Pandemic Influenza (A/H1N1) Vaccine Uptake among French Private General Practitioners: A Cross Sectional Study in 2010

Background

In July, 2009, French health authorities, like those in many other countries, decided to embark on a mass vaccination campaign against the pandemic A(H1N1) influenza. Private general practitioners (GPs) were not involved in this campaign. We studied GPs’ pandemic vaccine (pvaccine) uptake, quantified the relative contribution of its potential explanatory factors and studied whether their own vaccination choice was correlated with their recommendations to patients about pvaccination.

Methodology/Principal Findings

In this cross-sectional telephone survey, professional investigators interviewed an existing panel of randomly selected private GPs (N = 1431; response rate at inclusion in the panel: 36.8%; participation rate in the survey: 100%). The main outcome variable was GPs’ own pvaccine uptake. We used an averaging multi-model approach to quantify the relative contribution of factors associated with their vaccination. The pvaccine uptake rate was 61% (95%CI = 58.3–63.3). Four independent factors contributed the most to this rate (partial Nagelkerke’s R²): history of previous vaccination against seasonal influenza (14.5%), perception of risks and efficacy of the pvaccine (10.8%), opinions regarding the organization of the vaccination campaign (7.1%), and perception of the pandemic''s severity (5.2%). Overall, 71.3% (95%CI = 69.0–73.6) of the participants recommended pvaccination to young adults at risk and 40.1% (95%CI = 37.6–42.7) to other young adults. GPs’ own pvaccination was strongly predictive of their recommendation to both young adults at risk (OR = 9.6; 95%CI = 7.2–12.6) and those not at risk (OR = 8.5; 95%CI = 6.4–11.4).

Conclusions/Significance

These results suggest that around 60% of French private GPs followed French authorities’ recommendations about vaccination of health care professionals against the A(H1N1) influenza. They pinpoint priority levers for improving preparedness for future influenza pandemics. Besides encouraging GPs'' own uptake of regular vaccination against seasonal influenza, providing GPs with clear information about the risks and efficacy of any new pvaccine and involving them in the organization of any future vaccine campaign may improve their pvaccine uptake.     

Productive Infection of Human Skeletal Muscle Cells by Pandemic and Seasonal Influenza A(H1N1) Viruses

Besides the classical respiratory and systemic symptoms, unusual complications of influenza A infection in humans involve the skeletal muscles. Numerous cases of acute myopathy and/or rhabdomyolysis have been reported, particularly following the outbreak of pandemic influenza A(H1N1) in 2009. The pathogenesis of these influenza-associated myopathies (IAM) remains unkown, although the direct infection of muscle cells is suspected. Here, we studied the susceptibility of cultured human primary muscle cells to a 2009 pandemic and a 2008 seasonal influenza A(H1N1) isolate. Using cells from different donors, we found that differentiated muscle cells (i. e. myotubes) were highly susceptible to infection by both influenza A(H1N1) isolates, whereas undifferentiated cells (i. e. myoblasts) were partially resistant. The receptors for influenza viruses, α2-6 and α2-3 linked sialic acids, were detected on the surface of myotubes and myoblasts. Time line of viral nucleoprotein (NP) expression and nuclear export showed that the first steps of the viral replication cycle could take place in muscle cells. Infected myotubes and myoblasts exhibited budding virions and nuclear inclusions as observed by transmission electron microscopy and correlative light and electron microscopy. Myotubes, but not myoblasts, yielded infectious virus progeny that could further infect naive muscle cells after proteolytic treatment. Infection led to a cytopathic effect with the lysis of muscle cells, as characterized by the release of lactate dehydrogenase. The secretion of proinflammatory cytokines by muscle cells was not affected following infection. Our results are compatible with the hypothesis of a direct muscle infection causing rhabdomyolysis in IAM patients.     

Mortality Burden of the 2009 A/H1N1 Influenza Pandemic in France: Comparison to Seasonal Influenza and the A/H3N2 Pandemic

Background

The mortality burden of the 2009 A/H1N1 pandemic remains unclear in many countries due to delays in reporting of death statistics. We estimate the age- and cause-specific excess mortality impact of the pandemic in France, relative to that of other countries and past epidemic and pandemic seasons.

Methods

We applied Serfling and Poisson excess mortality approaches to model weekly age- and cause-specific mortality rates from June 1969 through May 2010 in France. Indicators of influenza activity, time trends, and seasonal terms were included in the models. We also reviewed the literature for country-specific estimates of 2009 pandemic excess mortality rates to characterize geographical differences in the burden of this pandemic.

Results

The 2009 A/H1N1 pandemic was associated with 1.0 (95% Confidence Intervals (CI) 0.2–1.9) excess respiratory deaths per 100,000 population in France, compared to rates per 100,000 of 44 (95% CI 43–45) for the A/H3N2 pandemic and 2.9 (95% CI 2.3–3.7) for average inter-pandemic seasons. The 2009 A/H1N1 pandemic had a 10.6-fold higher impact than inter-pandemic seasons in people aged 5–24 years and 3.8-fold lower impact among people over 65 years.

Conclusions

The 2009 pandemic in France had low mortality impact in most age groups, relative to past influenza seasons, except in school-age children and young adults. The historical A/H3N2 pandemic was associated with much larger mortality impact than the 2009 pandemic, across all age groups and outcomes. Our 2009 pandemic excess mortality estimates for France fall within the range of previous estimates for high-income regions. Based on the analysis of several mortality outcomes and comparison with laboratory-confirmed 2009/H1N1 deaths, we conclude that cardio-respiratory and all-cause mortality lack precision to accurately measure the impact of this pandemic in high-income settings and that use of more specific mortality outcomes is important to obtain reliable age-specific estimates.     

Pandemic swine influenza virus (H1N1): A threatening evolution

“Survival of the fittest” is an old axiom laid down by the great evolutionist Charles Darwin and microorganisms seem to have exploited this statement to a great extent. The ability of viruses to adapt themselves to the changing environment has made it possible to inhabit itself in this vast world for the past millions of years. Experts are well versed with the fact that influenza viruses have the capability to trade genetic components from one to the other within animal and human population. In mid April 2009, the Centers for Disease Control and Prevention and the World Health Organization had recognized a dramatic increase in number of influenza cases. These current 2009 infections were found to be caused by a new strain of influenza type A H1N1 virus which is a re-assortment of several strains of influenza viruses commonly infecting human, avian, and swine population. This evolution is quite dependent on swine population which acts as a main reservoir for the reassortment event in virus. With the current rate of progress and the efforts of heath authorities worldwide, we have still not lost the race against fighting this virus. This article gives an insight to the probable source of origin and the evolutionary progress it has gone through that makes it a potential threat in the future, the current scenario and the possible measures that may be explored to further strengthen the war against pandemic.     

Assessing Antigenic Drift of Seasonal Influenza A(H3N2) and A(H1N1)pdm09 Viruses

Under selective pressure from the host immune system, antigenic epitopes of influenza virus hemagglutinin (HA) have continually evolved to escape antibody recognition, termed antigenic drift. We analyzed the genomes of influenza A(H3N2) and A(H1N1)pdm09 virus strains circulating in Thailand between 2010 and 2014 and assessed how well the yearly vaccine strains recommended for the southern hemisphere matched them. We amplified and sequenced the HA gene of 120 A(H3N2) and 81 A(H1N1)pdm09 influenza virus samples obtained from respiratory specimens and calculated the perfect-match vaccine efficacy using the p_epitope model, which quantitated the antigenic drift in the dominant epitope of HA. Phylogenetic analysis of the A(H3N2) HA1 genes classified most strains into genetic clades 1, 3A, 3B, and 3C. The A(H3N2) strains from the 2013 and 2014 seasons showed very low to moderate vaccine efficacy and demonstrated antigenic drift from epitopes C and A to epitope B. Meanwhile, most A(H1N1)pdm09 strains from the 2012–2014 seasons belonged to genetic clades 6A, 6B, and 6C and displayed the dominant epitope mutations at epitopes B and E. Finally, the vaccine efficacy for A(H1N1)pdm09 (79.6–93.4%) was generally higher than that of A(H3N2). These findings further confirmed the accelerating antigenic drift of the circulating influenza A(H3N2) in recent years.     

Self-Reported Use of Personal Protective Equipment among Chinese Critical Care Clinicians during 2009 H1N1 Influenza Pandemic

BACKGROUND: Critically ill patients with 2009 H1N1 influenza are often treated in intensive care units (ICUs), representing significant risk of nosocomial transmission to critical care clinicians and other patients. Despite a large body of literature and guidelines recommending infection control practices, numerous barriers have been identified in ICUs, leading to poor compliance to the use of personal protective equipment (PPE). The use of PPE among critical care clinicians has not been extensively evaluated, especially during the pandemic influenza. This study examined the knowledge, attitudes, and self-reported behaviors, and barriers to compliance with the use of PPE among ICU healthcare workers (HCWs) during the pandemic influenza. METHODOLOGY/PRINCIPAL FINDINGS: A survey instrument consisting of 36 questions was developed and mailed to all HCWs in 21 ICUs in 17 provinces in China. A total of 733 physicians, nurses, and other professionals were surveyed, and 650 (88.7%) were included in the analysis. Fifty-six percent of respondents reported having received training program of pandemic influenza before they cared for H1N1 patients, while 77% reported to have adequate knowledge of self and patient protection. Only 18% of respondents were able to correctly identify all components of PPE, and 55% reported high compliance (>80%) with PPE use during patient care. In multivariate analysis, vaccination for 2009 H1N1 influenza, positive attitudes towards PPE use, organizational factors such as availability of PPE in ICU, and patient information of influenza precautions, as well as reprimand for noncompliance by the supervisors were associated with high compliance, whereas negative attitudes towards PPE use and violation of PPE use were independent predictors of low compliance. CONCLUSION/SIGNIFICANCE: Knowledge and self-reported compliance to recommended PPE use among Chinese critical care clinicians is suboptimal. The perceived barriers should be addressed in order to close the significant gap between perception and knowledge or behavior.     

Assessment of Seasonal Influenza A Virus-Specific CD4 T-Cell Responses to 2009 Pandemic H1N1 Swine-Origin Influenza A Virus

Very limited evidence has been reported to show human adaptive immune responses to the 2009 pandemic H1N1 swine-origin influenza A virus (S-OIV). We studied 17 S-OIV peptides homologous to immunodominant CD4 T epitopes from hemagglutinin (HA), neuraminidase (NA), nuclear protein (NP), M1 matrix protein (MP), and PB1 of a seasonal H1N1 strain. We concluded that 15 of these 17 S-OIV peptides would induce responses of seasonal influenza virus-specific T cells. Of these, seven S-OIV sequences were identical to seasonal influenza virus sequences, while eight had at least one amino acid that was not conserved. T cells recognizing epitopes derived from these S-OIV antigens could be detected ex vivo. Most of these T cells expressed memory markers, although none of the donors had been exposed to S-OIV. Functional analysis revealed that specific amino acid differences in the sequences of these S-OIV peptides would not affect or partially affect memory T-cell responses. These findings suggest that without protective antibody responses, individuals vaccinated against seasonal influenza A may still benefit from preexisting cross-reactive memory CD4 T cells reducing their susceptibility to S-OIV infection.The outbreak of H1N1 swine-origin influenza A virus (S-OIV) in April 2009 has raised a new threat to public health (5, 6). This novel virus (with A/California/04/09 H1N1 as a prototypic strain) not only replicated more efficiently but also caused more severe pathological lesions in the lungs of infected mice, ferrets, and nonhuman primates than a currently circulating human H1N1 virus (9). Similarly, human patients with influenza-like illness who tested negative for S-OIV had a milder clinical course than those who tested positive (13). Another major concern is the lack of immune protection against S-OIV in the human population. Initial serum analysis indicated that cross-reactive antibodies to this novel viral strain were detected in only one-third of people over 60 years of age, while humoral immune responses in the population under 60 years of age were rarely detected (3, 8). In addition, vaccination with recent seasonal influenza vaccines induced little or no cross-reactive antibody responses to S-OIV in any age group (3, 8).Only a few studies address whether preexisting seasonal influenza A virus-specific memory T cells cross-react with antigenic peptides derived from S-OIV (7). In the absence of preexisting cross-reactive neutralizing antibodies, it is likely that T-cell-mediated cellular immunity contributes to viral clearance and reduces the severity of symptoms, although virus-specific T cells cannot directly prevent the establishment of infection (10). Greenbaum and colleagues recently compared published T-cell epitopes for seasonal influenza viruses with S-OIV antigens (Ags) using a computational approach (7). Several seasonal H1N1 epitopes were found to be identical to S-OIV sequences. This implies that seasonal flu-specific memory T cells circulating in the peripheral blood of vaccinated and/or previously infected individuals are able to recognize their S-OIV homologues.The first objective of this study was to determine the extent of cross-reactivity of seasonal H1N1 influenza A virus-specific CD4 T cells with S-OIV epitopes, especially those less conserved peptide sequences. We chose 17 immunodominant DR4-restricted T-cell epitopes derived from a seasonal H1N1 strain, compared the binding of these epitopes and their S-OIV homologous peptides to DR4, tested the ability of S-OIV peptides to drive seasonal influenza virus-specific T-cell proliferation in vitro, and estimated the frequency of S-OIV cross-reactive T cells in the periphery of noninfected donors. We found that most homologous S-OIV peptides were able to activate seasonal H1N1 virus-specific CD4 T cells. The second objective was to compare the antigen dosage requirement to activate those T cells. By assessing the alternations in the functional avidities (of T cells to the cognate peptide and S-OIV homologue) due to amino acid differences in S-OIV peptides, we showed how those cross-reactive CD4 T cells differentially responded to the antigenic peptides derived from seasonal H1N1 virus or S-OIV. This study leads to the conclusion that previous exposure to seasonal H1N1 viral antigens will generate considerable levels of memory CD4 T cells cross-reactive with S-OIV.     

Genomic Signature and Mutation Trend Analysis of Pandemic (H1N1) 2009 Influenza A Virus

A novel swine-origin pandemic influenza A(H1N1) virus (H1N1pdm, also referred to as S-OIV) was identified as the causative agent of the 21^st century''s first influenza pandemic, but molecular features conferring its ability of human-to-human transmission has not been identified. Here we compared the protein sequences of 2009 H1N1pdm strains with those causing other pandemics and the viruses isolated from humans, swines and avians, and then analyzed the mutation trend of the residues at the signature and non-signature positions, which are species- and non-species-associated, respectively, in the proteins of H1N1pdm during the pandemic of 2009. We confirmed that the host-specific genomic signatures of 2009 H1N1pdm, which are mainly swine-like, were highly identical to those of the 1918 H1N1pdm. During the short period of time when the pandemic alert level was raised from phase 4 to phase 6, one signature residue at the position of NP-100 mutated from valine to isoleucine. Four non-signature residues, at positions NA-91, NA-233, HA-206, and NS1-123, also changed during the epidemic in 2009. All these mutant residues, except that at NA-91, are located in the viral functional domains, suggesting that they may play roles in the human adaption and virulence of 2009 H1N1pdm.     

Transmission of Pandemic H1N1 Influenza Virus and Impact of Prior Exposure to Seasonal Strains or Interferon Treatment

Novel swine-origin influenza viruses of the H1N1 subtype were first detected in humans in April 2009. As of 12 August 2009, 180,000 cases had been reported globally. Despite the fact that they are of the same antigenic subtype as seasonal influenza viruses circulating in humans since 1977, these viruses continue to spread and have caused the first influenza pandemic since 1968. Here we show that a pandemic H1N1 strain replicates in and transmits among guinea pigs with similar efficiency to that of a seasonal H3N2 influenza virus. This transmission was, however, partially disrupted when guinea pigs had preexisting immunity to recent human isolates of either the H1N1 or H3N2 subtype and was fully blocked through daily intranasal administration of interferon to either inoculated or exposed animals. Our results suggest that partial immunity resulting from prior exposure to conventional human strains may blunt the impact of pandemic H1N1 viruses in the human population. In addition, the use of interferon as an antiviral prophylaxis may be an effective way to limit spread in at-risk populations.A pandemic of novel swine-origin influenza virus (H1N1) is developing rapidly. As of 12 August 2009, nearly 180,000 cases had been reported to the WHO from around the globe (36). Sustained human-to-human transmission has furthermore been observed in multiple countries, prompting the WHO to declare a public health emergency of international concern and to raise the pandemic alert level to phase 6 (7).Swine are a natural host of influenza viruses, and although sporadic incidences of human infection with swine influenza viruses occur (8, 9, 14, 29, 35), human-to-human transmission is rare. H1N1 influenza viruses have likely circulated in swine since shortly after the 1918 human influenza pandemic (38). From the 1930s, when a swine influenza virus was first isolated, to the late 1990s, this classical swine lineage has remained relatively stable antigenically (34). In the late 1990s, however, genetic reassortment between a human H3N2 virus, a North American avian virus, and a classical swine influenza virus produced a triple reassortant virus, which subsequently spread among North American swine (34). Further reassortment events involving human influenza viruses led to the emergence in pigs of triple reassortants of the H1N1 and H1N2 subtypes (34). None of these swine viruses have demonstrated the potential for sustained human-to-human transmission.The swine-origin influenza viruses now emerging in the human population possess a previously uncharacterized constellation of eight genes (28). The NA and M segments derive from a Eurasian swine influenza virus lineage, having entered pigs from the avian reservoir around 1979, while the HA, NP, and NS segments are of the classical swine lineage and the PA, PB1, and PB2 segments derive from the North American triple reassortant swine lineage (13). This unique combination of genetic elements (segments from multiple swine influenza virus lineages, some of them derived from avian and human influenza viruses) may account for the improved fitness of pandemic H1N1 viruses, relative to that of previous swine isolates, in humans.Several uncertainties remain about how this outbreak will develop over time. Although the novel H1N1 virus has spread over a broad geographical area, the number of people known to be infected remains low in many countries, which could be due, at least in part, to the lack of optimal transmission of influenza viruses outside the winter season; thus, it is unclear at this point whether the new virus will become established in the long term. Two major factors will shape the epidemiology of pandemic H1N1 viruses in the coming months and years: the intrinsic transmissibility of the virus and the degree of protection offered by previous exposure to seasonal human strains. Initial estimates of the reproductive number (R₀) have been made based on the epidemiology of the virus to date and suggest that its rate of spread is intermediate between that of seasonal flu and that of previous pandemic strains (3, 11). However, more precise estimates of R₀ will depend on better surveillance data in the future. The transmission phenotype of pandemic H1N1 viruses in a ferret model was also recently reported and was found to be similar to (16, 27) or less efficient (25) than that of seasonal H1N1 strains. The reason for this discrepancy in the ferret model is unclear.Importantly, in considering the human population, the impact of immunity against seasonal strains on the transmission potential of pandemic H1N1 viruses is not clear. According to conventional wisdom, an influenza virus must be of a hemagglutinin (HA) subtype which is novel to the human population in order to cause a pandemic (18, 38). Analysis of human sera collected from individuals with diverse influenza virus exposure histories has indicated that in those born in the early part of the 20th century, neutralizing activity against A/California/04/09 (Cal/04/09) virus is often present (16). Conversely, serological analyses of ferret postinfection sera (13) and human pre- and postvaccination sera (4a) revealed that neutralizing antibodies against recently circulating human H1N1 viruses do not react with pandemic H1N1 isolates. These serological findings may explain the relatively small number of cases seen to date in individuals greater than 65 years of age (6). Even in the absence of neutralizing antibodies, however, a measure of immune protection sufficient to dampen transmission may be present in a host who has recently experienced seasonal influenza (10). If, on the other hand, transmission is high and immunity is low, then pandemic H1N1 strains will likely continue to spread rapidly through the population. In this situation, a range of pharmaceutical interventions will be needed to dampen the public health impact of the pandemic.Herein we used the guinea pig model (4, 21-24, 26, 30) to assess the transmissibility of the pandemic H1N1 strains Cal/04/09 and A/Netherlands/602/09 (NL/602/09) relative to that of previous human and swine influenza viruses. To better mimic the human situation, we then tested whether the efficiency of transmission is decreased by preexisting immunity to recent human H1N1 or H3N2 influenza viruses. Finally, we assessed the efficacy of intranasal treatment with type I interferon (IFN) in limiting the replication and transmission of pandemic H1N1 viruses.     

Evidence of Cross-Reactive Immunity to 2009 Pandemic Influenza A Virus in Workers Seropositive to Swine H1N1 Influenza Viruses Circulating in Italy

Background

Pigs play a key epidemiologic role in the ecology of influenza A viruses (IAVs) emerging from animal hosts and transmitted to humans. Between 2008 and 2010, we investigated the health risk of occupational exposure to swine influenza viruses (SIVs) in Italy, during the emergence and spread of the 2009 H1N1 pandemic (H1N1pdm) virus.

Methodology/Principal Findings

Serum samples from 123 swine workers (SWs) and 379 control subjects (Cs), not exposed to pig herds, were tested by haemagglutination inhibition (HI) assay against selected SIVs belonging to H1N1 (swH1N1), H1N2 (swH1N2) and H3N2 (swH3N2) subtypes circulating in the study area. Potential cross-reactivity between swine and human IAVs was evaluated by testing sera against recent, pandemic and seasonal, human influenza viruses (H1N1 and H3N2 antigenic subtypes). Samples tested against swH1N1 and H1N1pdm viruses were categorized into sera collected before (n. 84 SWs; n. 234 Cs) and after (n. 39 SWs; n. 145 Cs) the pandemic peak. HI-antibody titers ≥10 were considered positive. In both pre-pandemic and post-pandemic peak subperiods, SWs showed significantly higher swH1N1 seroprevalences when compared with Cs (52.4% vs. 4.7% and 59% vs. 9.7%, respectively). Comparable HI results were obtained against H1N1pdm antigen (58.3% vs. 7.7% and 59% vs. 31.7%, respectively). No differences were found between HI seroreactivity detected in SWs and Cs against swH1N2 (33.3% vs. 40.4%) and swH3N2 (51.2 vs. 55.4%) viruses. These findings indicate the occurrence of swH1N1 transmission from pigs to Italian SWs.

Conclusion/Significance

A significant increase of H1N1pdm seroprevalences occurred in the post-pandemic peak subperiod in the Cs (p<0.001) whereas SWs showed no differences between the two subperiods, suggesting a possible occurrence of cross-protective immunity related to previous swH1N1 infections. These data underline the importance of risk assessment and occupational health surveillance activities aimed at early detection and control of SIVs with pandemic potential in humans.     

Comparison of Temporal and Spatial Dynamics of Seasonal H3N2, Pandemic H1N1 and Highly Pathogenic Avian Influenza H5N1 Virus Infections in Ferrets

Humans may be infected by different influenza A viruses-seasonal, pandemic, and zoonotic-which differ in presentation from mild upper respiratory tract disease to severe and sometimes fatal pneumonia with extra-respiratory spread. Differences in spatial and temporal dynamics of these infections are poorly understood. Therefore, we inoculated ferrets with seasonal H3N2, pandemic H1N1 (pH1N1), and highly pathogenic avian H5N1 influenza virus and performed detailed virological and pathological analyses at time points from 0.5 to 14 days post inoculation (dpi), as well as describing clinical signs and hematological parameters. H3N2 infection was restricted to the nose and peaked at 1 dpi. pH1N1 infection also peaked at 1 dpi, but occurred at similar levels throughout the respiratory tract. H5N1 infection occurred predominantly in the alveoli, where it peaked for a longer period, from 1 to 3 dpi. The associated lesions followed the same spatial distribution as virus infection, but their severity peaked between 1 and 6 days later. Neutrophil and monocyte counts in peripheral blood correlated with inflammatory cell influx in the alveoli. Of the different parameters used to measure lower respiratory tract disease, relative lung weight and affected lung tissue allowed the best quantitative distinction between the virus groups. There was extra-respiratory spread to more tissues-including the central nervous system-for H5N1 infection than for pH1N1 infection, and to none for H3N2 infection. This study shows that seasonal, pandemic, and zoonotic influenza viruses differ strongly in the spatial and temporal dynamics of infection in the respiratory tract and extra-respiratory tissues of ferrets.     

A Qualitative Study of Vaccine Acceptability and Decision Making among Pregnant Women in Morocco during the A (H1N1) pdm09 Pandemic

Vaccination uptake of pregnant women in Morocco during the A (H1N1) pdm09 pandemic was lower than expected. A qualitative study using open-ended questions was developed to explore the main determinants of acceptance and non-acceptance of the monovalent A (H1N1) pdm09 vaccine among pregnant women in Morocco and to identify information sources that influenced their decision-making process. The study sample included 123 vaccinated and unvaccinated pregnant women who were in their second or third trimester between December 2009 and March 2010. They took part in 14 focus group discussions and eight in-depth interviews in the districts of Casablanca and Kenitra. Thematic qualitative analysis identified reasons for vaccine non-acceptance: (1) fear of the monovalent A (H1N1) pdm09 vaccine, (2) belief in an A (H1N1) pdm09 pandemic conspiracy, (3) belief in the inapplicability of the monovalent A (H1N1) pdm09 vaccine to Moroccans, (4) lack of knowledge of the monovalent A (H1N1) pdm09 vaccine, and (5) challenges of vaccination services/logistics. Reasons for vaccine acceptance included: (1) perceived benefits and (2) modeling. Decision-making was strongly influenced by family, community, mass media, religious leaders and health providers suggesting that broad communication efforts should also be used to advocate for vaccination. Meaningful communication for future vaccine campaigns must consider these context-specific findings. As cultural and religious values are shared across many Arab countries, these findings may also provide valuable insights for seasonal influenza vaccine planning in the Middle East and North Africa region at large.     

Household Transmission of Influenza A(H1N1)pdm09 in the Pandemic and Post-Pandemic Seasons

Background

The transmission of influenza viruses occurs person to person and is facilitated by contacts within enclosed environments such as households. The aim of this study was to evaluate secondary attack rates and factors associated with household transmission of laboratory-confirmed influenza A(H1N1)pdm09 in the pandemic and post-pandemic seasons.

Methods

During the 2009–2010 and 2010–2011 influenza seasons, 76 sentinel physicians in Navarra, Spain, took nasopharyngeal and pharyngeal swabs from patients diagnosed with influenza-like illness. A trained nurse telephoned households of those patients who were laboratory-confirmed for influenza A(H1N1)pdm09 to ask about the symptoms, risk factors and vaccination status of each household member.

Results

In the 405 households with a patient laboratory-confirmed for influenza A(H1N1)pdm09, 977 susceptible contacts were identified; 16% of them (95% CI 14–19%) presented influenza-like illness and were considered as secondary cases. The secondary attack rate was 14% in 2009–2010 and 19% in the 2010–2011 season (p = 0.049), an increase that mainly affected persons with major chronic conditions. In the multivariate logistic regression analysis, the risk of being a secondary case was higher in the 2010–2011 season than in the 2009–2010 season (adjusted odds ratio: 1.72; 95% CI 1.17–2.54), and in children under 5 years, with a decreasing risk in older contacts. Influenza vaccination was associated with lesser incidence of influenza-like illness near to statistical significance (adjusted odds ratio: 0.29; 95% CI 0.08–1.03).

Conclusion

The secondary attack rate in households was higher in the second season than in the first pandemic season. Children had a greater risk of infection. Preventive measures should be maintained in the second pandemic season, especially in high-risk persons.     

Detection of Extensive Cross-Neutralization between Pandemic and Seasonal A/H1N1 Influenza Viruses Using a Pseudotype Neutralization Assay

Background

Cross-immunity between seasonal and pandemic A/H1N1 influenza viruses remains uncertain. In particular, the extent that previous infection or vaccination by seasonal A/H1N1 viruses can elicit protective immunity against pandemic A/H1N1 is unclear.

Methodology/Principal Findings

Neutralizing titers against seasonal A/H1N1 (A/Brisbane/59/2007) and against pandemic A/H1N1 (A/California/04/2009) were measured using an HIV-1-based pseudovirus neutralization assay. Using this highly sensitive assay, we found that a large fraction of subjects who had never been exposed to pandemic A/H1N1 express high levels of pandemic A/H1N1 neutralizing titers. A significant correlation was seen between neutralization of pandemic A/H1N1 and neutralization of a standard seasonal A/H1N1 strain. Significantly higher pandemic A/H1N1 neutralizing titers were measured in subjects who had received vaccination against seasonal influenza in 2008–2009. Higher pandemic neutralizing titers were also measured in subjects over 60 years of age.

Conclusions/Significance

Our findings reveal that the extent of protective cross-immunity between seasonal and pandemic A/H1N1 influenza viruses may be more important than previously estimated. This cross-immunity could provide a possible explanation of the relatively mild profile of the recent influenza pandemic.     

Mutations in Hemagglutinin and Polymerase Alter the Virulence of Pandemic A(H1N1) Influenza Virus

To study the pathogenicity factors of the pandemic A(H1N1) influenza virus, a number of mutant variants of the A/Hamburg/5/2009 (H1N1)pdm09 strain were obtained through passage in chicken embryos, mouse lungs, and MDCK cell culture. After 17 lung-to-lung passages of the A/Hamburg/5/2009 in mice, the minimum lethal dose of the derived variant decreased by five orders of magnitude compared to that of the parental virus. This variant differed from the original virus by nine amino acid residues in the following viral proteins: hemagglutinin (HA), neuraminidase (NA), and components of the polymerase complex. Additional passaging of the intermediate variants and cloning made it possible to obtain pairs of strains that differed by a single amino acid substitution. Comparative analysis of replicative activity, receptor specificity, and virulence of these variants revealed two mechanisms responsible for increased pathogenicity of the virus for mice. Thus, (1) substitutions in HA (Asp225Gly or Gln226Arg) and compensatory mutation decreasing the charge of HA (Lys123Asn, Lys157Asn, Gly158Glu, Asn159Asp, or Lys212Met) altered viral receptor-binding specificity and restored the functional balance between HA and NA; (2) Phe35Leu substitution in the PA protein increased viral polymerase activity.