For a short time the Brazilian city of Manaus, in the heart of the Amazon rainforest, offered a glimmer of hope in the search for herd immunity from Covid-19.
After a devastating wave in May killed about 3,400 people and infected many more, the prevalence of the virus subsided rapidly, leading some scientists to theorise that the city of 2m had reached a form of collective immunity.
That hypothesis is now in doubt as a resurgence in cases in Manaus poses fresh challenges to the authorities and difficult questions for the scientists and policymakers worldwide who have been edging towards herd immunity policies as an alternative to harsh lockdowns.
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