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A sharp drop in coronavirus cases in the state of Amazonas, raises the question about herd immunity.

Image Credit: Shutterstock

In the Brazilian city of Manaus, the number of coronavirus cases decreased significantly, which has brought into light whether there is collective immunity/herd immunity.

Herd immunity refers to the populations’ resistance to the spread of the virus due to an immunity built up from either infection or vaccination.

Manaus is the largest city in the Amazonas region with a population of 2,260,788. The city was one of the worst-hit regions in Brazil during the coronavirus peak, with images of mass graves, hospital corridors filled with corpses were becoming a common sight. During the early stages of the pandemic, a high number of people were getting infected and a lot were succumbing to the virus. In Manaus, no lockdown was ever imposed or any other strict containment measure, and people were not adhering to social distancing rules. Brazil’s President Jair Bolsonaro who also got infected with the virus, dismissed it as a ‘little flu’ and demanded all regional leaders to do business as usual. Most regional leaders ignored his request, and imposed their own quarantine measures. In Manaus, the State Public Prosecutor tried to impose a lockdown in April, in an attempt to contain the virus. Unfortunately, a judge denied the request due to insufficient information.

During the peak of the virus, there were around 130 burials a day, and due to a shortage of coffins, mass graves were dug. The city usually sees 35 burials a day in exceptional circumstances like in a prison riot.

A mass grave in the Nossa Senhora Aparecida Cemetery in Manaus in May. Image Credit: Felipe Dana/AP

In peak days during the months of May and June, Amazonas had an average of 1,500 cases, and Manaus around 1000 cases. As of yesterday, only 201 cases were reported in the region. There were days during May and June where the number of cases exceeded 2000 in the Amazonas. So far, the Amazonas reported a total of 120,060 cases and 3,639 deaths since the pandemic began. There were 3,300 deaths in Manaus alone. This means that one in 685 residents have been killed by the coronavirus in Manaus. The indigenous communities were the worst affected

Hospitalizations of patients with coronavirus have decreased from more than 1,300 in May to less than 300 in August. Deaths in the city of Manaus fell from a peak of 150 in May, to nearly zero in August. The situation improved so much that Manaus closed its field hospital, Hospital de Campanha Municipal, which opened specifically for COVID-19. This hospital was once overwhelmed with the number of patients.

A graph shows the number of patients dying from the virus in the Amazonas region has stabilised, after peaking in May with 100 fatalities every day

The following graph shows the number of COVID-19 deaths in Manaus, clearly showing a peak during May with around 150 deaths, and then the number of deaths stabilized in the following months. Image Credit:

As such, the factors that are resulting in a decreasing number of cases is still unclear. It is thought that changed behaviours and individual community characteristics have contributed to a decrease in the spread of the virus. Also, the city of Manaus is testing far more than it did in the beginning of the pandemic, and it was initially thought, that for a collective immunity to be reached by a population, 60-70% of the population must be exposed to the virus. This is not the case in Manaus, as it never got past 20%. Manaus is not an isolated case. The city of Guayaquil in Ecuador is another example, and it also never got past 33%. Sweden and Arizona, USA are another prime examples.

According to Jarbas Barbosa da Silva, an assistant director of the Pan American Health Organization said that ‘The hypotheses -and this is just a hypothesis – is that the peak we had in Manaus was very strong, and there was such widespread community transmission that it may have produced some kind of collective immunity.’

The herd immunity that experts believe Manaus is close to achieving, or maybe that it achieved, is linked to the high number of people that got infected. He suggested that the fall in cases may be attributed to a ‘natural dynamic’ rather than the effect of a preventive measure. It is imperative to note that so far, it can not be ruled out that the low number of cases could be due to an under-reporting of the cases.

Mr. Barbosa said that Manaus ‘paid a very large price’ to get here. Mr. Barbosa said ‘This was not a strategy. It was a tragedy.’ According to researchers during a live outbreak, collective immunity works differently. As the number of potential victims shrinks, the transmission of the virus decreases until it is no more. Once the number of potential victims have all been infected, a resurgence of the virus would be unlikely. By this time the number of people infected would be very high.

Regardless of the decreased number of cases in countries or regions that were initially hit hard, many researchers have been reluctant to conclude that herd immunity has been achieved in these areas, and that the worst has passed. This is due to two reasons. It is unknown how long the immunity lasts, and the virus can mutate.

An environmental health scientist at Columbia University, Jeffrey Shaman, believes that the worst is not over yet. ‘In Manaus, maybe we’re done with it, and that’s it. I would love that as well. But the reality is that it’s wishful thinking. It’s confirmation bias. We can’t pick evidence we hope is true. We have to be very careful about this because it could blow up in your face very quickly.’

The Mayor of Manaus, Virgilio hopes that the scientists proposing that a herd immunity has been achieved, are right. He believes that the medical system in Manaus already failed once. If a second wave comes, he knows that it won’t be good. He said ‘Our capacities would be overwhelmed.’

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