Epidemiology of 2009 pandemic influenza A (H1N1) in the United States

Signs and symptoms predicting influenza in children: a matched case-control analysis of prospectively collected clinical data

We aimed to determine whether there are signs or symptoms that could help clinicians to distinguish between influenza and other respiratory infections. The clinical data for this matched case-control analysis were derived from a 2-year prospective cohort study of respiratory infections among children aged≤13 years. At any signs of respiratory infection, the children were examined and nasal swabs were obtained for virologic analyses. Cases were 353 children with laboratory-confirmed influenza and controls were 353 children with respiratory symptoms who tested negative for influenza. Cases and controls were matched for gender, age, and timing of the visit. In the multivariate conditional logistic regression analyses, fever was the only sign that independently predicted influenza virus infection, with odds ratios ranging from 13.55 (95% confidence interval [CI], 6.90-26.63) to 50.10 (95% CI, 16.25-154.45), depending on the degree of fever. In all analyses, the predictive capability of fever increased with incremental elevations in the child's temperature. The likelihood ratio of fever≥40.0°C in predicting influenza was 6.00 (95% CI, 2.80-12.96). Among unselected children seen as outpatients during influenza outbreaks, fever is the only reliable predictor of influenza virus infection. The optimal use of influenza-specific antiviral drugs in children may require virologic confirmation.

Clinical Characteristics of Pandemic Influenza A (H1N1) 2009 Pediatric Infection in Busan and Gyeongsangnam-do: One Institution

Background

This study investigated the clinical characteristics and risk factors of the severity of pandemic influenza A (H1N1) 2009 infection in pediatric patients in Busan and Gyeongsangnam-do.

Methods

Cases of influenza A (H1N1) 2009 in patients under the age of 18 years, confirmed by reverse transcription polymerase chain reaction, at Pusan National University Hospital and Pusan National University Yangsan Hospital from the last week of August 2009 through the last week of February 2010 were retrospectively analyzed.

Results

Of the 3,777 confirmed cases of influenza A (H1N1) 2009, 2,200 (58.2%) were male and 1,577 (41.8%) were female. The average age of the patients was 8.4±4.8 years. The total cases peaked during 44th to 46th week. Most of the patients were in the 5- to 9-year-old age group. Oseltamivir was administered to 2,959 (78.3%) of the patients. 221 patients (5.9%) were hospitalized, age an average of 6.7±4.5 years. The average duration of hospitalization was 7.4±5.6 days. One hundred cases (45.2%) had pneumonia. Risk factors for hospitalization included male gender, <2 years of age, and underlying disease. Children with asthma were at very high risk of hospitalization, over 20 times the non-asthmatic children (odds ratio [OR], 21.684; confidence interval [CI], 13.295~39.791). Likewise the children with neurologic deficits faced a 16 times higher risk (OR, 15.738; CI, 7.961~31.111). Ten of the patients (4.5%) were admitted to the intensive care unit, and eight (3.6%) required mechanical ventilation.

Conclusion

Of the pediatric patients with pandemic influenza A (H1N1) 2009, most of the patients were in the 5- to 9-year-old age group. Risk factors for hospitalization included male gender, <2 years of age, and underlying disease. The most common complication was pneumonia. The very high risk of severe morbidity in children with asthma or neurologic disease shows the critical importance of targeted vaccine coverage, special awareness and swift care by both guardians and primary care providers.

HI responses induced by seasonal influenza vaccination are associated with clinical protection and with seroprotection against non-homologous strains

Vaccination against influenza induces homologous as well as cross-specific hemagglutination inhibiting (HI) responses. Induction of cross-specific HI responses may be essential when the influenza strain does not match the vaccine strain, or even to confer a basic immune response against a pandemic influenza virus. We carried out a clinical study to evaluate the immunological responses after seasonal vaccination in healthy adults 18-60 years of age, receiving the yearly voluntary vaccination during the influenza season 2006/2007. Vaccinees of different age groups were followed for laboratory confirmed influenza (LCI) and homologous HI responses as well as cross-specific HI responses against the seasonal H1N1 strain of 2008 and pandemic H1N1 virus of 2009 (H1N1pdm09) were determined. Homologous HI titers that are generally associated with protection (i.e. seroprotective HI titers ≥40) were found in more than 70% of vaccinees. In contrast, low HI titers before and after vaccination were significantly associated with seasonal LCI. Cross-specific HI titers ≥40 against drifted seasonal H1N1 were found in 69% of vaccinees. Cross-specific HI titers ≥40 against H1N1pdm09 were also significantly induced, especially in the youngest age group. More specifically, cross-specific HI titers ≥40 against H1N1pdm09 were inversely correlated with age. We did not find a correlation between the subtype of influenza which was circulating at the age of birth of the vaccinees and cross-specific HI response against H1N1pdm09. These data indicate that the HI titers before and after vaccination determine the vaccination efficacy. In addition, in healthy adults between 18 and 60 years of age, young adults appear to be best able to mount a cross-protective HI response against H1N1pdm09 or drifted seasonal influenza after seasonal vaccination.

Obesity and respiratory infections: does excess adiposity weigh down host defense?

The number of overweight and obese individuals has dramatically increased in the US and other developed nations during the past 30 years. While type II diabetes and cardiovascular disease are well recognized co-morbid conditions associated with obesity, recent reports have demonstrated a greater severity of illness in obese patients due to influenza during the 2009 H1N1 pandemic. Consistent with these reports, diet-induced obesity has been shown to impair anti-viral host defense in murine models of influenza infection. However, the impact of obesity on the risk of community-acquired and nosocomial pneumonia in human patients is not clear. Relatively few studies have evaluated the influence of diet-induced obesity in murine models of bacterial infections of the respiratory tract. Obese leptin deficient humans and leptin and leptin-receptor deficient mice exhibit greater susceptibility to respiratory infections suggesting a requirement for leptin in the pulmonary innate and adaptive immune response to infection. In contrast to these studies, we have observed that obese leptin receptor signaling mutant mice are resistant to pneumococcal pneumonia highlighting the complex interaction between leptin receptor signaling and immune function. Given the increased prevalence of obesity and poor responsiveness of obese individuals to vaccination against influenza, the development of novel immunization strategies for this population is warranted. Additional clinical and animal studies are needed to clarify the relationship between increased adiposity and susceptibility to community-acquired and nosocomial pneumonia.

The spread of influenza A(H1N1)pdm09 virus in Madagascar described by a sentinel surveillance network

Background

The influenza A(H1N1)pdm09 virus has been a challenge for public health surveillance systems in all countries. In Antananarivo, the first imported case was reported on August 12, 2009. This work describes the spread of A(H1N1)pdm09 in Madagascar.

Methods

The diffusion of influenza A(H1N1)pdm09 in Madagascar was explored using notification data from a sentinel network. Clinical data were charted to identify peaks at each sentinel site and virological data was used to confirm viral circulation.

Results

From August 1, 2009 to February 28, 2010, 7,427 patients with influenza-like illness were reported. Most patients were aged 7 to 14 years. Laboratory tests confirmed infection with A(H1N1)pdm09 in 237 (33.2%) of 750 specimens. The incidence of patients differed between regions. By determining the epidemic peaks we traced the diffusion of the epidemic through locations and time in Madagascar. The first peak was detected during the epidemiological week 47-2009 in Antananarivo and the last one occurred in week 07-2010 in Tsiroanomandidy.

Conclusion

Sentinel surveillance data can be used for describing epidemic trends, facilitating the development of interventions at the local level to mitigate disease spread and impact.

Factors mediating seasonal and influenza A (H1N1) vaccine acceptance among ethnically diverse populations in the urban south

OBJECTIVE

We examined the acceptability of the influenza A (H1N1) and seasonal vaccinations immediately following government manufacture approval to gauge potential product uptake in minority communities. We studied correlates of vaccine acceptance including attitudes, beliefs, perceptions, and influenza immunization experiences, and sought to identify communication approaches to increase influenza vaccine coverage in community settings.

METHODS

Adults ≥18 years participated in a cross-sectional survey from September through December 2009. Venue-based sampling was used to recruit participants of racial and ethnic minorities.

RESULTS

The sample (N=503) included mostly lower income (81.9%, n=412) participants and African Americans (79.3%, n=399). Respondents expressed greater acceptability of the H1N1 vaccination compared to seasonal flu immunization (t=2.86, p=0.005) although H1N1 vaccine acceptability was moderately low (38%, n=191). Factors associated with acceptance of the H1N1 vaccine included positive attitudes about immunizations [OR=0.23, CI (0.16, 0.33)], community perceptions of H1N1 [OR=2.15, CI (1.57, 2.95)], and having had a flu shot in the past 5 years [OR=2.50, CI (1.52, 4.10). The factors associated with acceptance of the seasonal flu vaccine included positive attitudes about immunization [OR=0.43, CI (0.32, 0.59)], community perceptions of H1N1 [OR=1.53, CI (1.16, 2.01)], and having had the flu shot in the past 5 years [OR=3.53, CI (2.16, 5.78)]. Participants were most likely to be influenced to take a flu shot by physicians [OR=1.94, CI (1.31, 2.86)]. Persons who obtained influenza vaccinations indicated that Facebook (χ(2)=11.7, p=0.02) and Twitter (χ(2)=18.1, p=0.001) could be useful vaccine communication channels and that churches (χ(2)=21.5, p<0.001) and grocery stores (χ(2)=21.5, p<0.001) would be effective "flu shot stops" in their communities.

CONCLUSIONS

In this population, positive vaccine attitudes and community perceptions, along with previous flu vaccination, were associated with H1N1 and seasonal influenza vaccine acceptance. Increased immunization coverage in this community may be achieved through physician communication to dispel vaccine conspiracy beliefs and discussion about vaccine protection via social media and in other community venues.

Knowledge, risk perceptions, and preventive precautions among Hong Kong students during the 2009 influenza A (H1N1) pandemic

This study reports the knowledge, risk perceptions, and preventive precautions among 288 Hong Kong secondary school students during the 2009 influenza A (H1N1) pandemic. Only 28.1% of the students expressed knowledge of all 3 major routes of H1N1 transmission, and 58.1% perceived a high risk of H1N1 infection. Adoption of 4-5 preventive precautions was not prevalent (40.9%) and was significantly associated with female sex, better knowledge of transmission, and higher perceived risk of infection.

Influenza virus H1N1pdm09 infections in the young and old: evidence of greater antibody diversity and affinity for the hemagglutinin globular head domain (HA1 Domain) in the elderly than in young adults and children

The H1N1 2009 influenza virus (H1N1pdm09) pandemic had several unexpected features, including low morbidity and mortality in older populations. We performed in-depth evaluation of antibody responses generated following H1N1pdm09 infection of naïve ferrets and of 130 humans ranging from the very young (0 to 9 years old) to the very old (70 to 89 years old). In addition to hemagglutination inhibition (HI) titers, we used H1N1pdm09 whole-genome-fragment phage display libraries (GFPDL) to evaluate the antibody repertoires against internal genes, hemagglutinin (HA), and neuraminidase (NA) and also measured antibody affinity for antigenic domains within HA. GFPDL analyses of H1N1pdm09-infected ferrets demonstrated gradual development of antibody repertoires with a focus on M1 and HA1 by day 21 postinfection. In humans, H1N1pdm09 infection in the elderly (>70 years old) induced antibodies with broader epitope recognition in both the internal genes and the HA1 receptor binding domain (RBD) than for the younger age groups (0 to 69 years). Importantly, post-H1N1 infection serum antibodies from the elderly demonstrated substantially higher avidity for recombinant HA1 (rHA1) (but not HA2) than those from younger subjects (50% versus <22% 7 M urea resistance, respectively) and lower antibody dissociation rates using surface plasmon resonance. This is the first study in humans that provides evidence for a qualitatively superior antibody response in the elderly following H1N1pdm09 infection, indicative of recall of long-term memory B cells or long-lived plasma cells. These findings may help explain the age-related morbidity and mortality pattern observed during the H1N1pdm09 pandemic.

Investigating the effect of high spring incidence of pandemic influenza A(H1N1) on early autumn incidence

A pandemic H1N1 infection wave in the USA occurred during spring 2009. Some hypothesized that for regions affected by the spring wave, an autumn outbreak would be less likely or delayed compared to unaffected regions because of herd immunity. We investigated this hypothesis using the Outpatient Influenza-like Illness (ILI) Network, a collaboration among the Centers for Disease Control and Prevention, health departments, and care providers. We evaluated the likelihood of high early autumn incidence given high spring incidence in core-based statistical areas (CBSAs). Using a surrogate incidence measure based on influenza-related illness ratios, we calculated the odds of high early autumn incidence given high spring incidence. CBSAs with high spring ILI ratios proved more likely than unaffected CBSAs to have high early autumn ratios, suggesting that elevated spring illness did not protect against early autumn increases. These novel methods are applicable to planning and studies involving other infectious diseases.

The ins and outs of universal childhood influenza vaccination

Influenza viruses continue to cause disease of varying severity among humans. People with underlying disease and the elderly are at increased risk of developing severe disease after infection with an influenza virus. As effective and safe vaccines are available, the WHO has recommended vaccinating these groups against influenza annually. In addition to this recommendation, public health authorities of a number of countries have recently recommended vaccinating all healthy children aged 6-59 months against influenza. Here, we review the currently available data concerning the burden of disease in children, the economical impact of implementing universal vaccination of children, the efficacy of currently available influenza virus vaccines, the theoretical concerns regarding preventing immunity otherwise induced by infections with seasonal influenza viruses, and finally, how to address these concerns.

Impact of the 2009 influenza pandemic on pneumococcal pneumonia hospitalizations in the United States

BACKGROUND

Infection with influenza virus increases the risk for developing pneumococcal disease. The A/H1N1 influenza pandemic in autumn 2009 provided a unique opportunity to evaluate this relationship.

METHODS

Using weekly age-, state-, and cause-specific hospitalizations from the US State Inpatient Databases of the Healthcare Cost and Utilization Project 2003-2009, we quantified the increase in pneumococcal pneumonia hospitalization rates above a seasonal baseline during the pandemic period.

RESULTS

We found a significant increase in pneumococcal hospitalizations from late August to mid-December 2009, which corresponded to the timing of highest pandemic influenza activity. Individuals aged 5-19 years, who have a low baseline level of pneumococcal disease, experienced the largest relative increase in pneumococcal hospitalizations (ratio, 1.6 [95% confidence interval {CI}, 1.4-1.7]), whereas the largest absolute increase was observed among individuals aged 40-64 years. In contrast, there was no excess disease in the elderly. Geographical variation in the timing of excess pneumococcal hospitalizations matched geographical patterns for the fall pandemic influenza wave.

CONCLUSIONS

The 2009 influenza pandemic had a significant impact on the rate of pneumococcal pneumonia hospitalizations, with the magnitude of this effect varying between age groups and states, mirroring observed variations in influenza activity.

Prior infections with seasonal influenza A/H1N1 virus reduced the illness severity and epidemic intensity of pandemic H1N1 influenza in healthy adults

Background. A new influenza A/H1N1 (pH1N1) virus emerged in April 2009, proceeded to spread worldwide, and was designated as an influenza pandemic. A/H1N1 viruses had circulated in 1918–1957 and 1977–2009 and were in the annual vaccine during 1977–2009.

Methods. Serum antibody to the pH1N1 and seasonal A/H1N1 viruses was measured in 579 healthy adults at enrollment (fall 2009) and after surveillance for illness (spring 2010). Subjects reporting with moderate to severe acute respiratory illness had illness and virus quantitation for 1 week; evaluations for missed illnesses were conducted over holiday periods and at the spring 2010 visit.

Results. After excluding 66 subjects who received pH1N1 vaccine, 513 remained. Seventy-seven had reported with moderate to severe illnesses; 31 were infected with pH1N1 virus, and 30 with a rhinovirus. Determining etiology from clinical findings was not possible, but fever and prominent myalgias favored influenza and prominent rhinorrhea favored rhinovirus. Tests of fall and spring antibody indicated pH1N1 infection of 23% had occurred, with the rate decreasing with increasing anti-pH1N1 antibody; a similar pattern was seen for influenza-associated illness. A reducing frequency of pH1N1 infections was also seen with increasing antibody to the recent seasonal A/H1N1 virus (A/Brisbane/59/07). Preexisting antibody to pH1N1 virus, responses to a single vaccine dose, a low infection-to-illness ratio, and a short duration of illness and virus shedding among those with influenza indicated presence of considerable preexisting immunity to pH1N1 in the population.

Conclusions. The 2009 A/H1N1 epidemic among healthy adults was relatively mild, most likely because of immunity from prior infections with A/H1N1 viruses.

High incidence of severe influenza among individuals over 50 years of age

Age-specific epidemiological data on asymptomatic, symptomatic, and severe infections are essential for public health policies on combating influenza. In this study, we incorporated data on microbiologically confirmed infections and seroprevalence to comprehensively describe the epidemiology of pandemic H1N1 2009 influenza. Seroprevalence was determined from 1,795 random serum samples collected in our hospital in January 2007 (before the first wave of the pandemic) and March 2010 (after the second wave). Data on microbiologically confirmed infection and severe cases were obtained from the Centre for Health Protection in Hong Kong. Severe cases were most common in the 51- to 60-year-old age group. The microbiologically confirmed incidence rate was highest for children aged ≤10 years and dropped sharply for the adult population (ρ = -1.0; P < 0.01), but the incidence rate for severe disease was highest for the 51- to 60-year-old age group. For the 51- to 60-year-old age group, the seroprevalence was similar to that for the younger age groups, but the proportion of severe cases relative to seroprevalence was significantly higher than that for 11- to 50-year-old age groups. As judged from the percentage of specimens positive for other respiratory viruses compared with that for pandemic H1N1 virus, the impact of symptomatic disease due to pandemic H1N1 virus was higher than that for other respiratory viruses in people aged ≤50 years. In conclusion, the 51- to 60-year-old age group, which had the highest overall incidence and the highest rate of severe disease but is currently not considered by the World Health Organization to be an at-risk group, should be prioritized for influenza vaccination in areas where universal influenza vaccination is not practiced.

Response to the 2009 pandemic: effect on influenza control in wealthy and poor countries

The declaration by the World Health Organization (WHO) that appearance of a swine-origin novel influenza virus in 2009 represented a pandemic was based on previously adopted guidelines and the new International Health Regulations. Severity of the pandemic was not part of the definition used, but it was stated to be less than severe at the time of declaration. It was necessary, when there was still uncertainty about the overall impact of the pandemic, for vaccine production to begin to have timely availability. Countries arranged to have vaccine for their populations, and WHO attempted to secure supplies for under-resourced countries. The world had been concerned that the next pandemic might be a severe one, based on the specter of avian influenza with a case fatality of up to 80% in humans. After it was clear that the 2009 pandemic was not severe, there were accusations, especially in Europe, that countries had secured vaccine supplies mainly to benefit the manufacturers. Such charges, even when refuted, may undermine public confidence in the process which assures vaccine supply and availability of vaccine for seasonal use. Production of pandemic vaccine is conditioned on the supply of seasonal influenza vaccine; it is unrealistic to expect vaccine to be available for pandemic use when none is used for seasonal influenza. This particularly applies to poorer counties. They have traditionally not recognized that influenza is a problem, although this attitude is changing. As we go forward, we need to keep in mind the global nature of the threat of influenza. Had the 2009 pandemic been more severe, demand would have been greater and poorer counties would have had little vaccine to meet their needs. Only by taking a broad view of influenza on an annual basis can vaccine supplies be ensured for all countries of the world.

Preliminary results of 2009 pandemic influenza surveillance in the United States using the Aggregate Hospitalization and Death Reporting Activity

Please cite this paper as: Jhung et al. (2011) Preliminary results of 2009 pandemic influenza surveillance in the United States using the Aggregate Hospitalization and Death Reporting Activity. Influenza and Other Respiratory Viruses 5(5), 321–327.

Background  To augment established influenza surveillance systems in the United States, the Centers for Disease Control and Prevention and the Council of State and Territorial Epidemiologists implemented the Aggregate Hospitalization and Death Reporting Activity (AHDRA) in August 2009. The AHDRA was designed to meet increased demands for timely and detailed information describing illness severity during the 2009 H1N1 influenza A (pH1N1) pandemic response.

Objectives  We describe the implementation of AHDRA and provide preliminary results from this new surveillance activity.

Methods  All 50 US states were asked to report influenza‐associated hospitalizations and deaths to AHDRA each week using either a laboratory‐confirmed or syndromic surveillance definition. Aggregate counts were used to calculate age‐specific weekly and cumulative rates per 100 000, and laboratory‐confirmed reports were used to estimate the age distribution of pH1N1 influenza‐associated hospitalizations and deaths.

Results  From August 30, 2009, through April 6, 2010, AHDRA identified 41 689 laboratory‐confirmed influenza‐associated hospitalizations and 2096 laboratory‐confirmed influenza‐associated deaths. Aggregate Hospitalization and Death Reporting Activity rates peaked earlier than hospitalization and death rates seen in previous influenza seasons with other surveillance systems, and the age distribution of cases revealed a tendency for hospitalizations and deaths to occur in persons <65 years for age.

Conclusions  Aggregate Hospitalization and Death Reporting Activity laboratory‐confirmed reports provided important information during the 2009 pandemic response. Aggregate Hospitalization and Death Reporting Activity syndromic reports were marked by low representativeness and specificity and were therefore less useful. The AHDRA was implemented quickly and may be a useful surveillance system to monitor severe illness during future influenza pandemics.

A comparison of the epidemiology and clinical presentation of seasonal influenza A and 2009 pandemic influenza A (H1N1) in Guatemala

BACKGROUND

A new influenza A (H1N1) virus was first found in April 2009 and proceeded to cause a global pandemic. We compare the epidemiology and clinical presentation of seasonal influenza A (H1N1 and H3N2) and 2009 pandemic influenza A (H1N1) (pH1N1) using a prospective surveillance system for acute respiratory disease in Guatemala.

METHODOLOGY/FINDINGS

Patients admitted to two public hospitals in Guatemala in 2008-2009 who met a pneumonia case definition, and ambulatory patients with influenza-like illness (ILI) at 10 ambulatory clinics were invited to participate. Data were collected through patient interview, chart abstraction and standardized physical and radiological exams. Nasopharyngeal swabs were taken from all enrolled patients for laboratory diagnosis of influenza A virus infection with real-time reverse transcription polymerase chain reaction. We identified 1,744 eligible, hospitalized pneumonia patients, enrolled 1,666 (96%) and tested samples from 1,601 (96%); 138 (9%) had influenza A virus infection. Surveillance for ILI found 899 eligible patients, enrolled 801 (89%) and tested samples from 793 (99%); influenza A virus infection was identified in 246 (31%). The age distribution of hospitalized pneumonia patients was similar between seasonal H1N1 and pH1N1 (P = 0.21); the proportion of pneumonia patients <1 year old with seasonal H1N1 (39%) and pH1N1 (37%) were similar (P = 0.42). The clinical presentation of pH1N1 and seasonal influenza A was similar for both hospitalized pneumonia and ILI patients. Although signs of severity (admission to an intensive care unit, mechanical ventilation and death) were higher among cases of pH1N1 than seasonal H1N1, none of the differences was statistically significant.

CONCLUSIONS/SIGNIFICANCE

Small sample sizes may limit the power of this study to find significant differences between seasonal influenza A and pH1N1. In Guatemala, influenza, whether seasonal or pH1N1, appears to cause severe disease mainly in infants; targeted vaccination of children should be considered.