As Brazil crossed the threshold of 130,000 COVID-19 related deaths at the start of September, a new study has reported a major step in the progression of the country’s epidemic. Published on the medRxiv platform on 21st September 2020 and awaiting validation from a scientific committee, it suggests that the drastic decline in deaths in the city of Manaus (in the country’s north-west) is proof of herd immunity.More than half of the inhabitants affectedIn April and May, at the height of the pandemic, the metropolis of nearly 1.8 million inhabitants nestled among the Amazon rainforest found itself overwhelmed by the scale of the infections it was experiencing. Although non-essential businesses shut down, authorities did not implement general, nationwide lockdowns. Hospitals were overwhelmed by the death toll and cemeteries didn’t have enough time to dig all the necessary graves.But last June, the death toll unexpectedly dropped, leaving public health experts wondering if there was actually anyone left to be infected. The new research, carried out on thousands of data samples taken since March, finally seems to indicate that 44 to 66% of the population contracted the virus between the height of the crisis in mid-May and the end of summer. As a result, they would now have Sars-Cov-2 antibodies.Scientists are still in the process of determining the herd immunity threshold for COVID-19; the number of people who are considered ‘immune' (after infection or as a result of a vaccine) that would be needed to curve the epidemic. Estimations show it would need to reach around 40 to 60% of the population. Therefore, herd immunity by natural infection would potentially have been reached in Manaus, slowing the spread of the disease.Ephemeral antibodiesHowever, the numbers are on the rise again in this Brazilian city. In fact, according to researchers, ‘antibodies against SARS-CoV-2 decay quickly, a few months after infection’ which would explain the resurgence in Manaus. It would also prove once again just how important it is to take measures seriously in order to understand the evolution of the disease and establish the limits of herd immunity by natural infection, since we do not yet know if it is possible to be re-infected.As Reuters reported, on Friday 25th September, due to the fear of a second deadly wave, local authorities banned parties and gatherings and limited opening hours for restaurants and shops for 30 days. Doctor André Patricio Almeida, an infectious disease specialist at the city’s Heitor Vieira Dourado Tropical Medicine Foundation, summed up the situation:It’s a bit premature to speak of herd immunity from a new disease that we are still learning about, but we are seeing people with mild cases that appear to have anti-body immunity that can last for 2-3 months.