Purpose: The COVID-19 pandemic has increased challenges to intimate partner violence (IPV) service provision. This study aimed to explore administrative perspectives on the impacts of the COVID-19 pandemic on United States regional and national IPV service organizations. Methods: We interviewed 35 administrators working within state, regional, or national organizations addressing IPV. Interview domains included (1) organizational response to COVID-19, including communication and supporting employees and partner agencies, (2) impact on marginalized communities, and (3) resource needs. We used a hybrid deductive-inductive approach and thematic analysis for coding and analysis. Results: We identified four key themes: (1) COVID-19 worsened pre-existing challenges and created new challenges at multiple levels within IPV service organizations; (2) IPV service organizations initiated multi-level initiatives to support IPV survivors, their staff, their organization, and their member/partner agencies; (3) Organizations identified changes that should continue beyond the pandemic; and (4) Systemic racism compounded the impact of COVID-19 on IPV survivors and IPV service agencies. Conclusions: Findings suggest that (1) multi-level responses are needed for robust support of IPV survivors during and beyond the pandemic and (2) a syndemic model that addresses underlying structural inequities may strengthen efforts to support IPV survivors during a pandemic or other large-scale disaster. © 2023, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.

Policymakers make decisions about COVID-19 management in the face of considerable uncertainty. We convened multiple modeling teams to evaluate reopening strategies for a mid-sized county in the United States, in a novel process designed to fully express scientific uncertainty while reducing linguistic uncertainty and cognitive biases. For the scenarios considered, the consensus from 17 distinct models was that a second outbreak will occur within 6 months of reopening, unless schools and non-essential workplaces remain closed. Up to half the population could be infected with full workplace reopening; non-essential business closures reduced median cumulative infections by 82%. Intermediate reopening interventions identified no win-win situations; there was a trade-off between public health outcomes and duration of workplace closures. Aggregate results captured twice the uncertainty of individual models, providing a more complete expression of risk for decision-making purposes.

Policymakers must make management decisions despite incomplete knowledge and conflicting model projections. Little guidance exists for the rapid, representative, and unbiased collection of policy-relevant scientific input from independent modeling teams. Integrating approaches from decision analysis, expert judgment, and model aggregation, we convened multiple modeling teams to evaluate COVID-19 reopening strategies for a mid-sized United States county early in the pandemic. Projections from seventeen distinct models were inconsistent in magnitude but highly consistent in ranking interventions. The 6-mo-ahead aggregate projections were well in line with observed outbreaks in mid-sized US counties. The aggregate results showed that up to half the population could be infected with full workplace reopening, while workplace restrictions reduced median cumulative infections by 82%. Rankings of interventions were consistent across public health objectives, but there was a strong trade-off between public health outcomes and duration of workplace closures, and no win-win intermediate reopening strategies were identified. Between-model variation was high; the aggregate results thus provide valuable risk quantification for decision making. This approach can be applied to the evaluation of management interventions in any setting where models are used to inform decision making. This case study demonstrated the utility of our approach and was one of several multimodel efforts that laid the groundwork for the COVID-19 Scenario Modeling Hub, which has provided multiple rounds of real-time scenario projections for situational awareness and decision making to the Centers for Disease Control and Prevention since December 2020.

OBJECTIVE: Viral load suppression (VLS) is critical in reducing morbidity and mortality associated with HIV as well as minimizing the likelihood of HIV transmission to uninfected persons. The objective of this study was to identify factors associated with VLS among people living with HIV (PLWH) on antiretroviral (ARV) therapy to inform HIV programme strategies in Nigeria. METHODS: Adult participants, aged 15-64 years, from the 2018 Nigeria HIV/AIDS Indicator and Impact Survey (NAIIS), who self-reported to be a PLWH or had detectable ARVs, were analysed to examine factors associated with VLS defined as HIV RNA <1000 copies/mL. NAIIS measured HIV prevalence, viral load, ARV and hepatitis B in PLWH. Logistic regression models were used and reported weighted prevalence. RESULTS: Of 1322 participants, 949 (68.25%) were women and 1287 (96.82%) had detectable ARVs. The median age was 39.31 [interquartile range (IQR): 31.47-47.63] years. Prevalence of VLS was 80.88%. Compared with participants with detectable ARVs, those with undetectable ARVs in their blood specimens had lower odds of VLS [adjusted odds ratio (aOR) = 0.24, 95% confidence interval (CI): 0.08-0.64). Coinfection with hepatitis B and nonnucleoside reverse transcriptase inhibitor metabolites were also associated with lower odds of VLS. Older people (45-54 vs 15-24 years) had increased odds of VLS (aOR = 2.81, 95% CI: 1.14-6.90). CONCLUSION: Young people and those with undetectable ARVs had lower odds of virological suppression. Targeted interventions focusing on young people and adherence to medication are needed to achieve the UNAIDS 95-95-95 goals for HIV epidemic control.

OBJECTIVE: Evidence regarding effectiveness of interleukin-1 receptor antagonism in Multisystem Inflammatory Syndrome in Children (MIS-C) is lacking. We characterized variation in initial treatment with anakinra and evaluated cardiovascular outcomes associated with adding anakinra to standard initial therapy. METHODS: We conducted a retrospective cohort study of MIS-C cases in a U.S. surveillance registry November 2020-December 2021. Day 0 was the first calendar day of immunomodulatory treatment. Factors associated with initial anakinra use (days 0-1) were identified. We compared cases ages 2-20 years receiving intravenous immunoglobulin (IVIG) and glucocorticoids vs. anakinra plus IVIG and/or glucocorticoids (days 0-1), using inverse probability weighting to balance severity. Primary outcomes were vasopressor requirement (day 3) and impaired left ventricular ejection fraction (days 3-4). The secondary outcome was 50% reduction in C-reactive protein (day 3). RESULTS: Among 1516 MIS-C cases (44 sites), 193 (13%) received anakinra alone or with other immunomodulators as initial treatment (range 0-74% by site). Site accounted for 59% of residual variance in anakinra use. After balancing severity, initial treatment with anakinra plus IVIG and/or glucocorticoids (N=121) vs. IVIG and glucocorticoids (N=389) was not associated with significant differences in vasopressor requirement (25.6% vs. 20.1%; RR 1.27, 95% CI [0.88-1.84]), ventricular dysfunction (33.7% vs. 25.7%; RR 1.31, 95% CI [0.98-1.75]), or C-reactive protein reduction. CONCLUSIONS: We identified substantial variation in initial anakinra use in a real-world population of children with MIS-C, but no average short-term improvement in cardiovascular outcomes associated with early addition of anakinra to IVIG and/or glucocorticoids compared to IVIG and glucocorticoids alone.

BACKGROUND: HIV drug resistance (HIVDR) surveillance is an important tool to monitor threats to progress towards epidemic control. The characterization of HIVDR in Nigeria at the national level is needed to inform both clinical decisions and population-level HIV policy strategies. This study uses data obtained from the Nigeria HIV/AIDS Indicator and Impact Survey (NAIIS) to describe the prevalence and distribution of HIVDR in Nigeria. METHODS: NAIIS was a cross-sectional, population-based survey of households throughout Nigeria in 2018. NAIIS was designed to provide estimates of HIV prevalence and related health indicators from a nationally representative sample. The study population included participants aged 15-64 years who tested positive for HIV, had a viral load at least 1000 copies/ml, and had available HIV drug resistance genotypes. HIV isolates were genotyped to detect drug resistance mutations. Individual characteristics of study participants associated with HIVDR were identified using a weighted multivariable logistic regression model. RESULTS: Of 1355 respondents with available HIV genotypes, 293 (19%) had evidence of drug-resistant mutations (DRMs) that conferred resistance to at least one antiretroviral drug. The majority of DRMs observed conferred resistance to NNRTIs (17.6%) and NRTIs (11.2%). HIVDR was associated with being ART-experienced, longer duration on ART, and lower CD4+ count but not sociodemographic characteristics. CONCLUSION: The population level DRM prevalence in Nigeria was consistent with what would be expected in a mature HIV treatment landscape. The continued roll out of dolutegravir-anchored regimens should mitigate the impact of NNRTI resistance on population viral load suppression and progress towards epidemic control.

BACKGROUND: Information on HIV drug resistance (HIVDR) prevalence in people newly diagnosed with HIV is limited. We implemented a cross-sectional study to estimate HIVDR prevalence among pregnant women recently infected with HIV in Malawi. METHODS: The HIVDR study was nested within a routine antenatal clinic (ANC) sentinel surveillance survey. Dried blood spot samples were tested for recent infection using a limiting antigen antibody assay together with HIV viral load testing. HIV-1 protease and reverse transcriptase were sequenced using Sanger sequencing. Drug susceptibility was predicted using Stanford HIVdb algorithm (version 8.9). Weighted analysis was performed in Stata 15.1. RESULTS: Of the 21,642 pregnant women enrolled in the ANC survey, 8.4% (1826/21,642) tested HIV positive. Of these, 5.0% (92/1826) had recent HIV infection, and 90.2% (83/92) were tested by PCR. The amplification and sequencing success rate was 57.8% (48/83). The prevalence of any HIVDR was 14.6% (5/45) (95% CI: 4.7-36.8%), all of which indicated HIVDR to nonnucleoside reverse transcriptase inhibitors (NNRTIs). HIVDR to nucleoside reverse transcriptase inhibitors was 7.9% (2/45) (95% CI: 1.4-34.6%). Resistance to protease inhibitors currently in use in Malawi was not observed. CONCLUSIONS: Despite the low number of cases with presumed TDR, our study hints that resistance to NNRTIs was high, above the 10% target for regimen change. Further investigation is needed to establish the exact magnitude of presumed TDR among women recently infected with HIV. These findings support the transition to an integrase inhibitor-based first-line regimen for patients initiating or on ART.

BACKGROUND: Medication for Opioid Use Disorder (MOUD) has been shown to be a safe, cost-effective intervention that successfully lowers risk of opioid overdose. However, access to and use of MOUD has been limited. Our objective was to explore attitudes, opinions, and beliefs regarding MOUD among healthcare and social service providers in a community highly impacted by the opioid overdose epidemic. METHODS: As part of a larger ethnographic study examining neighborhoods in Allegheny County, PA, with the highest opioid overdose death rates, semi-structured qualitative in-person and telephone interviews were conducted with forty-five providers treating persons with opioid use disorders in these communities. An open coding approach was used to code interview transcripts followed by thematic analysis. RESULTS: Three major themes were identified related to MOUD from the perspectives of our provider participants. Within a variety of health and substance use service roles and settings, provider reflections revealed: (1) different opinions about MOUD as a transition to abstinence or as a long-term treatment; (2) perceived lack of uniformity and dissemination of accurate information of MOUD care, permitting differences in care, and (3) observed barriers to entry and navigation of MOUD, including referrals as a "word-of-mouth insider system" and challenges of getting patients MOUD services when they need it. CONCLUSIONS: Even in communities hard hit by the opioid overdose epidemic, healthcare providers' disagreement about the standard of care for MOUD can be a relevant obstacle. These insights can inform efforts to improve MOUD treatment and access for people with opioid use disorders.

BACKGROUND: The COVID-19 pandemic has impacted children and young people experiencing child abuse and neglect. Child Protective Services (CPS) has played an important role in supporting children and families during the COVID-19 pandemic. Few studies to-date have evaluated the impact of the pandemic on CPS caseworkers and administrators in the United States. OBJECTIVES: We conducted interviews to explore CPS caseworkers' and administrators' experiences working and serving families during the pandemic. METHODS: Participants were U.S.-based CPS caseworkers and administrators. We conducted semi-structured virtual interviews with participants and used an inductive thematic analysis approach. RESULTS: We conducted 37 interviews. Participants discussed how the COVID-19 pandemic has changed the way they conduct investigations and provide services to families in the CPS system. Several services were adapted to occur virtually, providing challenges and unique opportunities. Participants also described the personal barriers they faced during the pandemic, including working remotely, experiencing burnout, and challenges obtaining personal protective equipment. Finally, participants shared creative solutions they engaged in to support children and families during the COVID-19 pandemic, including expanding collaborations with other community-based organizations. DISCUSSION: This study suggests the important role that CPS has played during the pandemic and challenges individual CPS workers felt, in terms of both experiencing burnout and difficulty obtaining personalized protective equipment. Inclusion of the CPS system in emergency preparedness planning for future pandemics or natural disasters will ensure continuation of these vital services.

Relatively few studies have considered the impact of the COVID-19 pandemic on intimate partner violence (IPV) advocates or the agencies where they work. In this study, based on United States IPV advocates' experiences working with survivors during the COVID-19 pandemic, we conducted interviews to explore: 1) personal challenges and resilience working as IPV advocates during the COVID-19 pandemic; 2) how agencies adapted to the pandemic to support IPV survivors and advocates; and 3) specific needs and challenges of culturally-specific agencies. We conducted semi-structured interviews with 53 IPV advocates from June to November 2020. Participants were included if they worked directly with survivors, identified as an IPV advocate, worked at a US-based agency, and spoke and understood English. We created a sampling matrix to ensure adequate representation from IPV advocates serving survivors from communities which have been marginalized. Interviews were conducted through a virtual platform by a trained member of the research team. We used an inductive thematic analysis approach, with weekly coding meetings to resolve discrepancies in coding. Five themes emerged from the data: 1) IPV advocates described how working as an IPV advocate during the COVID-19 pandemic impacted them personally; 2) agencies developed new methods of addressing IPV advocates' needs; 3) agencies developed new solutions to address pandemic-related client needs; 4) transitioning advocacy work to virtual formats created challenges but also opportunities and; 5) pandemic limitations and impacts compounded pre-pandemic challenges for culturally specific agencies. IPV advocates are frontline workers who have played essential roles in adjusting services to meet survivor needs during the COVID-19 pandemic while simultaneously coping with pandemic impacts on themselves and their agencies. Developing inter-agency collaborations and promoting advocates' safety and wellbeing during future public health crises will help support IPV survivors.

Certain communities in the United States experience greater opioid-involved overdose mortality than others. Interventions to stem overdose benefit from contextual understandings of communities' needs and strengths in addressing the opioid crisis. This project aims to understand multiple stakeholder perspectives on the opioid epidemic in communities disproportionately affected by opioid-involved overdose mortality. Methods: We performed a rapid-cycle qualitative assessment study utilizing in-depth interviews with community stakeholders and observations of community meetings in eight communities in Allegheny County, Pennsylvania, USA, disproportionately impacted by opioid-involved overdose mortality. Stakeholder categories included: current and past illicit users of opioids; medical and social service providers; emergency medical services; law enforcement; spouse or other family members of illicit users/former users of opioids; government officials; school officials; community members. Content analysis was utilized to identify themes and answer study questions. Regular feedback to stakeholders was provided to support targeted interventions. Results: We performed semi-structured, in-depth interviews with 130 community stakeholders and 29 community meeting observations in 2018-2019. Participants perceived similar economic and social determinants as origins of the opioid overdose epidemic including lack of economic resources, loss of jobs, transient populations and dilapidated housing. However, they differed in their awareness of and attention to the epidemic. Awareness was dependent on the visibility of opioid use (presence of paraphernalia litter, location of drug users, media coverage, and relationship to users). Overall, there was good knowledge of naloxone for opioid overdose reversal but less knowledge about local syringe services programs. Perceptions of harm reduction efforts were ambivalent. Conclusions: Members of communities impacted by the opioid epidemic perceived that economic downturn was a major factor in the opioid overdose epidemic. However, the varied beliefs within and between communities suggest that interventions need to be tailored according to the cultural norms of place.

Background: The impacts of opioid use disorder and opioid-involved overdose are known, but less is known about the contexts in which people first misuse opioids, and the motivations for continued misuse. Methods: In-depth interviews with 26 individuals in Allegheny County, Pennsylvania with current or past histories of opioid misuse were conducted. Narratives were analyzed to understand the circumstances and influences contributing to initial and continued misuse of opioids. Results: Participants described social and familial contexts that normalized or accepted opioid misuse-this often included their own use of other illicit substances prior to initiating opioids. Participants also described initial use of opioids as related to efforts to cope with physical pain. They also described recognizing and then seeking psychological/emotional benefits from opioids. All three of these themes often overlapped and intersected in these stories of starting opioid misuse. Conclusions: Opioid misuse stemmed from complex interacting influences involving coping with physical and psychological pain, perception that opioids are needed to feel "normal", and acceptance or normalization of opioid use. This suggests a multi-pronged approach to both prevention and treatment are needed.

Of the eighteen hemagglutinin (HA) subtypes (H1-H18) that have been identified in bats and aquatic birds, many HA subtypes have been structurally characterized. However, several subtypes (H8, H11 and H12) still require characterization. To better understand all of these HA subtypes at the molecular level, HA structures from an A(H4N6) (A/swine/Missouri/A01727926/2015), an A(H8N4) (A/turkey/Ontario/6118/1968), an A(H11N9) (A/duck/Memphis/546/1974), an A(H14N5) A/mallard/Gurjev/263/1982, and an A(H15N9) (A/wedge-tailed shearwater/Western Australia/2576/1979 were determined by X-ray crystallography at 2.2Å, 2.3Å, 2.8Å, 3.0Å and 2.5Å resolution, respectively. The interactions between these viruses and host receptors were studied utilizing glycan-binding analyses with their recombinant HA. The data show that all avian HAs retain their strict binding preference to avian receptors, whereas swine H4 has a weak human receptor binding. The molecular characterization and structural analyses of the HA from these zoonotic influenza viruses not only provide a deeper appreciation and understanding of the structure of all HA subtypes, but also re-iterate why continuous global surveillance is needed.

BACKGROUND: Human immunodeficiency virus (HIV)-infected persons are at a higher risk of severe influenza. Although we have shown that a standard-dose intradermal influenza vaccine versus a standard-dose intramuscular influenza vaccine does not result in differences in hemagglutination-inhibition titers in this population, a comprehensive examination of cell-mediated immune responses remains lacking. METHODS: Serological, antigen-specific B-cell, and interleukin 2-, interferon gamma-, and tumor necrosis factor alpha-secreting T-cell responses were assessed in 79 HIV-infected men and 79 HIV-uninfected men. RESULTS: The route of vaccination did not affect the immunoglobulin A and immunoglobulin G (IgG) plasmablast or memory B-cell response, although these were severely impaired in the group with a CD4+ T-cell count of <200 cells/muL. The frequencies of IgG memory B cells measured on day 28 after vaccination were highest in the HIV-uninfected group, followed by the group with a CD4+ T-cell count of >/=200 cells/muL and the group with a CD4+ T-cell count of <200 cells/muL. The route of vaccination did not affect the CD4+ or CD8+ T-cell responses measured at various times after vaccination. CONCLUSIONS: The route of vaccination had no effect on antibody responses, antibody avidity, T-cell responses, or B-cell responses in HIV-infected or HIV-uninfected subjects. With the serological and cellular immune responses to influenza vaccination being impaired in HIV-infected individuals with a CD4+ T-cell count of <200 cells/muL, passive immunization strategies need to be explored to protect this population. CLINICAL TRIALS REGISTRATION: NCT01538940.

The avian influenza A(H7N9) virus continues to cause human infections in China and is a major ongoing public health concern. Five epidemic waves of A(H7N9) infection have occurred since 2013, and the recent fifth epidemic wave saw the emergence of two distinct lineages with elevated numbers of human infection cases and broader geographic distribution of viral diseases compared to the first four epidemic waves. Moreover, highly pathogenic avian influenza (HPAI) A(H7N9) viruses were also isolated during the fifth epidemic wave. Here, we present a detailed structural and biochemical analysis of the surface hemagglutinin (HA) antigen from viruses isolated during this recent epidemic wave. Results highlight that when compared to the 2013 virus HAs, the fifth wave virus HAs remained a weak binder to human glycan receptor analogs. We also studied three mutations, V177K-K184T-G219S, that were recently reported to switch a 2013 A(H7N9)HA to human-type receptor specificity. Our results indicate that these mutations could also switch the H7 HA receptor preference to a predominantly human binding specificity for both fifth wave H7 HAs analyzed in this study.IMPORTANCE The A(H7N9) viruses circulating in China are of great public health concern. Herein, we report a molecular and structural study of the major surface proteins from several recent A(H7N9) influenza viruses. Our results improve the understanding of these evolving viruses and provide important information on their receptor preference that is central to ongoing pandemic risk assessment.

During 2014, a subclade 2.3.4.4 HPAI A(H5N8) virus caused poultry outbreaks around the world. In late 2014/early 2015 the virus was detected in wild birds in Canada and the U.S. and these viruses also gave rise to reassortant progeny, composed of viral RNA segments (vRNAs) from both Eurasian and North America lineages. In particular, viruses were found with N1, N2 and N8 neuraminidase vRNAs, and are collectively referred to as H5Nx viruses. In the U. S., more than 48 million domestic birds have been affected. Here, we present a detailed structural and biochemical analysis of the surface antigens from H5N1, H5N2 and H5N8 in addition to a recent human H5N6 virus. Our results with recombinant hemagglutinin reveal that these viruses have a strict avian receptor binding preference, while recombinantly expressed neuraminidases are sensitive to FDA approved and investigational antivirals. Although H5Nx viruses currently pose a low risk to humans, it is important to maintain surveillance of these circulating viruses, and to continually assess future changes that may increase their pandemic potential. IMPORTANCE: The H5Nx viruses emerging in North America, Europe, and Asia are of great public health concern. Herein, we report a molecular and structural study of the major surface proteins from several H5Nx influenza viruses. Our results improve the understanding of these new viruses and provide important information on their receptor preference and susceptibility to antivirals, which is central to pandemic risk assessment.

During 2013, three new avian influenza A virus subtypes, A(H7N9), A(H6N1), and A(H10N8), resulted in human infections. While the A(H7N9) virus resulted in a significant epidemic in China across 19 provinces and municipalities, both A(H6N1) and A(H10N8) viruses resulted in only a few human infections. This study focuses on the major surface glycoprotein hemagglutinins from both of these novel human viruses. The detailed structural and glycan microarray analyses presented here highlight the idea that both A(H6N1) and A(H10N8) virus hemagglutinins retain a strong avian receptor binding preference and thus currently pose a low risk for sustained human infections. IMPORTANCE: Human infections with zoonotic influenza virus subtypes continue to be a great public health concern. We report detailed structural analysis and glycan microarray data for recombinant hemagglutinins from A(H6N1) and A(H10N8) viruses, isolated from human infections in 2013, and compare them with hemagglutinins of avian origin. This is the first structural report of an H6 hemagglutinin, and our results should further the understanding of these viruses and provide useful information to aid in the continuous surveillance of these zoonotic influenza viruses.

A(H3N2) influenza viruses have circulated in humans since 1968, and antigenic drift of the hemagglutinin (HA) protein continues to be a driving force that allows the virus to escape the human immune response. Since the major antigenic sites of the HA overlap into the receptor binding site (RBS) of the molecule, the virus constantly struggles to effectively adapt to host immune responses, without compromising its functionality. Here, we have structurally assessed the evolution of the A(H3N2) virus HA RBS, using an established recombinant expression system. Glycan binding specificities of nineteen A(H3N2) influenza virus HAs, each a component of the seasonal influenza vaccine between 1968 and 2012, were analyzed. Results suggest that while its receptor-binding site has evolved from one that can bind a broad range of human receptor analogs to one with a more restricted binding profile for longer glycans, the virus continues to circulate and transmit efficiently among humans.

In late 2011, an A(H3N8) influenza virus infection resulted in the deaths of 162 New England harbor seals. Virus sequence analysis and virus receptor binding studies highlighted potential markers responsible for mammalian adaptation and a mixed receptor binding preference (Anthony et al. ; MBio 3: e00166-00112). Here we present a detailed structural and biochemical analysis of the surface antigens of this virus. Results obtained with recombinant proteins for both the hemagglutinin and neuraminidase, indicate a true avian receptor binding preference. Although the detection of this virus in new species highlights an increased potential for cross-species transmission with these viruses, our results indicate that the A(H3N8) virus currently poses a low risk to humans. IMPORTANCE: Cross-species transmission of zoonotic influenza viruses increases public health concerns. Here we report a molecular and structural study of the major surface proteins from an A(H3N8) influenza virus isolated from New England harbor seals. Results improve our understanding of these viruses as they evolve and provide important information to aid ongoing risk assessment analyses, as these zoonotic influenza viruses continue to circulate and adapt to new hosts.

The relative performance of ELISA using globular head domain (GH) and ectodomain hemagglutinins (HAs) as antigens to detect influenza A virus IgG antibody responses was assessed. Assay sensitivity and subtype cross-reactivity were evaluated using sera collected from recipients of monovalent H5N1 vaccine and A(H1N1)pdm09 virus-infected persons. Assay specificity was determined using collections of sera from either individuals unexposed to either H5N1 or A(H1N1)pdm09 viruses or exposed to H5N1 or A(H1N1)pdm09 viruses through vaccination or infection, respectively. ELISA using GH HA showed a similar degree of sensitivity, significantly higher specificity, and significantly lower subtype cross-reactivity compared to ELISA using ectodomain HA.

The non-covalent interactions that mediate trimerization of the influenza hemagglutinin (HA) are important determinants of its biological activities. Recent studies have demonstrated that mutations in the HA trimer interface affect the thermal and pH sensitivities of HA, suggesting a possible impact on vaccine stability (Farnsworth et al. 2011. Vaccine 29:: 1529-1533). We used size exclusion chromatography analysis of recombinant HA ectodomain to compare the differences among recombinant trimeric HA proteins from early 2009 pandemic H1N1 viruses, which dissociate to monomers, with those of more recent virus HAs that can be expressed as trimers. We analyzed differences amongst the HA sequences and identified inter-molecular interactions mediated by the residue at position 374 (HA0 numbering) of the HA2 sub-domain as critical for HA trimer stability. Crystallographic analyses of HA from the recent H1N1 virus A/Washington/5/2011 highlight the structural basis for this observed phenotype. It remains to be seen whether more recent viruses with this mutation will yield more stable vaccines in the future. IMPORTANCE: Hemagglutinins from the early 2009 H1N1 pandemic viruses are unable to maintain a trimeric complex when expressed in a recombinant system. However HAs from 2010 and 2011 strains are more stable and our work highlights the improvement in stability can be attributed to an E47K substitution in the HA2 subunit of the stalk that emerged naturally in the circulating viruses.