Typhoid fever is endemic in many parts of the world and remains a major public health concern in tropical and sub-tropical developing nations, including Fiji. To address high rates of typhoid fever, the Northern Division of Fiji is implementing a mass vaccination with typhoid conjugate vaccine (Vi-polysaccharide conjugated to tetanus toxoid) as a public health control measure in 2023. In this study we define the genomic epidemiology of S . Typhi in the Northern Division prior to island-wide vaccination, sequencing 85% (n=419) of the total cases from the Northern Division and Central Divisions of Fiji that occurred in the period 2017-2019. We found elevated rates of nucleotide polymorphisms in tviD and tviE genes (responsible for Vi-polysaccharide synthesis) relative to core genome levels within the Fiji endemic S . Typhi genotype 4.2. Expansion of these findings within a globally representative database of 12,382 S . Typhi (86 genotyphi clusters) showed evidence of convergent evolution of the same tviE mutations across the S . Typhi population, indicating that tvi selection has occurred both independently and globally. The functional impact of tvi mutations on the Vi-capsular structure and other phenotypic characteristics are presently unknown, yet commonly occurring tviE polymorphisms localise adjacent to predicted active site residues when overlayed against the predicted TviE protein structure. Given the central role of the Vi-polysaccharide in S . Typhi biology and vaccination, further integrated epidemiological, genomic, and phenotypic surveillance is required to determine the spread and functional implications of these mutations.

The common false dichotomy is that bacteria grow either intracellularly or extracellularly, and though there are some bacteria that have an almost viral-like dependency on host cells for metabolites, most can exploit both replicative niches. The facultative intracellular pathogens that preferentially replicate inside cells have evolved mechanisms that defeat or suppress the natural antibacterial cell biology processes associated with endosomal or phagosomal residency, and the nutritional limitations that might be brought about by entrapment within a lipid-bounded vacuole. This chapter describes how bacteria have adapted to intracellular replication and how they use sophisticated protein secretion apparatus and effector molecules, trigger uptake into cells, prevent killing by natural antibacterial processes, subvert host immunity, and replicate to cause some of the most important diseases of humankind, such as tuberculosis and dysentery.

The rapid increase and spread of Gram-negative bacteria resistant to many or all existing treatments threaten a return to the preantibiotic era. The presence of bacterial polysaccharides that impede the penetration of many antimicrobials and protect them from the innate immune system contributes to resistance and pathogenicity. No currently approved antibiotics target the polysaccharide regions of microbes. Here, describe monolaurin-based niosomes, the first lipid nanoparticles that can eliminate bacterial polysaccharides from hypervirulent Klebsiella pneumoniae, are described. Their combination with polymyxin B shows no cytotoxicity in vitro and is highly effective in combating K. pneumoniae infection in vivo. Comprehensive mechanistic studies have revealed that antimicrobial activity proceeds via a multimodal mechanism. Initially, lipid nanoparticles disrupt polysaccharides, then outer and inner membranes are destabilized and destroyed by polymyxin B, resulting in synergistic cell lysis. This novel lipidic nanoparticle system shows tremendous promise as a highly effective antimicrobial treatment targeting multidrug-resistant Gram-negative pathogens.

Classical Zn2+-dependent deac(et)ylases play fundamental regulatory roles in life and are well characterized in eukaryotes regarding their structures, substrates and physiological roles. In bacteria, however, classical deacylases are less well understood. We construct a Generalized Profile (GP) and identify thousands of uncharacterized classical deacylases in bacteria, which are grouped into five clusters. Systematic structural and functional characterization of representative enzymes from each cluster reveal high functional diversity, including polyamine deacylases and protein deacylases with various acyl-chain type preferences. These data are supported by multiple crystal structures of enzymes from different clusters. Through this extensive analysis, we define the structural requirements of substrate selectivity, and discovered bacterial de-d-/l-lactylases and long-chain deacylases. Importantly, bacterial deacylases are inhibited by archetypal HDAC inhibitors, as supported by co-crystal structures with the inhibitors SAHA and TSA, and setting the ground for drug repurposing strategies to fight bacterial infections. Thus, we provide a systematic structure-function analysis of classical deacylases in bacteria and reveal the basis of substrate specificity, acyl-chain preference and inhibition. In bacteria Zn2+-dependent deacylases are underexplored. Here, the authors identify bacterial deacylases, providing systemic structure-function analyses to reveal the basis of substrate specificity, acyl-chain preference and inhibition.

Background By acting as an environmental sensor, the ligand-induced transcription factor aryl hydrocarbon receptor (AhR) regulates acute innate and adaptive immune responses against pathogens. Here, we analyzed the function of AhR in a model for chronic systemic infection with attenuated Salmonella Typhimurium (STM).Methods Wild type and AhR-deficient mice were infected with the attenuated STM strain TAS2010 and analyzed for bacterial burden, host defense functions, and inflammatory stress erythropoiesis.Results AhR-deficient mice were highly susceptible to TAS2010 infection when compared with wild type mice, as demonstrated by reduced bacterial clearance and increased mortality. STM infection resulted in macrocytic anemia and enhanced splenomegaly with destruction of the splenic architecture in AhR-deficient mice. In addition, AhR-deficient mice displayed a major expansion of splenic immature red blood cells, indicative of infection-induced stress erythropoiesis. Elevated serum levels of erythropoietin and interleukin 6 upon infection, as well as increased numbers of splenic stress erythroid progenitors already in steady state, probably drive this effect and might cause the alterations in splenic immune cell compartments, thereby preventing an effective host defense against STM in AhR-deficient mice.Conclusions AhR-deficient mice fail to clear chronic TAS2010 infection due to enhanced stress erythropoiesis in the spleen and accompanying destruction of the splenic architecture. Aryl hydrocarbon receptor deficiency in mice results in increased susceptibility to chronic systemic Salmonella infection due to enhanced erythropoietin-driven stress erythropoiesis in the spleen and accompanying destruction of the splenic architecture.

Prior infection can generate protective immunity against subsequent infection, although the efficacy of such immunity can vary considerably. Live-attenuated vaccines (LAVs) are one of the most effective methods for mimicking this natural process, and analysis of their efficacy has proven instrumental in the identification of protective immune mechanisms. Here, we address the question of what makes a LAV efficacious by characterising immune responses to a LAV, termed TAS2010, which is highly protective (80-90%) against lethal murine salmonellosis, in comparison with a moderately protective (40-50%) LAV, BRD509. Mice vaccinated with TAS2010 developed immunity systemically and were protected against gut-associated virulent infection in a CD4+ T cell-dependent manner. TAS2010-vaccinated mice showed increased activation of Th1 responses compared with their BRD509-vaccinated counterparts, leading to increased Th1 memory populations in both lymphoid and non-lymphoid organs. The optimal development of Th1-driven immunity was closely correlated with the activation of CD11b+Ly6GnegLy6Chi inflammatory monocytes (IMs), the activation of which can be modulated proportionally by bacterial load in vivo. Upon vaccination with the LAV, IMs expressed T cell chemoattractant CXCL9 that attracted CD4+ T cells to the foci of infection, where IMs also served as a potent source of antigen presentation and Th1-promoting cytokine IL-12. The expression of MHC-II in IMs was rapidly upregulated following vaccination and then maintained at an elevated level in immune mice, suggesting IMs may have a role in sustained antigen stimulation. Our findings present a longitudinal analysis of CD4+ T cell development post-vaccination with an intracellular bacterial LAV, and highlight the benefit of inflammation in the development of Th1 immunity. Future studies focusing on the induction of IMs may reveal key strategies for improving vaccine-induced T cell immunity.

Immunity to systemic Salmonella infection depends on multiple effector mechanisms. Lymphocyte-derived interferon gamma (IFN-γ) enhances cell-intrinsic bactericidal capabilities to antagonize the hijacking of phagocytes as replicative niches for Salmonella. Programmed cell death (PCD) provides another means through which phagocytes fight against intracellular Salmonella. We describe remarkable levels of flexibility with which the host coordinates and adapts these responses. This involves interchangeable cellular sources of IFN-γ regulated by innate and adaptive cues, and the rewiring of PCD pathways in previously unknown ways. We discuss that such plasticity is likely the consequence of host–pathogen coevolution and raise the possibility of further functional overlap between these seemingly distinct processes.

Antibiotic resistance is driven by selection, but the degree to which a bacterial strain's evolutionary history shapes the mechanism and strength of resistance remains an open question. Here, we reconstruct the genetic and evolutionary mechanisms of carbapenem resistance in a clinical isolate of Klebsiella quasipneumoniae. A combination of short- and long-read sequencing, machine learning, and genetic and enzymatic analyses established that this carbapenem-resistant strain carries no carbapenemase-encoding genes. Genetic reconstruction of the resistance phenotype confirmed that two distinct genetic loci are necessary in order for the strain to acquire carbapenem resistance. Experimental evolution of the carbapenem-resistant strains in growth conditions without the antibiotic revealed that both loci confer a significant cost and are readily lost by de novo mutations resulting in the rapid evolution of a carbapenem-sensitive phenotype. To explain how carbapenem resistance evolves via multiple, low-fitness single-locus intermediates, we hypothesised that one of these loci had previously conferred adaptation to another antibiotic. Fitness assays in a range of drug concentrations show how selection in the antibiotic ceftazidime can select for one gene (bla(DHA-1)) potentiating the evolution of carbapenem resistance by a single mutation in a second gene (ompK36). These results show how a patient's treatment history might shape the evolution of antibiotic resistance and could explain the genetic basis of carbapenem-resistance found in many enteric-pathogens.

The safeguarding of Australian outdoor stone heritage is currently limited by a lack of information concerning mechanisms responsible for the degradation of the built heritage. In this study, the bacterial community colonizing the stone surface of an outdoor sculpture located at the Church of St. John the Evangelist in Melbourne was analysed, providing an overview of the patterns of microbial composition associated with stone in an anthropogenic context. Illumina MiSeq 16S rRNA gene sequencing together with confocal laser microscope investigations highlighted the bacterial community was composed of both phototrophic and chemotrophic microorganisms characteristic of stone and soil, and typical of arid, salty and urban environments. Cardinal exposure, position and surface geometry were the most important factors in determining the structure of the microbial community. The North-West exposed areas on the top of the sculpture with high light exposure gave back the highest number of sequences and were dominated by Cyanobacteria. The South and West facing in middle and lower parts of the sculpture received significantly lower levels of radiation and were dominated by Actinobacteria. Proteobacteria were observed as widespread on the sculpture. This pioneer research provided an in-depth investigation of the microbial community structure on a deteriorated artistic stone in the Australian continent and provides information for the identification of deterioration-associated microorganisms and/or bacteria beneficial for stone preservation.

Even in the setting of optimal resuscitation in high-income countries severe sepsis and septic shock have a mortality of 20–40%, with antibiotic resistance dramatically increasing this mortality risk. To develop a reference dataset enabling the identification of common bacterial targets for therapeutic intervention, we applied a standardized genomic, transcriptomic, proteomic and metabolomic technological framework to multiple clinical isolates of four sepsis-causing pathogens: Escherichia coli , Klebsiella pneumoniae species complex, Staphylococcus aureus and Streptococcus pyogenes . Exposure to human serum generated a sepsis molecular signature containing global increases in fatty acid and lipid biosynthesis and metabolism, consistent with cell envelope remodelling and nutrient adaptation for osmoprotection. In addition, acquisition of cholesterol was identified across the bacterial species. This detailed reference dataset has been established as an open resource to support discovery and translational research.

Despite the importance of encapsulation in bacterial pathogenesis, the biochemical mechanisms and forces that underpin retention of capsule by encapsulated bacteria are poorly understood. In Gram-negative bacteria, there may be interactions between lipopolysaccharide (LPS) core and capsule polymers, between capsule polymers with retained acyl carriers and the outer membrane, and in some bacteria, between the capsule polymers and Wzi, an outer membrane protein lectin. Our transposon studies in Klebsiella pneumoniae B5055 identified additional genes that, when insertionally inactivated, resulted in reduced encapsulation. Inactivation of the gene waaL, which encodes the ligase responsible for attaching the repeated O antigen of LPS to the LPS core, resulted in a significant reduction in capsule retention, measured by atomic force microscopy. This reduction in encapsulation was associated with increased sensitivity to human serum and decreased virulence in a murine model of respiratory infection and, paradoxically, with increased biofilm formation. The capsule in the WaaL mutant was physically smaller than that of the Wzi mutant of K. pneumoniae B5055. These results suggest that interactions between surface carbohydrate polymers may enhance encapsulation, a key phenotype in bacterial virulence, and provide another target for the development of antimicrobials that may avoid resistance issues associated with growth inhibition. IMPORTANCE Bacterial capsules, typically comprised of complex sugars, enable pathogens to avoid key host responses to infection, including phagocytosis. These capsules are synthesized within the bacteria, exported through the outer envelope, and then secured to the external surface of the organism by a force or forces that are incompletely described. This study shows that in the important hospital pathogen Klebsiella pneumoniae, the polysaccharide capsule is retained by interactions with other surface sugars, especially the repeated sugar molecule of the LPS molecule in Gram-negative bacteria known as "O antigen." This O antigen is joined to the LPS molecule by ligation, and loss of the enzyme responsible for ligation, a protein called WaaL, results in reduced encapsulation. Since capsules are essential to the virulence of many pathogens, WaaL might provide a target for new antimicrobial development, critical to the control of pathogens like K. pneumoniae that have become highly drug resistant.

IgA deficiency is the commonest immunodeficiency affecting up to 1 in 700 individuals. The effects of IgA deficiency are difficult to see in many individuals, are mild in many fewer and severe in fewer still. While monovalent IgA is found in serum, dimeric IgA is secreted through mucosal surfaces where it helps to maintain epithelial homeostasis. Studies with knockout mice have taught us that there are subtle inflammatory consequences of removing secretory IgA (sIgA), and the best explanation for these changes can be related by the loss of the ‘excretory’ immune system. The excretion of antigens is a logical process in regulating the immune system, given the long half-life of complement fixing antibodies. But the function of IgA as an immune or inflammation regulator may go beyond antigen removal.

ABSTRACT Antibiotic resistance is driven by selection, but how a bacterial strain’s evolutionary history shapes drug-resistance remains an open question. Here we reconstruct the genetic and evolutionary mechanisms of carbapenem resistance in a clinical isolate of Klebsiella . A combination of short and long read sequencing, machine learning, genetic and enzymatic analyses established that this carbapenem-resistant strain carries no carbapenemase-encoding genes. Genetic reconstruction of the resistance phenotype confirmed that two distinct genetic loci are necessary for the strain to acquire carbapenem resistance. Experimental evolution of the carbapenem-resistant strains in growth conditions without the antibiotic revealed that both loci confer a significant cost, and are readily lost by de novo mutation resulting in the rapid evolution of a carbapenem-sensitive phenotype. Thus, historical contingency - a patient’s treatment history - can shape the evolution of antibiotic resistance and suggests that the strategic combinations of antibiotics could direct the evolution of low-fitness, drug-resistant genotypes.

Klebsiella pneumoniae is an opportunistic Gram-negative pathogen that is a leading cause of healthcare-associated infections, including pneumonia, urinary tract infections, and sepsis. Essential to the colonization and infection by K. pneumoniae is the acquisition of nutrients, such as the transition metal ion zinc. Zinc has crucial structural and catalytic roles in the proteome of all organisms. Nevertheless, in excess, it has the potential to mediate significant toxicity by dysregulating the homeostasis of other transition elements, disrupting enzymatic processes, and perturbing metalloprotein cofactor acquisition. Here, we sought to elucidate the zinc detoxification mechanisms of K. pneumoniae, which remain poorly defined. Using the representative K. pneumoniae AJ218 strain, we showed that the P-type ATPase, ZntA, which is upregulated in response to cellular zinc stress, was the primary zinc efflux pathway. Deletion of zntA rendered K. pneumoniae AJ218 highly susceptible to exogenous zinc stress and manifested as an impaired growth phenotype and increased cellular accumulation of the metal. Loss of zntA also increased sensitivity to cadmium stress, indicating a role for this efflux pathway in cadmium resistance. Disruption of zinc homeostasis in the K. pneumoniae AJ218 ΔzntA strain also impacted manganese and iron homeostasis and was associated with increased production of biofilm. Collectively, this work showed the critical role of ZntA in K. pneumoniae zinc tolerance and provided a foundation for further studies on zinc homeostasis and the future development of novel antimicrobials to target this pathway. IMPORTANCE Klebsiella pneumoniae is a leading cause of healthcare-associated infections, including pneumonia, urinary tract infections, and sepsis. Treatment of K. pneumoniae infections is becoming increasingly challenging due to high levels of antibiotic resistance and the rising prevalence of carbapenem-resistant, extended-spectrum β-lactamases producing strains. Zinc is essential to the colonization and infection by many bacterial pathogens but toxic in excess. This work described the first dissection of the pathways associated with resisting extracellular zinc stress in K. pneumoniae. This study revealed that the P-type ATPase ZntA was highly upregulated in response to exogenous zinc stress and played a major role in maintaining bacterial metal homeostasis. Knowledge of how this major bacterial pathogen resists zinc stress provided a foundation for antimicrobial development studies to target and abrogate their essential function.

Klebsiella pneumoniae is a major cause of opportunistic healthcare-associated infections, which are increasingly complicated by the presence of extended-spectrum beta-lactamases (ESBLs) and carbapenem resistance. We conducted a year-long prospective surveillance study of K. pneumoniae clinical isolates in hospital patients. Whole-genome sequence (WGS) data reveals a diverse pathogen population, including other species within the K. pneumoniae species complex (18%). Several infections were caused by K. variicola/K. pneumoniae hybrids, one of which shows evidence of nosocomial transmission. A wide range of antimicrobial resistance (AMR) phenotypes are observed, and diverse genetic mechanisms identified (mainly plasmid-borne genes). ESBLs are correlated with presence of other acquired AMR genes (median n = 10). Bacterial genomic features associated with nosocomial onset are ESBLs (OR 2.34, p = 0.015) and rhamnose-positive capsules (OR 3.12, p < 0.001). Virulence plasmid-encoded features (aerobactin, hypermucoidy) are observed at low-prevalence (<3%), mostly in community-onset cases. WGS-confirmed nosocomial transmission is implicated in just 10% of cases, but strongly associated with ESBLs (OR 21, p < 1 × 10 −11 ). We estimate 28% risk of onward nosocomial transmission for ESBL-positive strains vs 1.7% for ESBL-negative strains. These data indicate that K. pneumoniae infections in hospitalised patients are due largely to opportunistic infections with diverse strains, with an additional burden from nosocomially-transmitted AMR strains and community-acquired hypervirulent strains.

ABSTRACT. Typhoid is an endemic in Fiji with increases observed since the early 2000s and frequent outbreaks reported. We assessed the diagnostic accuracy of currently available typhoid rapid diagnostic tests (RDTs) (TUBEX, Typhidot Rapid, and Test-It assay) to establish their performance against blood culture in Fiji and to examine their suitability for rapid typhoid outbreak identification. The performance of RDTs was assessed in the public health reference laboratory in Suva, Fiji, according to the manufacturers’ instructions. A simulation was used to examine the potential use of RDTs for attribution of a febrile illness outbreak to typhoid. For the diagnostic evaluation, 179 patients were included; 49 had blood culture–confirmed typhoid, 76 had fever as a result of non-typhoid etiologies, and 54 were age-matched community controls. The median (interquartile range) age was 29 (20–46) years. Of the participants, 92 (51.4%) were male and 131 (73.2%) were indigenous Fijians. The sensitivities of the tests were 77.6% for TUBEX, 75.5% for Typhidot Rapid, and 57.1% for Test-It assay. The Test-It assay had the highest specificity of 93.4%, followed by Typhidot Rapid 85.5% and TUBEX 60.5%. Typhidot Rapid had the best performance in the simulation for attribution of a febrile illness outbreak to typhoid. Typhoid RDTs performed suboptimally for individual patient diagnosis due to low sensitivity and variable specificity. We demonstrate that RDTs could be useful in the field for rapid attribution of febrile illness outbreaks to typhoid. Typhidot Rapid had the best combination of sensitivity, specificity, positive and negative predictive values, cost, and ease of use for this purpose.

Background Typhoid fever is endemic in some Pacific Island Countries including Fiji and Samoa yet genomic surveillance is not routine in such settings. Previous studies suggested imports of the global H58 clade of Salmonella enterica var Typhi (Salmonella Typhi) contribute to disease in these countries which, given the MDR potential of H58, does not auger well for treatment. The objective of the study was to define the genomic epidemiology of Salmonella Typhi in Fiji.Methods Genomic sequencing approaches were implemented to study the distribution of 255 Salmonella Typhi isolates from the Central Division of Fiji. We augmented epidemiological surveillance and Bayesian phylogenomic approaches with a multi-year typhoid case-control study to define geospatial patterns among typhoid cases.Findings Genomic analyses showed Salmonella Typhi from Fiji resolved into 2 non-H58 genotypes with isolates from the two dominant ethnic groups, the Indigenous (iTaukei) and non-iTaukei genetically indistinguishable. Low rates of international importation of clones was observed and overall, there were very low levels an antibiotic resistance within the endemic Fijian typhoid genotypes. Genomic epidemiological investigations were able to identify previously unlinked case clusters. Bayesian phylodynamic analyses suggested that genomic variation within the larger endemic Salmonella Typhi genotype expanded at discreet times, then contracted.Interpretation Cyclones and flooding drove 'waves' of typhoid outbreaks in Fiji which, through population aggregation, poor sanitation and water safety, and then mobility of the population, spread clones more widely. Minimal international importations of new typhoid clones suggest that targeted local intervention strategies may be useful in controlling endemic typhoid infection. These findings add to our understanding of typhoid transmission networks in an endemic island country with broad implications, particularly across Pacific Island Countries.Copyright (c) 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

SARS-CoV-2 infections were first reported in Wuhan, China, in 2019,1Huang C Wang Y Li X et al.Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China.Lancet. 2020; 395: 497-506Summary Full Text Full Text PDF PubMed Scopus (33099) Google Scholar and quickly became a global pandemic, as declared on March 11, 2020.2Cucinotta D Vanelli M WHO declares COVID-19 a pandemic.Acta Biomed. 2020; 91: 157-160PubMed Google Scholar SARS-CoV-2 is highly infectious3Petersen E Koopmans M Go U et al.Comparing SARS-CoV-2 with SARS-CoV and influenza pandemics.Lancet Infect Dis. 2020; 20: e238-e244Summary Full Text Full Text PDF PubMed Scopus (850) Google Scholar and COVID-19 is variable in its presentation, with many infected individuals, as detected by viral nucleic acid screening, being asymptomatic. In The Lancet, Zhenyu He and colleagues4He Z Ren L Yang J et al.Seroprevalence and humoral immune durability of anti-SARS-CoV-2 antibodies in Wuhan, China: a longitudinal, population-level, cross-sectional study.Lancet. 2021; 397: 1075-1084Summary Full Text Full Text PDF PubMed Scopus (175) Google Scholar report their cross-sectional study of serological responses of more than 9500 individuals from 3600 households in Wuhan, the early epicentre for the COVID-19 outbreak. The study was initiated shortly after lockdown in Wuhan ceased in April, 2020, with follow-up over two timepoints (June and October–December, 2020). In this cross-sectional, longitudinal serosurvey of Wuhan residents, the authors estimated the penetration of the virus into the community who were selected using a population-stratified, random-sampling approach. They also examined the development and durability of SARS-CoV-2 neutralising antibodies, the putative correlate of protection in COVID-19.5Garcia-Beltran WF Lam EC Astudillo MG et al.COVID-19-neutralizing antibodies predict disease severity and survival.Cell. 2021; 184: 476-488Summary Full Text Full Text PDF PubMed Scopus (448) Google Scholar In the Wuhan population sampled by He and colleagues, 532 (5·6%) of 9542 participants were positive for pan-immunoglobulins against SARS-CoV-2 at baseline, giving an adjusted seroprevalence of 6·92% (higher than an earlier report,6Xu X Sun J Nie S et al.Seroprevalence of immunoglobulin M and G antibodies against SARS-CoV-2 in China.Nat Med. 2020; 26: 1193-1195Crossref PubMed Scopus (309) Google Scholar which estimated a seroprevalence of 3·2–3·8%). More than 80% of those surveyed who were seropositive were asymptomatic. As a city of 11 million people with 9 million residents as estimated during lockdown,6Xu X Sun J Nie S et al.Seroprevalence of immunoglobulin M and G antibodies against SARS-CoV-2 in China.Nat Med. 2020; 26: 1193-1195Crossref PubMed Scopus (309) Google Scholar quick extrapolation of these estimates suggests that approximately 622 800 individuals in Wuhan would have been infected as of April, 2020, far exceeding the cumulative number of confirmed cases in Wuhan (50 333 as of April 17, 20207Health Commission of Hubei ProvinceEpidemic situation of new COVID-19 in Hubei Province on April 17, 2020.http://wjw.hubei.gov.cn/bmdt/ztzl/fkxxgzbdgrfyyq/xxfb/202004/t20200418_2234341.shtmlDate accessed: January 25, 2021Google Scholar). If the seroconversion rate is an accurate reflection of exposure to SARS-CoV-2, the apparent disparity between low case numbers and high seroconversion rate seems to suggest that most seroconverted individuals produced antibodies to SARS-CoV-2 after asymptomatic infection. Accordingly, by occupation group, the highest seroconversion rate was seen in health workers and in those with known COVID-19 contacts in the past 5 months. This observation suggests a relay of exposure that frequently resulted in asymptomatic transmission chains because, when the full population of Wuhan was screened in May, 2020, few additional, asymptomatic cases were detected compared with the known number of confirmed cases in April, 2020.8Cao S Gan Y Wang C et al.Post-lockdown SARS-CoV-2 nucleic acid screening in nearly ten million residents of Wuhan, China.Nat Commun. 2020; 115917Crossref PubMed Scopus (118) Google Scholar Although separating transmitting from non-transmitting asymptomatic individuals with COVID-19 is probably impractical, the transmission outcomes from these two patient cohorts would suggest that different management regimens might be considered. He and colleagues detected neutralising antibodies in 39·8% of the seropositive subgroup; encouragingly, the proportion was sustained over the 9-month study period, indicating that putative protection was durable when it occurred. Moreover, titres of neutralising antibodies were lower in individuals who had asymptomatic infections, compared with those with symptomatic infection and confirmed cases. The reason why those with asymptomatic disease developed lower neutralising antibody titres than those with symptomatic disease might be simply explained by different levels of exposure to SARS-CoV-2 antigens. Whether the levels of neutralising antibodies, or their transient appearance, also affected the so-called transmissibility potential would require further investigation. Efficient global management of COVID-19 will probably succeed or fail on the basis of the immunity induced by natural infection and, especially, vaccination. Given the relative paucity of neutralising antibodies through natural infection, the study by He and colleagues reinforces the need for effective COVID-19 vaccines in the population-level control of the disease. The extraordinary, rapid, and effective control measures implemented in Wuhan might have restricted the spread of the virus, but also reduced naturally acquired herd immunity by truncating the development of sustained neutralising antibodies. Although other national and local governments have used alternate and usually less effective strategies to control the spread of SARS-CoV-2, even in highly endemic communities the prevalence of disease is usually less 1%, but has increased to 23% in some communities.9Ripperger TJ Uhrlaub JL Watanabe M et al.Orthogonal SARS-CoV-2 serological assays enable surveillance of low-prevalence communities and reveal durable humoral immunity.Immunity. 2020; 53: 925-933Summary Full Text Full Text PDF PubMed Scopus (230) Google Scholar, 10Gidding HF Machalek DA Hendry AJ et al.Seroprevalence of SARS-CoV-2-specific antibodies in Sydney after the first epidemic wave in 2020.Med J Aust. 2021; (published online Feb 3.)https://doi.org/10.5694/mja2.50940Crossref PubMed Scopus (19) Google Scholar, 11Poustchi H Darvishian M Mohammadi Z et al.SARS-CoV-2 antibody seroprevalence in the general population and high-risk occupational groups across 18 cities in Iran: a population-based cross-sectional study.Lancet Infect Dis. 2020; (published online Dec 15.)https://doi.org/10.1016/S1473-3099(20)30858-6Summary Full Text Full Text PDF PubMed Scopus (112) Google Scholar, 12Bajema KL Wiegand RE Cuffe K et al.Estimated SARS-CoV-2 seroprevalence in the US as of September 2020.JAMA Intern Med. 2020; (published online Nov 24.)https://doi.org/10.1001/jamainternmed.2020.7976Crossref PubMed Scopus (190) Google Scholar He and colleagues' findings suggest that herd immunity will likely not develop after natural transmission in settings where infection control mechanisms are successfully introduced, underscoring the importance of effective vaccination strategies to control the spread of COVID-19. This study is an important milestone in the description of SARS-CoV-2 infection and our understanding of immunity in the pandemic. He and colleagues have provided a much deeper understanding of natural seroconversion in a key city in the pandemic and their findings also underscore the remarkable achievement of the Chinese public health system in controlling the Wuhan outbreak of COVID-19 at a time when testing, tracing, and treatment resources were much less developed. We declare no competing interests. Seroprevalence and humoral immune durability of anti-SARS-CoV-2 antibodies in Wuhan, China: a longitudinal, population-level, cross-sectional study6·92% of a cross-sectional sample of the population of Wuhan developed antibodies against SARS-CoV-2, with 39·8% of this population seroconverting to have neutralising antibodies. Our durability data on humoral responses indicate that mass vaccination is needed to effect herd protection to prevent the resurgence of the epidemic. Full-Text PDF