After initially containing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), many European and Asian countries had a resurgence of COVID-19 consistent with a large proportion of the population remaining susceptible to the virus after the first epidemic wave.1Lucy CO Verity R Watson OJ et al.Have deaths from COVID-19 in Europe plateaued due to herd immunity?.Lancet. 2020; 395: e110-e111Summary Full Text Full Text PDF PubMed Scopus (50) Google Scholar By contrast, in Manaus, Brazil, a study of blood donors indicated that 76% (95% CI 67–98) of the population had been infected with SARS-CoV-2 by October, 2020.2Buss LF Prete CA Abrahim CMM et al.Three-quarters attack rate of SARS-CoV-2 in the Brazilian Amazon during a largely unmitigated epidemic.Science. 2020; 371: 288-292Crossref PubMed Scopus (290) Google Scholar High attack rates of SARS-CoV-2 were also estimated in population-based samples from other locations in the Amazon Basin—eg, Iquitos, Peru 70% (67–73).3Álvarez-Antonio C Meza-Sánchez G Calampa C et al.Seroprevalence of anti-SARS-CoV-2 antibodies in Iquitos, Loreto, Peru.MedRxiv. 2021; (published online 20.) (preprint)https://doi.org/10.1101/2021.01.17.21249913Google Scholar The estimated SARS-CoV-2 attack rate in Manaus would be above the theoretical herd immunity threshold (67%), given a basic case reproduction number (R0) of 3.4Fontanet A Cauchemez S COVID-19 herd immunity: where are we?.Nat Rev Immunol. 2020; 20: 583-584Crossref PubMed Scopus (373) Google Scholar In this context, the abrupt increase in the number of COVID-19 hospital admissions in Manaus during January, 2021 (3431 in Jan 1–19, 2021, vs 552 in Dec 1–19, 2020) is unexpected and of concern (figure).5Fundação de Vigilância em Saúde do AmazonasCOVID-19 no Amazonas. Dados epidemiológicos e financeiros das ações de combate à COVID-19. Publicações.http://www.fvs.am.gov.br/publicacoesDate accessed: January 20, 2021Google Scholar, 6Prefeitura de ManausPublicações. COVID-19.http://www.manaus.am.gov.br/noticia/Date accessed: January 20, 2021Google Scholar, 7Fundação de Vigilância em Saúde do AmazonasIndex of media publicacao.http://www.fvs.am.gov.br/media/publicacaoDate accessed: January 20, 2021Google Scholar, 8Marcelo Oliveira capyvaraPopular repositories. GitHub, 2021https://github.com/capyvaraDate accessed: January 20, 2021Google Scholar, 9Nelson BW Instituto Nacional de Pesquisas da Amazônia (INPA)Excess deaths Manaus.https://t.co/6g4HHEAuNYDate: 2021Date accessed: January 20, 2021Google Scholar, 10Parag KV Cowling BJ Donnelly CA et al.Deciphering early-warning signals of the elimination and resurgence potential of SARS-CoV-2 from limited data at multiple scales.MedRxiv. 2020; (published online Jan 5.) (preprint)https://doi.org/10.1101/2020.11.23.20236968Google Scholar After a large epidemic that peaked in late April, 2020, COVID-19 hospitalisations in Manaus remained stable and fairly low for 7 months from May to November, despite the relaxation of COVID-19 control measures during that period (figure). There are at least four non-mutually exclusive possible explanations for the resurgence of COVID-19 in Manaus. First, the SARS-CoV-2 attack rate could have been overestimated during the first wave, and the population remained below the herd immunity threshold until the beginning of December, 2020. In this scenario, the resurgence could be explained by greater mixing of infected and susceptible individuals during December. The 76% estimate of past infection2Buss LF Prete CA Abrahim CMM et al.Three-quarters attack rate of SARS-CoV-2 in the Brazilian Amazon during a largely unmitigated epidemic.Science. 2020; 371: 288-292Crossref PubMed Scopus (290) Google Scholar might have been biased upwards due to adjustments to the observed 52·5% (95% CI 47·6–57·5) seroprevalence in June, 2020, to account for antibody waning. However, even this lower bound should confer important population immunity to avoid a larger outbreak. Furthermore, comparisons of blood donors with census data showed no major difference in a range of demographic variables,2Buss LF Prete CA Abrahim CMM et al.Three-quarters attack rate of SARS-CoV-2 in the Brazilian Amazon during a largely unmitigated epidemic.Science. 2020; 371: 288-292Crossref PubMed Scopus (290) Google Scholar and the mandatory exclusion of donors with symptoms of COVID-19 is expected to underestimate the true population exposure to the virus. Reanalysis and model comparison11Tassila S Pybus O França R et al.Coronavirus prevalence in Brazilian Amazon and Sao Paulo city [data set].Dryad. 2020; (published online Dec 8.)http://doi.org/10.5061/dryad.c59zw3r5nGoogle Scholar by independent groups will help inform the best-fitting models for antibody waning and the representativeness of blood donors. Second, immunity against infection might have already begun to wane by December, 2020, because of a general decrease in immune protection against SARS-CoV-2 after a first exposure. Waning of anti-nucleocapsid IgG antibody titres observed in blood donors2Buss LF Prete CA Abrahim CMM et al.Three-quarters attack rate of SARS-CoV-2 in the Brazilian Amazon during a largely unmitigated epidemic.Science. 2020; 371: 288-292Crossref PubMed Scopus (290) Google Scholar might reflect a loss of immune protection, although immunity to SARS-CoV-2 depends on a combination of B-cell and T-cell responses.12Dan JM Mateu J Kato Y et al.Immunological memory to SARS-CoV-2 assessed for up to 8 months after infection.Science. 2021; (published online Jan 6.)https://doi.org/10.1126/science.abf4063Crossref PubMed Scopus (1835) Google Scholar A study of UK health-care workers13Lumley SF O'Donnell D Stoesser NE et al.Antibody status and incidence of SARS-CoV-2 infection in health care workers.N Engl J Med. 2020; (published online Dec 23.)https://doi.org/10.1056/NEJMoa2034545Crossref PubMed Scopus (640) Google Scholar showed that reinfection with SARS-CoV-2 is uncommon up to 6 months after the primary infection. However, most of the SARS-CoV-2 infections in Manaus occurred 7–8 months before the resurgence in January, 2021; this is longer than the period covered by the UK study,13Lumley SF O'Donnell D Stoesser NE et al.Antibody status and incidence of SARS-CoV-2 infection in health care workers.N Engl J Med. 2020; (published online Dec 23.)https://doi.org/10.1056/NEJMoa2034545Crossref PubMed Scopus (640) Google Scholar but nonetheless suggests that waning immunity alone is unlikely to fully explain the recent resurgence. Moreover, population mobility in Manaus decreased from mid-November, 2020, with a sharp reduction in late December, 2020,14ODS Atlas AmazonasInloco. Índice de Isolamento Social das cidades do Amazonas. Dados cedidos pela empresa Inloco.https://datastudio.google.com/s/o1rTqejYd_4Date accessed: January 20, 2021Google Scholar suggesting that behavioural change does not account for the resurgence of hospitalisations. Third, SARS-CoV-2 lineages might evade immunity generated in response to previous infection.15Greaney AJ Loes AN Crawford KHD et al.Comprehensive mapping of mutations to the SARS-CoV-2 receptor-binding domain that affect recognition by polyclonal human serum antibodies.BioRxiv. 2021; (published online Jan 4.) (preprint)https://doi.org/10.1101/2020.12.31.425021PubMed Google Scholar Three recently detected SARS-CoV-2 lineages (B.1.1.7, B.1.351, and P.1), are unusually divergent and each possesses a unique constellation of mutations of potential biological importance.16Faria NR Claro IM Candido D et al.Genomic characterisation of an emergent SARS-CoV-2 lineage in Manaus: preliminary findings. Virological.https://virological.org/t/genomic-characterisation-of-an-emergent-sars-cov-2-lineage-in-manaus-preliminary-findings/586Date: January, 2021Date accessed: January 20, 2021Google Scholar, 17Rambaut A Loman N Pybus OG et al.Preliminary genomic characterisation of an emergent SARS-CoV-2 lineage in the UK defined by a novel set of spike mutations. Virological.https://virological.org/t/preliminary-genomic-characterisation-of-an-emergent-sars-cov-2-lineage-in-the-uk-defined-by-a-novel-set-of-spike-mutations/563Date: December, 2020Date accessed: January 20, 2021Google Scholar, 18Tegally H Wilkinson E Giovanetti M et al.Emergence and rapid spread of a new severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) lineage with multiple spike mutations in South Africa.MedRxiv. 2020; (published online Dec 22.) (preprint)https://doi.org/10.1101/2020.12.21.20248640Google Scholar Of these, two are circulating in Brazil (B.1.1.7 and P.1) and one (P.1) was detected in Manaus on Jan 12, 2021.16Faria NR Claro IM Candido D et al.Genomic characterisation of an emergent SARS-CoV-2 lineage in Manaus: preliminary findings. Virological.https://virological.org/t/genomic-characterisation-of-an-emergent-sars-cov-2-lineage-in-manaus-preliminary-findings/586Date: January, 2021Date accessed: January 20, 2021Google Scholar One case of SARS-CoV-2 reinfection has been associated with the P.1 lineage in Manaus19Naveca F da Costa C Nascimento V et al.SARS-CoV-2 reinfection by the new variant of concern (VOC) P.1 in Amazonas, Brazil. Virological.https://virological.org/t/sars-cov-2-reinfection-by-the-new-variant-of-concern-voc-p-1-in-amazonas-brazil/596Date: Jan 18, 2021Date accessed: January 20, 2021Google Scholar that accrued ten unique spike protein mutations, including E484K and N501K.16Faria NR Claro IM Candido D et al.Genomic characterisation of an emergent SARS-CoV-2 lineage in Manaus: preliminary findings. Virological.https://virological.org/t/genomic-characterisation-of-an-emergent-sars-cov-2-lineage-in-manaus-preliminary-findings/586Date: January, 2021Date accessed: January 20, 2021Google Scholar Moreover, the newly classified P.2 lineage (sublineage of B.1.128 that independently accrued the spike E484K mutation) has now been detected in several locations in Brazil, including Manaus.20Naveca F Nascimento V Souza V et al.Phylogenetic relationship of SARS-CoV-2 sequences from Amazonas with emerging Brazilian variants harboring mutations E484K and N501Y in the Spike protein. Virological.https://virological.org/t/phylogenetic-relationship-of-sars-cov-2-sequences-from-amazonas-with-emerging-brazilian-variants-harboring-mutations-e484k-and-n501y-in-the-spike-protein/585Date: Jan 11, 2021Date accessed: January 20, 2021Google Scholar P.2 variants with the E484K mutation have been detected in two people who have been reinfected with SARS-CoV-2 in Brazil,21Nonaka VCK Franco MM Gräf T et al.Genomic evidence of a SARS-Cov-2 reinfection case with E484K spike mutation in Brazil.Preprints. 2021; (published online Jan 6.) (preprint)https://doi.org/10.20944/preprints202101.0132.v1Google Scholar, 22Resende PC Bezerra JF Vasconcelos RHT et al.Spike E484K mutation in the first SARS-CoV-2 reinfection case confirmed in Brazil, 2020. Virological.https://virological.org/t/spike-e484k-mutation-in-the-first-sars-cov-2-reinfection-case-confirmed-in-brazil-2020/584Date: Jan 10, 2021Date accessed: January 20, 2021Google Scholar and there is in-vitro evidence that the presence of the E484K mutation reduces neutralisation by polyclonal antibodies in convalescent sera.15Greaney AJ Loes AN Crawford KHD et al.Comprehensive mapping of mutations to the SARS-CoV-2 receptor-binding domain that affect recognition by polyclonal human serum antibodies.BioRxiv. 2021; (published online Jan 4.) (preprint)https://doi.org/10.1101/2020.12.31.425021PubMed Google Scholar Fourth, SARS-CoV-2 lineages circulating in the second wave might have higher inherent transmissibility than pre-existing lineages circulating in Manaus. The P.1 lineage was first discovered in Manaus.16Faria NR Claro IM Candido D et al.Genomic characterisation of an emergent SARS-CoV-2 lineage in Manaus: preliminary findings. Virological.https://virological.org/t/genomic-characterisation-of-an-emergent-sars-cov-2-lineage-in-manaus-preliminary-findings/586Date: January, 2021Date accessed: January 20, 2021Google Scholar In a preliminary study, this lineage reached a high frequency (42%, 13 of 31) among genome samples obtained from COVID-19 cases in December, 2020, but was absent in 26 samples collected in Manaus between March and November, 2020.16Faria NR Claro IM Candido D et al.Genomic characterisation of an emergent SARS-CoV-2 lineage in Manaus: preliminary findings. Virological.https://virological.org/t/genomic-characterisation-of-an-emergent-sars-cov-2-lineage-in-manaus-preliminary-findings/586Date: January, 2021Date accessed: January 20, 2021Google Scholar Thus far, little is known about the transmissibility of the P.1 lineage, but it shares several independently acquired mutations with the B.1.1.7 (N501Y) and the B.1.325 (K417N/T, E484K, N501Y) lineages circulating in the UK and South Africa, which seem to have increased transmissibility.18Tegally H Wilkinson E Giovanetti M et al.Emergence and rapid spread of a new severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) lineage with multiple spike mutations in South Africa.MedRxiv. 2020; (published online Dec 22.) (preprint)https://doi.org/10.1101/2020.12.21.20248640Google Scholar Contact tracing and outbreak investigation data are needed to better understand relative transmissibility of this lineage. The new SARS-CoV-2 lineages may drive a resurgence of cases in the places where they circulate if they have increased transmissibility compared with pre-existing circulating lineages and if they are associated with antigenic escape. For this reason, the genetic, immunological, clinical, and epidemiological characteristics of these SARS-CoV-2 variants need to be quickly investigated. Conversely, if resurgence in Manaus is due to waning of protective immunity, then similar resurgence scenarios should be expected in other locations. Sustained serological and genomic surveillance in Manaus and elsewhere is a priority, with simultaneous monitoring for SARS-CoV-2 reinfections and implementation of non-pharmaceutical interventions. Determining the efficacy of existing COVID-19 vaccines against variants in the P.1 lineage and other lineages with potential immune escape variants is also crucial. Genotyping viruses from COVID-19 patients who were not protected by vaccination in clinical trials would help us to understand if there are lineage-specific frequencies underlying reinfection. The protocols and findings of such studies should be coordinated and rapidly shared wherever such variants emerge and spread. Since rapid data sharing is the basis for the development and implementation of actionable disease control measures during public health emergencies, we are openly sharing in real-time monthly curated serosurvey data from blood donors through the Brazil–UK Centre for Arbovirus Discovery, Diagnosis, Genomics and Epidemiology (CADDE) Centre GitHub website and will continue to share genetic sequence data and results from Manaus through openly accessible data platforms such as GISAID and Virological. NRF reports funding from Wellcome Trust, the Royal Society, and the UK Medical Research Council. CAP reports grants from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior Brasil and FAPESP. NMF reports grants from the UK Medical Research Council, the UK National Institute of Health Research, Community Jameel, NIH NIGMS, Janssen Pharmaceuticals, the Bill & Melinda Gates Foundation, and Gavi, the Vaccine Alliance. The other authors declare no competing interests.