Exceptionally low mortality despite widespread COVID-19 infection among Indigenous Tsimane and Moseten of Bolivia.

We report on the prevalence of and mortality from SARS-CoV-2 among the Tsimane and Moseten, two Indigenous subsistence groups in Bolivian Amazonia, and show that they evidence the lowest infection fatality rate ever reported. These populations have minimal access to medical care, a history of high parasite and pathogen exposures, and upregulated immune responses compared to industrialized populations. Between July and December of 2020, and again March - May of 2021, 85.4% of the Tsimane population aged 17+ were interviewed, and 42.0% provided blood samples. Among the Moseten aged 17+, 38.3% were interviewed and 38.6% provided blood samples. Blood samples were analyzed for SARS-CoV-2 antibodies with a mix of point-of-care IgG/IgM and ELISA with seroneutralization. The pandemic experience of these groups during the initial 2020 wave contrasts sharply with that observed in more industrialized contexts: while 71% and 63% of the Tsimane and Moseten populations were infected with SARS-CoV-2, mortality was minimal, with infection fatality ratios of 0.009% and 0.095%, respectively. The mortality rate for the Tsimane is 1/23rd the expectation based on rates observed elsewhere in the world. Disease severity-as measured by length of illness, ELISA antibody titers, and virus neutralizing test titers-did not increase markedly with age. We propose that low fatality, despite high transmissibility and non-negligible severity, may be due to a more vigorous and effective immune response to infection, with implications for future global pandemics.