The rainforest city of Manaus in Brazil was the first in the country to be struck by the pandemic. The virus rapidly spread, and by October last year it was estimated that 76% of the population had been infected – a number higher than the theoretical threshold for herd immunity. Yet, in January 2021, cases surged and the health system was once again overwhelmed, with hospitals running out of oxygen and doctors and nurses required to carry out manual ventilation. To find out what might be behind this second wave, Sarah Boseley speaks to the Guardian’s Latin America editor, Tom Phillips, and Dr Deepti Gurdasani, asking why Manaus has been hit twice and what it might mean for our understanding of immunity, new viral variants, and the path through the pandemic. This podcast was amended on 2nd February 2021 to correct errors in the scripting. We incorrectly stated that the city of Manaus is situated only by the Amazon River, and that the Amazon River flows into the Pacific Ocean. Help support our independent journalism at theguardian.com/sciencepod