Epidemiological and clinical trends of imported strongyloidiasis in a referral international health unit, Barcelona, Spain: A 12-year period experience.

BACKGROUND: Imported strongyloidiasis in non-endemic countries has increasingly been diagnosed. The aim of the present study is to describe the main epidemiological and clinical characteristics of patients with imported strongyloidiasis attended in a referral International Health Unit and to detect trend changes over a 12-year period. METHODS: This is an observational retrospective study including all imported strongyloidiasis cases seen at the International Health Unit Vall d'Hebron-Drassanes (Barcelona, Spain) from January 2009 to December 2020. Epidemiological and clinical characteristics from included patients were collected. RESULTS: Overall, 865 cases of imported strongyloidiasis were diagnosed, of whom 472 (54.6 %) were men and mean age was 38.7 (SD 13.4) years. Most cases were diagnosed in migrants (830, 96 %). The distribution of the geographic origin was: Latin America (561, 67.6 %), Sub-Saharan Africa (148, 17.8 %), Asia (113, 13.6 %), North Africa (5, 0.6 %), Eastern Europe (2, 0.2 %), and North America (1, 0.1 %). The main reasons for consultation at the Unit were screening of health status (371, 42.9 %), laboratory test alteration (367, 42.4 %), gastrointestinal symptoms (56, 6.5 %), cutaneous symptoms (26, 3 %), and other clinical symptoms (45, 5.2 %). An increase in the number of cases was observed in the last years of the study period. CONCLUSIONS: Imported strongyloidiasis has increasingly been diagnosed in our referral unit, mostly due to screening strategies implementation. Most of the patients were young migrants coming from Latin America, with no symptoms at the time of diagnosis. The optimization of screening strategies will increase the detection and treatment of cases, reducing potential complications.

Genetic Characterization of Extensively Drug-Resistant Shigella sonnei Infections, Spain, 2021-2022.

In 2022, the United Kingdom reported an increase in drug resistance in Shigella sonnei isolates. We report 33 cases in Spain genetically related to the UK cases and 4 cases with similar antimicrobial resistance profiles infected with genetically distant strains. Our results suggest circulation of multiple genetic clusters of multidrug-resistant S. sonnei in Spain.

Unexpected Loa loa Finding in an Asymptomatic Patient From The Gambia: A Case Report.

A 17-year-old asymptomatic male from The Gambia presented for a routine health examination after migration to Spain. Laboratory diagnosis confirmed the presence of Loa loa microfilariae. This unusual finding emphasizes the importance of screening in newly arrived migrants and the need of an extended anamnesis including migratory route and previous travels.

Post-infectious irritable bowel syndrome following a diagnosis of traveller's diarrhoea: a comprehensive characterization of clinical and laboratory parameters.

BACKGROUND: Prolonged or recurrent gastrointestinal symptoms may persist after acute traveller's diarrhoea (TD), even after adequate treatment of the primary cause. This study aims to describe the epidemiological, clinical and microbiological characteristics of patients with post-infectious irritable bowel syndrome (PI-IBS) after returning from tropical or subtropical areas. METHODS: We conducted a retrospective study of patients presenting between 2009 and 2018 at the International Health referral centre in Barcelona with persistent gastrointestinal symptoms following a diagnosis of TD. PI-IBS was defined as the presence of persistent or recurrent gastrointestinal manifestations for at least 6 months after the diagnosis of TD, a negative stool culture for bacterial pathogens and a negative ova and parasite exam after targeted treatment. Epidemiological, clinical and microbiological variables were collected. RESULTS: We identified 669 travellers with a diagnosis of TD. Sixty-eight (10.2%) of these travellers, mean age 33 years and 36 (52.9%) women, developed PI-IBS. The most frequently visited geographical areas were Latin America (29.4%) and the Middle East (17.6%), with a median trip duration of 30 days (IQR 14-96). A microbiological diagnosis of TD was made in 32 of these 68 (47%) patients, 24 (75%) of whom had a parasitic infection, Giardia duodenalis being the most commonly detected parasite (n = 20, 83.3%). The symptoms persisted for a mean of 15 months after diagnosis and treatment of TD. The multivariate analysis revealed that parasitic infections were independent risk factors for PI-IBS (OR 3.0, 95%CI 1.2-7.8). Pre-travel counselling reduced the risk of PI-IBS (OR 0.4, 95%CI 0.2-0.9). CONCLUSIONS: In our cohort, almost 10% of patients with travellers' diarrhoea developed persistent symptoms compatible with PI-IBS. Parasitic infections, mainly giardiasis, seem to be associated with PI-IBS.

Increasing trend of antimicrobial resistance in Shigella associated with MSM transmission in Barcelona, 2020-21: outbreak of XRD Shigella sonnei and dissemination of ESBL-producing Shigella flexneri.

BACKGROUND: Several countries have recently reported the detection of ESBL-producing Shigella sonnei associated with transmission among MSM. In a previous study by our group, 2.8% of Shigella spp. obtained from MSM in Barcelona between 2015 and 2019 were ESBL producers. OBJECTIVES: To describe and characterize the emerging ESBL-producing Shigella spp. associated with sexual transmission among MSM detected from 2020 to 2021 in Barcelona, elucidating their connectivity with contemporaneous ESBL-producing Shigella spp. from other countries. RESULTS: From 2020 to 2021, we identified that among MSM, 68% of S. sonnei were XDR harbouring blaCTX-M-27 and 14% of Shigella flexneri were MDR harbouring blaCTX-M-27. WGS analysis showed that the ESBL-producing S. sonnei were part of a monophyletic cluster, which included isolates responsible for the prolonged outbreak occurring in the UK. Our data also reveal the first emergence and clonal dissemination of ESBL-producing and fluoroquinolone-resistant S. flexneri 2a among MSM. CONCLUSIONS: We report an increasing trend of antimicrobial resistance in Shigella spp. among MSM in Barcelona since 2021, mainly as a consequence of the dissemination of XDR ESBL-producing S. sonnei, previously reported in the UK. These results highlight the importance of international collaborative surveillance of MDR/XDR S. sonnei and S. flexneri for rapid identification of their emergence and the prevention of the transmission of these pathogens.

Evaluation of Two Different Strategies for Schistosomiasis Screening in High-Risk Groups in a Non-Endemic Setting.

A consensus on the recommended screening algorithms for schistosomiasis in asymptomatic high-risk subjects in non-endemic areas is lacking. The objective of this study was to evaluate the real-life performance of direct microscopy and ELISA serology for schistosomiasis screening in a high-risk population in a non-endemic setting. A retrospective cohort study was conducted in two out-patient Tropical Medicine units in Barcelona (Spain) from 2014 to 2017. Asymptomatic adults arriving from the Sub-Saharan region were included. Schistosomiasis screening was conducted according to clinical practice following a different strategy in each setting: (A) feces and urine direct examination plus S. mansoni serology if non-explained eosinophilia was present and (B) S. mansoni serology plus uroparasitological examination as the second step in case of a positive serology. Demographic, clinical and laboratory features were collected. Schistosomiasis cases, clinical management and a 24 month follow-up were recorded for each group. Four-hundred forty individuals were included. The patients were mainly from West African countries. Fifty schistosomiasis cases were detected (11.5% group A vs. 4 % group B, p = 0.733). When both microscopic and serological techniques were performed, discordant results were recorded in 18.4% (16/88). Schistosomiasis cases were younger (p < 0.001) and presented eosinophilia and elevated IgE (p < 0.001) more frequently. Schistosomiasis is a frequent diagnosis among high-risk populations. Serology achieves a similar performance to direct diagnosis for the screening of schistosomiasis in a high-risk population.

Imported Strongyloides stercoralis infection and diabetes mellitus and other metabolic diseases: Is there any association?

OBJECTIVES: Strongyloidiasis is a nematode infection caused by Strongyloides stercoralis. Previous studies have addressed the possibility of the parasite to establish a complex relationship with the host that could affect the risk of developing diabetes mellitus or modify its presentation. This study aims to evaluate the potential impact of strongyloidiasis in diabetes mellitus and other metabolic diseases. METHODS: Case-control observational retrospective study that included 95 S. stercoralis-infected patients and 83 non-infected individuals. Epidemiological and clinical variables were retrieved from medical records, and a statistical analysis was carried out to explore any association between strongyloidiasis and diabetes mellitus and other metabolic diseases. RESULTS: Most of the patients were men (99, 55.60%) with a mean age of 42.53 ± SD 14 years. Twelve (6.70%) patients were diabetic; 30 (16.90%) presented arterial hypertension; 28 (15.70%) had dyslipidaemia; and 10 (5.60%) had thyroid pathology. When comparing patients with strongyloidiasis and uninfected patients, no differences were found regarding diabetes mellitus or other metabolic diseases. CONCLUSIONS: The results obtained in the present study do not confirm any type of association between strongyloidiasis and diabetes mellitus or other metabolic diseases.

Opportunistic Community Screening of Chronic Chagas Disease Using a Rapid Diagnosis Test in Pharmacies in Barcelona (Catalonia, Spain): Study Protocol and Pilot Phase Results.

Objectives: This study aimed to report the protocol and results from the pilot phase of an opportunistic CP-based CD screening program in Barcelona, Spain. Methods: Three strategies according to recruitment approach were designed: passive, active and active-community. The study process consisted of signing the informed consent form, recording the patient's data in a web-based database system, and performing the rapid test and blood collection on dry paper. Results: Nineteen pharmacies participated and 64 patients were included during the pilot phase of the study. The rapid diagnostic test (RDT) was positive in 2/64 (3.13%) cases. Of the 49 DBS samples that arrived at the laboratory, 22 (45%) were collected incorrectly. After quantitative and qualitative assessment of the program, the dry paper sample and passive strategy were ruled out. Conclusion: DBS sampling and the passive strategy are not suitable for CD screening in community pharmacies. There is a need to expand the number of participating pharmacies and individuals to determine whether conducting a RDT in community pharmacies is an effective screening method to increase access to CD diagnosis in a non-endemic area.

COVID-19: an opportunity of systematic integration for Chagas disease. Example of a community-based approach within the Bolivian population in Barcelona.

BACKGROUND: As a Neglected Tropical Disease associated with Latin America, Chagas Disease (CD) is little known in non-endemic territories of the Americas, Europe and Western Pacific, making its control challenging, with limited detection rates, healthcare access and consequent epidemiological silence. This is reinforced by its biomedical characteristics-it is usually asymptomatic-and the fact that it mostly affects people with low social and financial resources. Because CD is mainly a chronic infection, which principally causes a cardiomyopathy and can also cause a prothrombotic status, it increases the risk of contracting severe COVID-19. METHODS: In order to get an accurate picture of CD and COVID-19 overlapping and co-infection, this operational research draws on community-based experience and participative-action-research components. It was conducted during the Bolivian elections in Barcelona on a representative sample of that community. RESULTS: The results show that 55% of the people interviewed had already undergone a previous T. cruzi infection screening-among which 81% were diagnosed in Catalonia and 19% in Bolivia. The prevalence of T. cruzi infection was 18.3% (with 3.3% of discordant results), the SARS-CoV-2 22.3% and the coinfection rate, 6%. The benefits of an integrated approach for COVID-19 and CD were shown, since it only took an average of 25% of additional time per patient and undoubtedly empowered the patients about the co-infection, its detection and care. Finally, the rapid diagnostic test used for COVID-19 showed a sensitivity of 89.5%. CONCLUSIONS: This research addresses CD and its co-infection, through an innovative way, an opportunity of systematic integration, during the COVID-19 pandemic.

Epidemiological and clinical profile of adult patients with diarrhoea after international travel attended in an International Health referral center.

BACKGROUND: The aim of the study is to describe the epidemiological, clinical, and microbiological characteristics of patients with diarrhoea after their return from a trip to tropical and subtropical areas. METHODS: Retrospective study of patients with travel-related diarrhoea attended International Health referral center. Travel diarrhoea was defined as the presence of three or more liquid stools per day, or liquid stools more often than is normal for the individual, during travel or within two weeks after returning. Epidemiological, clinical and microbiological variables were collected. RESULTS: 669 patients were included, 393 (58.7%) were female, with a mean age of 33 (SD 10.7) years. Abdominal pain was present in 59.6% (n = 399), and fever in 44.7% (n = 299). In 43% (n = 280) cases the etiological agent was found. Giardia duodenalis, Enteropathogenic Escherichia coli, and Enterotoxigenic Escherichia coli were the most frequent identified causative agents. Parasitic cause of the diarrhoea was associated to a longer duration of the travel, longer duration of symptoms, and having received pre-travel counseling. CONCLUSIONS: In our cohort, that represents a group of travellers presenting prolonged symptoms after travel, the most frequent causes of diarrhoea were parasitic infections being the most prevalent Giardia duodenalis. This information could be relevant in order to improve travel-related diarrhoea management protocols in this type of patients.

Changes in the microbiological diagnosis and epidemiology of cutaneous leishmaniasis in real-time PCR era: A six-year experience in a referral center in Barcelona.

BACKGROUND: Leishmaniasis is a neglected disease caused by different species of the protozoa Leishmania spp. Cutaneous lesions are the most common clinical manifestation. This disease is prevalent in tropical and subtropical areas, including the Mediterranean basin. In Spain, Leishmania (L.) infantum is the only endemic species, but imported cases are often diagnosed. Different classical parasitological methods can be performed for cutaneous leishmaniasis (CL) diagnosis; but currently molecular techniques serve as a relevant tool for the detection and characterization of Leishmania parasites. We aimed to evaluate clinical and epidemiological characteristics of CL diagnosed patients by real-time PCR in a tertiary hospital over a six-year period. METHODOLOGY/PRINCIPAL FINDINGS: Clinical, epidemiological and microbiological data were retrospectively collected and analyzed. In our study, CL was confirmed in 59 (31.4%) out of 188 patients by real-time PCR, showing an increase over recent years: 11 cases of CL between 2014 and 2016 and 48 between 2017 and 2019. Real-time PCR was performed on skin swabs and/or biopsies samples, with a positivity of 38.5% and 26.5%, respectively. Results were 100% concordant when biopsy and skin swab were performed simultaneously. L. (L.) infantum was the most frequent species detected (50%), followed by L. (L.) major (45%) and Viannia subgenus (5%), which were detected only in imported cases. L. (L.) major was almost entirely detected in travelers/migrants from Morocco. Multiple and atypical skin lesions were more common in imported cases than in autochthonous cases (44.4% vs. 21.8%). CONCLUSIONS/SIGNIFICANCE: An increase in both autochthonous and imported CL cases has been observed in past years in our hospital. Molecular techniques assist in improving CL diagnosis and characterization of the Leishmania species, mainly in imported cases.

Pneumocystis jirovecii genetic diversity in a Spanish tertiary hospital.

Pneumocystis jirovecii is associated with non-noxious colonization or severe pneumonia in immunocompromised hosts. Epidemiological investigations have been hampered by the lack of a standardized typing scheme. Thus, only partial molecular data on Spanish P. jirovecii cases are available. Recently, a new ISHAM consensus multilocus sequence typing scheme (MLST) targeting ß-TUB, mt26S, CYB, and SOD with a publicly accessible database has been launched to overcome this problem. The molecular epidemiology of P. jirovecii from immunocompromised patients either colonized (n = 50) or having pneumonia (n = 36) seen between 2014 and 2018 at a single center in Barcelona, Spain, was studied. The new ISHAM consensus MSLT scheme was used to investigate the local epidemiology and identify possible unnoticed outbreaks. Mutations in the DHPS gene, not included in the scheme but giving information about potential sulfa treatment failure, were also studied. The study assigned 32 sequence types (ST) to 72.2% pneumonia and 56% colonization cases. The most frequent STs were ST21 (18.5%), ST22 (14.8%), and ST37(14.8%). For non-unique STs, ST3, ST30 and ST31 were found only in pneumonia cases, whereas ST27 was associated exclusively to colonizations. Despite 38 patients sharing similar STs, only two were involved in a potential cross transmission event. No DHPS mutations were identified. The new consensus typing scheme was useful to ascertain the molecular epidemiology of P. jirovecii in our center revealing a high genetic diversity and the potential association of specific STs to colonization and pneumonia cases. LAY SUMMARY: A newly described MLST scheme aims at providing a standardized tool to study and compare Pneumocystis jirovecii epidemiology. A high diversity among P. jirovecii isolates from patients in Barcelona, Spain, and a potential association between specific STs and infection/colonization were identified.

Analytical Evaluation of Dried Blood Spot and Rapid Diagnostic Test as a New Strategy for Serological Community Screening for Chronic Chagas Disease.

Background: Chagas disease is a public health problem not only in Latin America, but also in other regions, including Spain, due to migration movements. Conventional serological diagnosis requires an invasive sample (plasma or serum) and a well-equipped laboratory. To circumvent those limitations, blood samples dried on filter paper (DBS) or Rapid Diagnostic Test (RDT) could be a practical alternative to reference protocol for serological screening in epidemiological studies. We evaluated the usefulness of dried blood sampling and a rapid diagnostic test (Trypanosoma Detect™) for the detection of antibodies against T. cruzi for their use in community-based screening. Methodology/Principal Findings: A total of 162 stored paired whole-blood and serum samples from Latin American migrants and 25 negative-control blood samples were included. Diagnosis of chronic Chagas disease was performed in serum according to WHO algorithms. Blood samples were retrospectively collected as dried spots and then analyzed using two different serological techniques, enzyme-linked immunosorbent assay (ELISA) and electrochemiluminescence immunoassay (E-CLIA). Whole-blood samples were also used to evaluate a rapid diagnostic test based on immunochromatography. A better correlation with conventional serum was observed in dried blood elutes using E-CLIA than ELISA (97% vs. 77% sensitivity, respectively). Both assays reported 100% specificity. The median cut-off index values of E-CLIA for dried blood were significantly lower than those for serum (138.1 vs. 243.3, P<0.05). The Trypanosoma Detect™ test presented a sensitivity and specificity of 89.6% and 100%, respectively. Conclusions: The detection of antibodies against T. cruzi in dried blood samples shows a higher sensitivity when using E-CLIA compared with ELISA. Trypanosoma Detect™ is easier to use but has a lower sensitivity. Hence, we propose a sequential strategy based on performing the rapid test first, and a negative result will be confirmed by DBS-ECLIA for use in community Chagas disease screening programs.

Circulation of multi-drug-resistant Shigella sonnei and Shigella flexneri among men who have sex with men in Barcelona, Spain, 2015-2019.

BACKGROUND: In high-income countries, shigellosis is mainly found in travellers to high-risk regions or in men who have sex with men (MSM). This study investigated the genomic characteristics and the features of antimicrobial resistance of MSM-associated Shigella flexneri and Shigella sonnei circulating in Barcelona, Spain, elucidating their connectivity with contemporaneous Shigella spp. from other countries. METHODS: Antimicrobial susceptibility, whole-genome sequencing, genomic characterization and phylogenetic analysis were performed in MSM-associated Shigella spp. recovered from 2015 to 2019. Reference genomes of MSM-associated Shigella spp. were included for contextualization and to determine their connection with international outbreaks. RESULTS: In total, 44 S. flexneri and 26 S. sonnei were identified among MSM. Overall, 80% showed resistance to azithromycin, 65.7% showed resistance to trimethoprim-sulphamethoxazole and 32.8% showed resistance to ciprofloxacin; 27.1% were resistant to all three antimicrobials. mphA and/or ermB, and qnrS and mutations in the quinolone resistance determining regions were found in the azithromycin- and ciprofloxacin-resistant isolates, respectively. Additionally, two isolates carried blaCTX-M-27. Single-nucleotide-polymorphism-based analysis revealed that the isolates were organized into different lineages, most of which were closely related to dominant MSM-associated lineages described previously in the UK and Australia. CONCLUSIONS: This study investigated the circulation of lineages of S. flexneri and S. sonnei among MSM in Spain that were mainly resistant to first-/second-line oral treatments, and closely related to dominant MSM-associated lineages described previously in the UK and Australia. These data reinforce the urgent need for the implementation of public health measures focusing on the early detection and prevention of transmission of this emerging pathogen, which is contributing to the antimicrobial resistance crisis in sexually transmitted infections.

Differential Diagnosis of Fungal Pneumonias vs. Tuberculosis in AIDS Patients by Using Two New Molecular Methods.

Opportunistic fungal pneumonias (OFP) are the main cause of death in AIDS patients worldwide. Diagnosis of these infections is often late as tuberculosis (TB) is frequently the first suspicion. In addition, diagnostic tools have limitations and are unavailable in disadvantaged regions. To perform the differential diagnosis of the main fungi causing OFP in AIDS patients (Histoplasma capsulatum, Cryptococcus neoformans/C. gattii and Pneumocystis jirovecii) vs. the Mycobacterium tuberculosis complex (MTBC), two new assays were developed: (i) a multiplex real-time PCR (MRT-PCR) and (ii) a simple and cost-effective method based on real-time PCR and the analysis of melting curves after amplification (MC-PCR). Both of the techniques were optimized and standardized "in vitro", showing a suitable reproducibility (CV ranged between 1.84 and 3.81% and 1.41 and 4.83%, respectively), a 100% specificity and detection limits between 20 and 2 fg of genomic DNA per 20 µL of reaction. A validation study was performed by retrospectively using 42 clinical samples from 37 patients with proven fungal infection or TB, and 33 controls. The overall sensitivity for the MRT-PCR assay and the MC-PCR assay was 88% and 90.4%, respectively. Both techniques were fast, sensitive and reproducible, allowing for the detection of these pathogens and the performance of a differential diagnosis.

Blastocystis sp. Carriage and Irritable Bowel Syndrome: Is the Association Already Established?

BACKGROUND: The aim of the present study is to describe the occurrence of Blastocystis sp. detection among asymptomatic subjects and patients with irritable bowel syndrome in order to evaluate the potential association between irritable bowel syndrome and the parasitic infection. METHODS: Cross-sectional study where adult patients with irritable bowel syndrome diagnosed according to Rome IV criteria were included. A control group was formed by asymptomatic subjects older than 18 years. Exclusion criteria were: immunosuppressive condition or having received any drug with demonstrated activity against Blastocystis sp. within the last 6 months before study inclusion. Epidemiological and clinical information was collected from all included participants. Two stool samples were obtained from all participants: one sample for microscopic examination and one sample for Blastocystis sp. PCR detection. Blastocystis sp. infection was defined by the positivity of any of the diagnostic techniques. RESULTS: Seventy-two participants were included (36 asymptomatic subjects and 36 patients with irritable bowel syndrome). Thirty-five (48.6%) were men, and median age of participants was 34 (IQR 29-49) years. The overall rate of Blastocystis sp. carriage was 27.8% (20/72). The prevalence assessed through microscopic examination was 22.2% (16/72), while the prevalence measured by PCR was 15.3% (11/72). When comparing the presence of Blastocystis sp. between asymptomatic subjects and IBS patients, we did not find any statistically significant difference (36.1% vs. 19.4% respectively, p = 0.114). CONCLUSIONS: regarding the occurrence of Blastocystis sp., no differences were found between asymptomatic participants and patients with irritable bowel disease irrespective of the diagnostic technique performed.

Consensus Multilocus Sequence Typing Scheme for Pneumocystis jirovecii.

Pneumocystis jirovecii is an opportunistic human pathogenic fungus causing severe pneumonia mainly in immunocompromised hosts. Multilocus sequence typing (MLST) remains the gold standard for genotyping of this unculturable fungus. However, the lack of a consensus scheme impedes a global comparison, large scale population studies and the development of a global MLST database. To overcome this problem this study compared all genetic regions (19 loci) currently used in 31 different published Pneumocystis MLST schemes. The most diverse/commonly used eight loci, ß-TUB, CYB, DHPS, ITS1, ITS1/2, mt26S and SOD, were further assess for their ability to be successfully amplified and sequenced, and for their discriminatory power. The most successful loci were tested to identify genetically related and unrelated cases. A new consensus MLST scheme consisting of four genetically independent loci: ß-TUB, CYB, mt26S and SOD, is herein proposed for standardised P. jirovecii typing, successfully amplifying low and high fungal burden specimens, showing adequate discriminatory power, and correctly identifying suspected related and unrelated isolates. The new consensus MLST scheme, if accepted, will for the first time provide a powerful tool to investigate outbreak settings and undertake global epidemiological studies shedding light on the spread of this important human fungal pathogen.

Naturally occurring SARS-CoV-2 gene deletions close to the spike S1/S2 cleavage site in the viral quasispecies of COVID19 patients.

The SARS-CoV-2 spike (S) protein, the viral mediator for binding and entry into the host cell, has sparked great interest as a target for vaccine development and treatments with neutralizing antibodies. Initial data suggest that the virus has low mutation rates, but its large genome could facilitate recombination, insertions, and deletions, as has been described in other coronaviruses. Here, we deep-sequenced the complete SARS-CoV-2 S gene from 18 patients (10 with mild and 8 with severe COVID-19), and found that the virus accumulates deletions upstream and very close to the S1/S2 cleavage site (PRRAR/S), generating a frameshift with appearance of a stop codon. These deletions were found in a small percentage of the viral quasispecies (2.2%) in samples from all the mild and only half the severe COVID-19 patients. Our results suggest that the virus may generate free S1 protein released to the circulation. We suggest that natural selection has favoured a "Don't burn down the house" strategy, in which free S1 protein may compete with viral particles for the ACE2 receptor, thus reducing the severity of the infection and tissue damage without losing transmission capability.