'Herd Immunity' in Manaus?

We heard much about the possible benefits of 'herd immunity' at the start of the Covid-19 pandemic. Preliminary observations (they have not been through a peer review process) of the population in the Brazilian city of Manaus in the Amazon Rainforest may give us a glimpse into what it might look like (https://www.msn.com/en-gb/news/world/brazilian-amazon-city-of-manaus-may-have-reached-covid-19-herd-immunity-study-says/ar-BB19n1uB). The study basically examined blood samples and questionnaire responses from the city. Manaus, has a population of over 2 million, is in a relatively impoverished part of the country and is characterised by people living together in large groups. Most of the residents would be classed as 'young'. One should warn, initially, that simply looking at whether an individual's blood sample contains antibodies to the virus (is 'seropositive') does not reveal the whole picture about earlier exposure to the virus (the antibodies can rapidly 'fade' and T and B cells may still offer protection). Brazil had a dreadful Covid-19 pandemic (second only to the USA) and Manaus has had almost 2.5k deaths from the infection. The study suggests that around 66% of the population of the city are now seropositive, which would make it difficult for the virus to find new people to infect (if second infections are rare). Currently, Manaus is showing a very rapid re-openning of businesses, night clubs etc and there are fewer than 4 deaths per day, that are attributal to the virus. So, 'herd immunity' appears to work but at a dreadful cost. It doesn't appear to be something that should be entered into lightly without a vaccine.