The landscape of the COVID-19 pandemic in Poland emerging from epidemiological and genomic data.

The COVID-19 pandemic has profoundly affected all aspects of our lives. Through real-time monitoring and rapid vaccine implementation, we succeeded in suppressing the spread of the disease and mitigating its consequences. Finally, conclusions can be summarized and drawn. Here, we use the example of Poland, which was seriously affected by the pandemic. Compared to other countries, Poland has not achieved impressive results in either testing or vaccination, which may explain its high mortality (case fatality rate, CFR 1.94%). Through retrospective analysis of data collected by the COVID-19 Data Portal Poland, we found significant regional differences in the number of tests performed, number of cases detected, number of COVID-19-related deaths, and vaccination rates. The Masovian, Greater Poland, and Pomeranian voivodeships, the country's leaders in vaccination, reported high case numbers but low death rates. In contrast, the voivodeships in the eastern and southern parts of Poland (Subcarpathian, Podlaskie, Lublin, Opole), which documented low vaccination levels and low case numbers, had higher COVID-19-related mortality rates. The strong negative correlation between the CFR and the percentage of the population that was vaccinated in Poland supports the validity of vaccination. To gain insight into virus evolution, we sequenced more than 500 genomes and analyzed nearly 80 thousand SARS-CoV-2 genome sequences deposited in GISAID by Polish diagnostic centers. We showed that the SARS-CoV-2 variant distribution over time in Poland reflected that in Europe. Haplotype network analysis allowed us to follow the virus transmission routes and identify potential superspreaders in each pandemic wave.

Anti-SARS-CoV-2 Antibodies Level and COVID-19 Vaccine Boosters among Healthcare Workers with the Highest SARS-CoV-2 Infection Risk-Follow Up Study.

During the COVID-19 pandemic, several vaccines were developed to limit the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, due to SARS-CoV-2 mutations and uneven vaccination coverage among populations, a series of COVID-19 waves have been caused by different variants of concern (VOCs). Despite the updated vaccine formulations for the new VOC, the benefits of additional COVID-19 vaccine doses have raised many doubts, even among high-risk groups such as healthcare workers (HCWs). We examined the factors underlying hesitancy to receive COVID-19 booster vaccine doses and analysed the anti-SARS-CoV-2 IgG antibody response after booster vaccination among HCWs. Our study found that 42% of the HCWs were hesitant about the second booster dose, while 7% reported no intent to get vaccinated with any additional doses. As reasons for not vaccinating, participants most frequently highlighted lack of time, negative experiences with previous vaccinations, and immunity conferred by past infections. In addition, we found the lowest post-vaccination antibody titres among HCWs who did not receive any vaccine booster dose and the highest among HCWs vaccinated with two booster doses.

FAIR+E pathogen data for surveillance and research: lessons from COVID-19.

The COVID-19 pandemic has exemplified the importance of interoperable and equitable data sharing for global surveillance and to support research. While many challenges could be overcome, at least in some countries, many hurdles within the organizational, scientific, technical and cultural realms still remain to be tackled to be prepared for future threats. We propose to (i) continue supporting global efforts that have proven to be efficient and trustworthy toward addressing challenges in pathogen molecular data sharing; (ii) establish a distributed network of Pathogen Data Platforms to (a) ensure high quality data, metadata standardization and data analysis, (b) perform data brokering on behalf of data providers both for research and surveillance, (c) foster capacity building and continuous improvements, also for pandemic preparedness; (iii) establish an International One Health Pathogens Portal, connecting pathogen data isolated from various sources (human, animal, food, environment), in a truly One Health approach and following FAIR principles. To address these challenging endeavors, we have started an ELIXIR Focus Group where we invite all interested experts to join in a concerted, expert-driven effort toward sustaining and ensuring high-quality data for global surveillance and research.

Novel Molecular Consortia of Cannabidiol with Nonsteroidal Anti-Inflammatory Drugs Inhibit Emerging Coronaviruses' Entry.

The COVID-19 pandemic provoked a global health crisis and highlighted the need for new therapeutic strategies. In this study, we explore the potential of the molecular consortia of cannabidiol (CBD) and non-steroidal anti-inflammatory drugs (NSAIDs) as novel antiviral dual-target agents against SARS-CoV-2/COVID-19. CBD is a natural compound with a wide range of therapeutic activities, including antiviral and anti-inflammatory properties, while NSAIDs are commonly used to mitigate the symptoms of viral infections. Chemical modifications of CBD with NSAIDs were performed to obtain dual-target agents with enhanced activity against SARS-CoV-2. The synthesised compounds were characterised using spectroscopic techniques. The biological activity of three molecular consortia (CBD-ibuprofen, CBD-ketoprofen, and CBD-naproxen) was evaluated in cell lines transduced with vesicular stomatitis virus-based pseudotypes bearing the SARS-CoV-1 or SARS-CoV-2 spike proteins or infected with influenza virus A/Puerto Rico/8/34. The results showed that some CBD-NSAID molecular consortia have superior antiviral activity against SARS-CoV-1 and SARS-CoV-2, but not against the influenza A virus. This may suggest a potential therapeutic role for these compounds in the treatment of emerging coronavirus infections. Further studies are needed to investigate the efficacy of these compounds in vivo, and their potential use in clinical settings. Our findings provide a promising new approach to combatting current and future viral emergencies.

Mpox virus detection in the wastewater and the number of hospitalized patients in the Poznan metropolitan area, Poland.

OBJECTIVES: Wastewater-based epidemiology can determine the scale of a mpox epidemic and thus is a promising additional tool that can complete data gathered by the clinical monitoring approach and predict more accurately the development and progress of the current mpox outbreak. METHODS: We collected daily average samples from two wastewater treatment plants (WTPs): Central and Left-Bank, in Poznan, Poland from July to December 2022. The mpox DNA was detected using real-time polymerase chain reaction and compared with the number of hospitalizations. RESULTS: We detected the mpox DNA in the Central WTP in weeks 29, 43, and 47 and the Left-Bank WTP mostly from mid-September till the end of October. A total of 22 patients with mpox were reported by the public health authority from July to December 2022, with the highest number of hospitalized individuals from mid-July to mid-August. The mpox virus detection does not correlate with the number of hospitalizations in Poznan, Poland. CONCLUSION: Our results suggest that the scale of the mpox epidemic is underestimated, and many mpox virus-infected individuals are not identified by the public health authority.

Risk Factors and the Character of Clinical Course of the Echinococcus multilocularis Infection in Patients in Poland.

Alveolar echinococcosis (AE) is a chronic zoonotic disease caused by the larval form of Echinococcus multilocularis. In humans, it may become a serious chronic infection of the liver which resembles a slow malignant process leading to death when untreated. The aim of the study was an assessment of the risk factors of the E. multilocularis infections and the description of AE clinical course in the group of 36 patients with confirmed AE, hospitalized at the Department and Clinic of Tropical and Parasitic Diseases, Poznan University of Medical Sciences between 2013 and 2022. Among the study participants, most patients cultivated land, bred livestock, worked in the forest, or were employed in animal shelters. The E. multilocularis infection was diagnosed based on imaging and immunoassay techniques within 6 months in the majority of patients hospitalized in the Department. All patients hospitalized in the Department initiated anti-parasitic therapy at the moment of the diagnosis. Pharmacological treatment combined with surgery was applied in most of the study participants, who were presented with more advanced stages of infection. We conclude the following: 1. For humans in the risk group, regular abdominal imaging examinations and the detection of specific antibodies against E. multilocularis are recommended. 2. Regular screening tests in the hyperendemic areas of AE would increase the early detection of the disease and to improve the clinical prognosis in this extremely life-threatening parasitic disease.

Differences in BNT126b2 and ChAdOx1 Homologous Vaccination Antibody Response among Teachers in Poznan, Poland.

Children are among the best vectors to spread respiratory viruses, including emerging variants of SARS-CoV-2 due to the asymptomatic or relatively mild course of infection and simultaneously high titres of pathogens in the respiratory tract. Therefore, individuals who have constant contact with children, e.g., teachers should be vaccinated against COVID-19 as essential workers within the first phases of a vaccination campaign. In Poland, primary and secondary school teachers were vaccinated with ChAdOx1 from February 2021 with a three month interval between the two doses, while lecturers at medical universities, who are simultaneously healthcare workers, received the BNT126b2 vaccine from December 2020 with three weeks between the first and second doses. The aim of this study was to compare the antibody responses at two weeks and three months after vaccination and to estimate the vaccine effectiveness against COVID-19 among infection-naïve teachers vaccinated with mRNA and a vector vaccine. We found that the anti-SARS-CoV-2 spike protein antibodies were significantly higher among the lecturers but antibody waning was slower among the schoolteachers. However, those vaccinated with ChAdOx1 complained significantly more often of vaccine side effects. In addition, during the three months after the second vaccine dose no study participants were infected with SARS-CoV-2. The BNT126b2 vaccine gave higher antibody titres in comparison with ChAdOx1 but protection against COVID-19 in both cases was similar. Moreover, we did not find any anti-SARS-CoV-2 nucleoprotein antibodies at two weeks as well as at three months after vaccination among the study participants, which shows a very high vaccine effectiveness in the occupational group with a high SARS-CoV-2-infection risk.

The Longitudinal Analysis on the Anti-SARS-CoV-2 Antibodies among Healthcare Workers in Poland-Before and after BNT126b2 mRNA COVID-19 Vaccination.

One of the groups most vulnerable to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is healthcare workers (HCWs) who have direct contact with suspected and confirmed coronavirus diseases 2019 (COVID-19) patients. Therefore, this study aimed to (i) conduct a longitudinal analysis of the seroprevalence of SARS-CoV-2 infection among HCWs working in two healthcare units (HCUs) in Poland and (ii) identify anti-SARS-CoV-2 IgG antibody (Ab) response factors following infection and anti-COVID-19 vaccination. The overall seroprevalence increased from 0% at baseline in September 2020 to 37.8% in December 2020. It reached 100% in February 2021 after BNT126b2 (Pfizer New York, NY, USA/BioNTech Mainz, Germany) full vaccination and declined to 94.3% in September 2021. We observed significant differences in seroprevalence between the tested high- and low-risk infection HCUs, with the highest seropositivity among the midwives and nurses at the Gynecology and Obstetrics Ward, who usually have contact with non-infectious patients and may not have the proper training, practice and personal protective equipment to deal with pandemic infections, such as SARS-CoV-2. We also found that anti-SARS-CoV-2 Ab levels after coronavirus infection were correlated with disease outcomes. The lowest Ab levels were found among HCWs with asymptomatic coronavirus infections, and the highest were found among HCWs with severe COVID-19. Similarly, antibody response after vaccination depended on previous SARS-CoV-2 infection and its course: the highest anti-SARS-CoV-2 Ab levels were found in vaccinated HCWs after severe COVID-19. Finally, we observed an approximately 90-95% decrease in anti-SARS-CoV-2 Ab levels within seven months after vaccination. Our findings show that HCWs have the highest risk of SARS-CoV-2 infection, and due to antibody depletion, extra protective measures should be undertaken. In addition, in the context of the emergence of new pathogens with pandemic potential, our results highlight the necessity for better infectious disease training and regular updates for the low infection risk HCUs, where the HCWs have only occasional contact with infectious patients.

Secondary Structure of Influenza A Virus Genomic Segment 8 RNA Folded in a Cellular Environment.

Influenza A virus (IAV) is a member of the single-stranded RNA (ssRNA) family of viruses. The most recent global pandemic caused by the SARS-CoV-2 virus has shown the major threat that RNA viruses can pose to humanity. In comparison, influenza has an even higher pandemic potential as a result of its high rate of mutations within its relatively short (<13 kbp) genome, as well as its capability to undergo genetic reassortment. In light of this threat, and the fact that RNA structure is connected to a broad range of known biological functions, deeper investigation of viral RNA (vRNA) structures is of high interest. Here, for the first time, we propose a secondary structure for segment 8 vRNA (vRNA8) of A/California/04/2009 (H1N1) formed in the presence of cellular and viral components. This structure shows similarities with prior in vitro experiments. Additionally, we determined the location of several well-defined, conserved structural motifs of vRNA8 within IAV strains with possible functionality. These RNA motifs appear to fold independently of regional nucleoprotein (NP)-binding affinity, but a low or uneven distribution of NP in each motif region is noted. This research also highlights several accessible sites for oligonucleotide tools and small molecules in vRNA8 in a cellular environment that might be a target for influenza A virus inhibition on the RNA level.

Genetic tests based on the RT-PCR reaction in the diagnostics of SARS-CoV-2 infection.

INTRODUCTION: Since the SARS-CoV-2 emergence in 2019/2020, at least 158 million infections with this pathogen have been recorded, of which 3.29 million infected people have died. Due to the non-specific symptoms of SARS-CoV-2 infection, laboratory tests based on RT-PCR (reverse transcription and polymerase chain reaction) are mainly used in the diagnosis of COVID-19 disease. AIM: The aim of this study is to compare the molecular tests available on the Polish market for the diagnosis of SARS-CoV2 infection. RESULTS: Based on the data provided by the manufacturers and the performed laboratory analyses, we have shown that the available diagnostic kits differ mainly in the sensitivity and duration of the reaction. CONCLUSION: Due to the ongoing COVID-19 pandemic, the indicated parameters are key to effective control of the spread of SARS-CoV2, and therefore should be mainly taken into account when choosing and purchasing by diagnostic centres.

Prevalence of Anti-SARS-CoV-2 Antibodies in Poznan, Poland, after the First Wave of the COVID-19 Pandemic.

In comparison to other European countries, during the first months of the COVID-19 pandemic, Poland reported a relatively low number of confirmed cases of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections. To estimate the scale of the pandemic in Poland, a serosurvey of antibodies against SARS-CoV-2 was performed after the first wave of COVID-19 in Europe (March-May 2020). Within this study, we collected samples from 28 July to 24 September 2020 and, based on the ELISA results, we found that 1.67% (25/1500, 95% CI 1.13-2.45) of the Poznan (Poland) metropolitan area's population had antibodies against SARS-CoV-2 after the first wave of COVID-19. However, the presence of anti-SARS-CoV-2 IgG antibodies was confirmed with immunoblotting in 56% (14/25) samples, which finally resulted in a decrease in seroprevalence, i.e., 0.93% (14/1500, 95% CI 0.56-1.56). The positive anti-SARS-CoV-2 IgG results were associated with age, occupation involving constant contact with people, travelling abroad, non-compliance with epidemiological recommendations and direct contact with the novel coronavirus. Our findings confirm the low SARS-CoV-2 incidence in Poland and imply that the population had little herd immunity heading into the second and third wave of the pandemic, and therefore, that herd immunity contributed little to preventing the high numbers of SARS-CoV-2 infections and COVID-19-related deaths in Poland during these subsequent waves.

Case of Plasmodium knowlesi Malaria in Poland Linked to Travel in Southeast Asia.

We report a case of Plasmodium knowlesi malaria imported to central Europe from Southeast Asia. Laboratory suspicion of P. knowlesi infection was based on the presence of atypical developmental forms of the parasite in Giemsa-stained microscopic smears. We confirmed and documented the clinical diagnosis by molecular biology techniques.

Massive Cryptosporidium infections and chronic diarrhea in HIV-negative patients.

Protozoa of the genus Cryptosporidium are common parasites of domestic and wild animals-mammals, birds, reptiles, and fishes. The invasive forms are thick-walled oocysts, which can be present in water supplies, on fruits, vegetables, or in the soil contaminated with feces. In this work, we describe three cases of middle-aged persons with massive Cryptosporidium hominis infection and chronic diarrhea with no immunological abnormalities and no history of previous travels to tropical countries. The lesions discovered during colonoscopy within the large intestine-cryptitis and the histopathological changes were related to massive cryptosporidiosis. All these statements indicate necessity of parasitological stool examination in cases with chronic diarrhea in which no etiological agents are detected, but not only in HIV positive individuals. Parasite's eradication leads to symptom disappearance as well as improvement of histopathological mucosa alterations.

Multiscale modeling of influenza A virus replication in cell cultures predicts infection dynamics for highly different infection conditions.

Influenza A viruses (IAV) are commonly used to infect animal cell cultures for research purposes and vaccine production. Their replication is influenced strongly by the multiplicity of infection (MOI), which ranges over several orders of magnitude depending on the respective application. So far, mathematical models of IAV replication have paid little attention to the impact of the MOI on infection dynamics and virus yields. To address this issue, we extended an existing model of IAV replication in adherent MDCK cells with kinetics that explicitly consider the time point of cell infection. This modification does not only enable the fitting of high MOI measurements, but also the successful prediction of viral release dynamics of low MOI experiments using the same set of parameters. Furthermore, this model allows the investigation of defective interfering particle (DIP) propagation in different MOI regimes. The key difference between high and low MOI conditions is the percentage of infectious virions among the total virus particle release. Simulation studies show that DIP interference at a high MOI is determined exclusively by the DIP content of the seed virus while, in low MOI conditions, it is predominantly controlled by the de novo generation of DIPs. Overall, the extended model provides an ideal framework for the prediction and optimization of cell culture-derived IAV manufacturing and the production of DIPs for therapeutic use.

A Novel Type of Influenza A Virus-Derived Defective Interfering Particle with Nucleotide Substitutions in Its Genome.

Defective interfering particles (DIPs) replicate at the expense of coinfecting, fully infectious homologous virus. Typically, they contain a highly deleted form of the viral genome. Utilizing single-cell analysis, here we report the discovery of a yet-unknown DIP type, derived from influenza A viruses (IAVs), termed OP7 virus. Instead of deletions, the genomic viral RNA (vRNA) of segment 7 (S7) carried 37 point mutations compared to the reference sequence, affecting promoter regions, encoded proteins, and genome packaging signals. Coinfection experiments demonstrated strong interference of OP7 virus with IAV replication, manifested by a dramatic decrease in the infectivity of released virions. Moreover, an overproportional quantity of S7 in relation to other genome segments was observed, both intracellularly and in the released virus population. Concurrently, OP7 virions lacked a large fraction of other vRNA segments, which appears to constitute its defect in virus replication. OP7 virus might serve as a promising candidate for antiviral therapy. Furthermore, this novel form of DIP may also be present in other IAV preparations.IMPORTANCE Defective interfering particles (DIPs) typically contain a highly deleted form of the viral genome, rendering them defective in virus replication. Yet upon complementation through coinfection with fully infectious standard virus (STV), interference with the viral life cycle can be observed, leading to suppressed STV replication and the release of mainly noninfectious DIPs. Interestingly, recent research indicates that DIPs may serve as an antiviral agent. Here we report the discovery of a yet-unknown type of influenza A virus-derived DIP (termed "OP7" virus) that contains numerous point mutations instead of large deletions in its genome. Furthermore, the underlying principles that render OP7 virions interfering and apparently defective seem to differ from those of conventional DIPs. In conclusion, we believe that OP7 virus might be a promising candidate for antiviral therapy. Moreover, it exerts strong effects, both on virus replication and on the host cell response, and may have been overlooked in other IAV preparations.

TMPRSS11A activates the influenza A virus hemagglutinin and the MERS coronavirus spike protein and is insensitive against blockade by HAI-1.

The influenza virus hemagglutinin (HA) facilitates viral entry into target cells. Cleavage of HA by host cell proteases is essential for viral infectivity, and the responsible enzymes are potential targets for antiviral intervention. The type II transmembrane serine protease (TTSP) TMPRSS2 has been identified as an HA activator in cell culture and in the infected host. However, it is less clear whether TMPRSS2-related enzymes can also activate HA for spread in target cells. Moreover, the activity of cellular serine protease inhibitors against HA-activating TTSPs is poorly understood. Here, we show that TMPRSS11A, another member of the TTSP family, cleaves and activates the influenza A virus (FLUAV) HA and the Middle East respiratory syndrome coronavirus spike protein (MERS-S). Moreover, we demonstrate that TMPRSS11A is expressed in murine tracheal epithelium, which is a target of FLUAV infection, and in human trachea, suggesting that the protease could support FLUAV spread in patients. Finally, we show that HA activation by the TMPRSS11A-related enzymes human airway tryptase and DESC1, but not TMPRSS11A itself, is blocked by the cellular serine protease inhibitor hepatocyte growth factor activator inhibitor type-1 (HAI-1). Our results suggest that TMPRSS11A could promote FLUAV spread in target cells and that HA-activating TTSPs exhibit differential sensitivity to blockade by cellular serine protease inhibitors.

Non-human primate orthologues of TMPRSS2 cleave and activate the influenza virus hemagglutinin.

The cellular serine protease TMPRSS2, a member of the type II transmembrane serine protease (TTSP) family, cleaves and activates the hemagglutinin of influenza A viruses (FLUAV) in cell culture and is essential for spread of diverse FLUAV in mice. Non-human primates (NHP), in particular rhesus and cynomolgus macaques, serve as animal models for influenza and experimental FLUAV infection of common marmosets has recently also been reported. However, it is currently unknown whether the NHP orthologues of human TMPRSS2 cleave and activate FLUAV hemagglutinin and contribute to viral spread in respiratory tissue. Here, we cloned and functionally analyzed the macaque and marmoset orthologues of human TMPRSS2. In addition, we analyzed the macaque orthologues of human TMPRSS4 and HAT, which also belong to the TTSP family. We found that all NHP orthologues of human TMPRSS2, TMPRSS4 and HAT cleave and activate HA upon directed expression and provide evidence that endogenous TMPRSS2 is expressed in the respiratory epithelium of rhesus macaques. Finally, we demonstrate that a serine protease inhibitor active against TMPRSS2 suppresses FLUAV spread in precision-cut lung slices of human, macaque and marmoset origin. These results indicate that FLUAV depends on serine protease activity for spread in diverse NHP and in humans. Moreover, our findings suggest that macaques and marmosets may serve as models to study FLUAV activation by TMPRSS2 in human patients.

The Hemagglutinin of Bat-Associated Influenza Viruses Is Activated by TMPRSS2 for pH-Dependent Entry into Bat but Not Human Cells.

New World bats have recently been discovered to harbor influenza A virus (FLUAV)-related viruses, termed bat-associated influenza A-like viruses (batFLUAV). The internal proteins of batFLUAV are functional in mammalian cells. In contrast, no biological functionality could be demonstrated for the surface proteins, hemagglutinin (HA)-like (HAL) and neuraminidase (NA)-like (NAL), and these proteins need to be replaced by their human counterparts to allow spread of batFLUAV in human cells. Here, we employed rhabdoviral vectors to study the role of HAL and NAL in viral entry. Vectors pseudotyped with batFLUAV-HAL and -NAL were able to enter bat cells but not cells from other mammalian species. Host cell entry was mediated by HAL and was dependent on prior proteolytic activation of HAL and endosomal low pH. In contrast, sialic acids were dispensable for HAL-driven entry. Finally, the type II transmembrane serine protease TMPRSS2 was able to activate HAL for cell entry indicating that batFLUAV can utilize human proteases for HAL activation. Collectively, these results identify viral and cellular factors governing host cell entry driven by batFLUAV surface proteins. They suggest that the absence of a functional receptor precludes entry of batFLUAV into human cells while other prerequisites for entry, HAL activation and protonation, are met in target cells of human origin.

TMPRSS2 Isoform 1 Activates Respiratory Viruses and Is Expressed in Viral Target Cells.

The cellular protease TMPRSS2 cleaves and activates the influenza virus hemagglutinin (HA) and TMPRSS2 expression is essential for viral spread and pathogenesis in mice. Moreover, severe acute respiratory syndrome coronavirus (SARS-CoV) and other respiratory viruses are activated by TMPRSS2. However, previous studies on viral activation by TMPRSS2 focused on a 492 amino acids comprising form of the protein (isoform 2) while other TMPRSS2 isoforms, generated upon alternative splicing of the tmprss2 mRNA, have not been characterized. Here, we show that the mRNA encoding a TMPRSS2 isoform with an extended N-terminal cytoplasmic domain (isoform 1) is expressed in lung-derived cell lines and tissues. Moreover, we demonstrate that TMPRSS2 isoform 1 colocalizes with HA and cleaves and activates HA. Finally, we show that isoform 1 activates the SARS-CoV spike protein for cathepsin L-independent entry into target cells. Our results indicate that TMPRSS2 isoform 1 is expressed in viral target cells and might contribute to viral activation in the host.

Tetherin Sensitivity of Influenza A Viruses Is Strain Specific: Role of Hemagglutinin and Neuraminidase.

UNLABELLED: The expression of the antiviral host cell factor tetherin is induced by interferon and can inhibit the release of enveloped viruses from infected cells. The Vpu protein of HIV-1 antagonizes the antiviral activity of tetherin, and tetherin antagonists with Vpu-like activity have been identified in other viruses. In contrast, it is incompletely understood whether tetherin inhibits influenza A virus (FLUAV) release and whether FLUAV encodes tetherin antagonists. Here, we show that release of several laboratory-adapted FLUAV strains and a seasonal FLUAV strain is inhibited by tetherin, while pandemic FLUAV A/Hamburg/4/2009 is resistant. Studies with a virus-like particle system and analysis of reassortant viruses provided evidence that the viral hemagglutinin (HA) is an important determinant of tetherin antagonism but requires the presence of its cognate neuraminidase (NA) to inhibit tetherin. Finally, tetherin antagonism by FLUAV was dependent on the virion context, since retrovirus release from tetherin-positive cells was not rescued, and correlated with an HA- and NA-dependent reduction in tetherin expression. In sum, our study identifies HA and NA proteins of certain pandemic FLUAV as tetherin antagonists, which has important implications for understanding FLUAV pathogenesis. IMPORTANCE: Influenza A virus (FLUAV) infection is responsible for substantial global morbidity and mortality, and understanding how the virus evades the immune defenses of the host may uncover novel targets for antiviral intervention. Tetherin is an antiviral effector molecule of the innate immune system which can contribute to control of viral invasion. However, it has been unclear whether FLUAV is inhibited by tetherin and whether these viruses encode tetherin-antagonizing proteins. Our observation that several pandemic FLUAV strains can counteract tetherin via their HA and NA proteins identifies these proteins as novel tetherin antagonists and indicates that HA/NA-dependent inactivation of innate defenses may contribute to the efficient spread of pandemic FLUAV.