemic in Manaus 66 percent of the population was infected at some point during that time."

The team found transmission of SARS-CoV-2, the virus that causes COVID-19, increased quickly between March and April. It then declined between May and September, despite no measures being taken to slow the spread.

They said that while there may have been a change in behavior among the city residents that helped limit transmission, "the unusually high infection rate suggests that herd immunity played a significant role in determining the size of the epidemic."

The researchers also found that seroprevalence fell in July and August. The researchers say it is normal for antibodies to fall following infection and that their study was not designed to look at how long immunity lasts for. However, longer-term monitoring of the situation of Manaus could potentially provide an insight into this. "The significance of this in terms of protective immunity is still an open question," Buss said. "It will be important to continue to monitor the situation in Manaus to understand the issue better."

"Herd immunity is the proportion of the population that must be immune to an infection such that each case generates, on average, less than one new case. It is important to distinguish this from the final proportion of the population to become infected, which will be higher. Our study is an example of a largely unmitigated SARS-CoV-2 epidemic with an exceptionally high attack rate in which herd immunity may have contributed to the drop in new cases and deaths.

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Aerial view of graves in the Nossa Senhora Aparecida cemetery in Manaus in June. The city was overwhelmed with COVID-19 cases in March and April, leaving thousands dead. MICHAEL DANTAS/AFP via Getty Images

"As we emphasize in our preprint, our results should not be extrapolated directly to other contexts. Our results fit into the wider scientific discussion around herd immunity, which has largely relied on mathematical modeling, by providing empirical evidence of the extent of infection in one of the most severely affected areas of the world."

David Goldsmith, who recently wrote a commentary in the Journal of the Royal Society of Medicine about how Sweden—a herd immunity proponent—had failed to reach the level required for protection, said the latest findings show mitigation measures "hamper the rapid development of population herd immunity." But this rapid development, he said, comes at a price.

"From mortality studies we know that the price paid by vulnerable susceptible victims caught up in the COVID-19 maelstrom can be very high, so unrestrained viral infection will indeed burn itself out, but at a terrible cost," he told Newsweek in an email. "Now whether a 'slow fuse' approach to acquire herd immunity—as frequently claimed for Sweden—is a viable strategy remains very unclear, and again, fraught with risk."