Cost-effectiveness of 2009 pandemic influenza A(H1N1) vaccination in the United States

Background

Pandemic influenza A(H1N1) (pH1N1) was first identified in North America in April 2009. Vaccination against pH1N1 commenced in the U.S. in October 2009 and continued through January 2010. The objective of this study was to evaluate the cost-effectiveness of pH1N1 vaccination.

Methodology

A computer simulation model was developed to predict costs and health outcomes for a pH1N1 vaccination program using inactivated vaccine compared to no vaccination. Probabilities, costs and quality-of-life weights were derived from emerging primary data on pH1N1 infections in the US, published and unpublished data for seasonal and pH1N1 illnesses, supplemented by expert opinion. The modeled target population included hypothetical cohorts of persons aged 6 months and older stratified by age and risk. The analysis used a one-year time horizon for most endpoints but also includes longer-term costs and consequences of long-term sequelae deaths. A societal perspective was used. Indirect effects (i.e., herd effects) were not included in the primary analysis. The main endpoint was the incremental cost-effectiveness ratio in dollars per quality-adjusted life year (QALY) gained. Sensitivity analyses were conducted.

Results

For vaccination initiated prior to the outbreak, pH1N1 vaccination was cost-saving for persons 6 months to 64 years under many assumptions. For those without high risk conditions, incremental cost-effectiveness ratios ranged from $8,000–$52,000/QALY depending on age and risk status. Results were sensitive to the number of vaccine doses needed, costs of vaccination, illness rates, and timing of vaccine delivery.

Conclusions

Vaccination for pH1N1 for children and working-age adults is cost-effective compared to other preventive health interventions under a wide range of scenarios. The economic evidence was consistent with target recommendations that were in place for pH1N1 vaccination. We also found that the delays in vaccine availability had a substantial impact on the cost-effectiveness of vaccination.     

Response to 2009 pandemic influenza A (H1N1) vaccine in HIV-infected patients and the influence of prior seasonal influenza vaccination

Background

The immunogenicity of 2009 pandemic influenza A(H1N1) (pH1N1) vaccines and the effect of previous influenza vaccination is a matter of current interest and debate. We measured the immune response to pH1N1 vaccine in HIV-infected patients and in healthy controls. In addition we tested whether recent vaccination with seasonal trivalent inactivated vaccine (TIV) induced cross-reactive antibodies to pH1N1. (clinicaltrials.gov Identifier:{"type":"clinical-trial","attrs":{"text":"NCT01066169","term_id":"NCT01066169"}}NCT01066169)

Methods and Findings

In this single-center prospective cohort study MF59-adjuvanted pH1N1 vaccine (Focetria®, Novartis) was administered twice to 58 adult HIV-infected patients and 44 healthy controls in November 2009 (day 0 and day 21). Antibody responses were measured at baseline, day 21 and day 56 with hemagglutination-inhibition (HI) assay. The seroprotection rate (defined as HI titers ≥1∶40) for HIV-infected patients was 88% after the first and 91% after the second vaccination. These rates were comparable to those in healthy controls. Post-vaccination GMT, a sensitive marker of the immune competence of a group, was lower in HIV-infected patients. We found a high seroprotection rate at baseline (31%). Seroprotective titers at baseline were much more common in those who had received 2009–2010 seasonal TIV three weeks prior to the first dose of pH1N1 vaccine. Using stored serum samples of 51 HIV-infected participants we measured the pH1N1 specific response to 2009–2010 seasonal TIV. The seroprotection rate to pH1N1 increased from 22% to 49% after vaccination with 2009–2010 seasonal TIV. Seasonal TIV induced higher levels of antibodies to pH1N1 in older than in younger subjects.

Conclusion

In HIV-infected patients on combination antiretroviral therapy, with a median CD4+ T-lymphocyte count above 500 cells/mm³, one dose of MF59-adjuvanted pH1N1 vaccine induced a high seroprotection rate comparable to that in healthy controls. A second dose had a modest additional effect. Furthermore, seasonal TIV induced cross-reactive antibodies to pH1N1 and this effect was more pronounced in older subjects.     

Receptor recognition mechanism of human influenza A H1N1 (1918), avian influenza A H5N1 (2004), and pandemic H1N1 (2009) neuraminidase

Influenza A neuraminidase (NA) is a target for anti-influenza drugs. The function of this enzyme is to cleave a glycosidic linkage of a host cell receptor that links sialic acid (Sia) to galactose (Gal), to allow the virus to leave an infected cell and propagate. The receptor is an oligosaccharide on the host cell surface. There are two types of oligosaccharide receptor; the first, which is found mainly on avian epithelial cell surfaces, links Sia with Gal by an α2,3 glycosidic linkage; in the second, found mainly on human epithelial cell surfaces, linkage is via an α2,6 linkage. Some researchers believe that NAs from different viruses show selectivity for each type of linkage, but there is limited information available to confirm this hypothesis. To see if the linkage type is more specific to any particular NA, a number of NA-receptor complexes of human influenza A H1N1 (1918), avian influenza A H5N1 (2004), and a pandemic strain of H1N1 (2009) were constructed using homology modeling and molecular dynamics simulation. The results show that the two types of receptor analogues bound to NAs use different mechanisms. Moreover, it was found that a residue unique to avian virus NA is responsible for the recognition of the Siaα2,3Gal receptor, and a residue unique to human virus NA is responsible for the recognition of Siaα2,6Gal. We believe that this finding could explain how NAs of different virus origins always possess some unique residues.     

Susceptibility of antiviral drugs against 2009 influenza A (H1N1) virus

The recent outbreak of the novel strain of influenza A (H1N1) virus has raised a global concern of the future risk of a pandemic. To understand at the molecular level how this new H1N1 virus can be inhibited by the current anti-influenza drugs and which of these drugs it is likely to already be resistant to, homology modeling and MD simulations have been applied on the H1N1 neuraminidase complexed with oseltamivir, and the M2-channel with adamantanes bound. The H1N1 virus was predicted to be susceptible to oseltamivir, with all important interactions with the binding residues being well conserved. In contrast, adamantanes are not predicted to be able to inhibit the M2 function and have completely lost their binding with the M2 residues. This is mainly due to the fact that the M2 transmembrane of the new H1N1 strain contains the S31N mutation which is known to confer resistance to adamantanes.     

Killer-cell immunoglobulin-like receptors (KIR) in severe A (H1N1) 2009 influenza infections

Introduction of a novel influenza virus into the human population leads to the occurrence of pandemic events, such as the one caused by pandemic influenza A (H1N1) 2009 virus. The severity of infections caused by this virus in young adults was greater than that observed in patients with seasonal influenza. Fatal cases have been associated with an abnormal innate, proinflammatory immune response. A critical role for natural killer cells during the initial responses to influenza infections has been suggested. In this study, we assessed the association of killer-cell immunoglobulin-like receptors (KIRs) with disease severity by comparing KIR gene content in patients with mild and severe pandemic influenza virus infections to a control group. We found that activator (KIR3DS1 and KIR2DS5) and inhibitory (KIR2DL5) genes, encoded in group B haplotypes containing the cB01, cB03 and tB01 motifs, are associated with severe pandemic influenza A (H1N1) 2009 infections. Better understanding of how genetic variability contributes to influenza virus pathogenesis may help to the development of immune intervention strategies aiming at controlling the severity of disease.     

Seasonal FluMist vaccination induces cross-reactive T cell immunity against H1N1 (2009) influenza and secondary bacterial infections

T cell epitopes have been found to be shared by circulating, seasonal influenza virus strains and the novel pandemic H1N1 influenza infection, but the ability of these common epitopes to provide cross-protection is unknown. We have now directly tested this by examining the ability of live seasonal influenza vaccine (FluMist) to mediate protection against swine-origin H1N1 influenza virus infection. Naive mice demonstrated considerable susceptibility to H1N1 Cal/04/09 infection, whereas FluMist-vaccinated mice had markedly decreased morbidity and mortality. In vivo depletion of CD4(+) or CD8(+) immune cells after vaccination indicated that protective immunity was primarily dependent upon FluMist-induced CD4(+) cells but not CD8(+) T cells. Passive protection studies revealed little role for serum or mucosal Abs in cross-protection. Although H1N1 influenza infection of naive mice induced intensive phagocyte recruitment, pulmonary innate defense against secondary pneumococcal infection was severely suppressed. This increased susceptibility to bacterial infection was correlated with augmented IFN-γ production produced during the recovery stage of H1N1 influenza infection, which was completely suppressed in mice previously immunized with FluMist. Furthermore, susceptibility to secondary bacterial infection was decreased in the absence of type II, but not type I, IFN signaling. Thus, seasonal FluMist treatment not only promoted resistance to pandemic H1N1 influenza infection but also restored innate immunity against complicating secondary bacterial infections.     

Evolutionary pathways of the pandemic influenza A (H1N1) 2009 in the UK

The emergence of the influenza (H1N1) 2009 virus provided a unique opportunity to study the evolution of a pandemic virus following its introduction into the human population. Virological and clinical surveillance in the UK were comprehensive during the first and second waves of the pandemic in 2009, with extensive laboratory confirmation of infection allowing a detailed sampling of representative circulating viruses. We sequenced the complete coding region of the haemagglutinin (HA) segment of 685 H1N1 pandemic viruses selected without bias during two waves of pandemic in the UK (April-December 2009). Phylogenetic analysis showed that although temporal accumulation of amino acid changes was observed in the HA sequences, the overall diversity was less than that typically seen for seasonal influenza A H1N1 or H3N2. There was co-circulation of multiple variants as characterised by signature amino acid changes in the HA. A specific substitution (S203T) became predominant both in UK and global isolates. No antigenic drift occurred during 2009 as viruses with greater than four-fold reduction in their haemagglutination inhibition (HI) titre ("low reactors") were detected in a low proportion (3%) and occurred sporadically. Although some limited antigenic divergence in viruses with four-fold reduction in HI titre might be related to the presence of 203T, additional studies are needed to test this hypothesis.     

Prevalence of 2009 pandemic influenza A (H1N1) virus antibodies, Tampa Bay Florida--November-December, 2009

Background

In 2009, a novel influenza virus (2009 pandemic influenza A (H1N1) virus (pH1N1)) caused significant disease in the United States. Most states, including Florida, experienced a large fall wave of disease from September through November, after which disease activity decreased substantially. We determined the prevalence of antibodies due to the pH1N1 virus in Florida after influenza activity had peaked and estimated the proportion of the population infected with pH1N1 virus during the pandemic.

Methods

During November-December 2009, we collected leftover serum from a blood bank, a pediatric children''s hospital and a pediatric outpatient clinic in Tampa Bay Florida. Serum was tested for pH1N1 virus antibodies using the hemagglutination-inhibition (HI) assay. HI titers ≥40 were considered seropositive. We adjusted seroprevalence results to account for previously established HI assay specificity and sensitivity and employed a simple statistical model to estimate the proportion of seropositivity due to pH1N1 virus infection and vaccination.

Results

During the study time period, the overall seroprevalence in Tampa Bay, Florida was 25%, increasing to 30% after adjusting for HI assay sensitivity and specificity. We estimated that 5.9% of the population had vaccine-induced seropositivity while 25% had seropositivity secondary to pH1N1 virus infection. The highest cumulative incidence of pH1N1 virus infection was among children aged 5–17 years (53%) and young adults aged 18–24 years (47%), while adults aged ≥50 years had the lowest cumulative incidence (11–13%) of pH1N1 virus infection.

Conclusions

After the peak of the fall wave of the pandemic, an estimated one quarter of the Tampa Bay population had been infected with the pH1N1 virus. Consistent with epidemiologic trends observed during the pandemic, the highest burdens of disease were among school-aged children and young adults.     

Genetic diversity of the 2009 pandemic influenza A(H1N1) viruses in Finland

Background

In Finland, the first infections caused by the 2009 pandemic influenza A(H1N1) virus were identified on May 10. During the next three months almost all infections were found from patients who had recently traveled abroad. In September 2009 the pandemic virus started to spread in the general population, leading to localized outbreaks and peak epidemic activity was reached during weeks 43–48.

Methods/Results

The nucleotide sequences of the hemagglutinin (HA) and neuraminidase (NA) genes from viruses collected from 138 patients were determined. The analyzed viruses represented mild and severe infections and different geographic regions and time periods. Based on HA and NA gene sequences, the Finnish pandemic viruses clustered in four groups. Finnish epidemic viruses and A/California/07/2009 vaccine virus strain varied from 2–8 and 0–5 amino acids in HA and NA molecules, respectively, giving a respective maximal evolution speed of 1.4% and 1.1%. Most amino acid changes in HA and NA molecules accumulated on the surface of the molecule and were partly located in antigenic sites. Three severe infections were detected with a mutation at HA residue 222, in two viruses with a change D222G, and in one virus D222Y. Also viruses with change D222E were identified. All Finnish pandemic viruses were sensitive to oseltamivir having the amino acid histidine at residue 275 of the neuraminidase molecule.

Conclusions

The Finnish pandemic viruses were quite closely related to A/California/07/2009 vaccine virus. Neither in the HA nor in the NA were changes identified that may lead to the selection of a virus with increased epidemic potential or exceptionally high virulence. Continued laboratory-based surveillance of the 2009 pandemic influenza A(H1N1) is important in order to rapidly identify drug resistant viruses and/or virus variants with potential ability to cause severe forms of infection and an ability to circumvent vaccine-induced immunity.     

The seroprevalence of pandemic influenza H1N1 (2009) virus in China

Background

Mainland China experienced pandemic influenza H1N1 (2009) virus (pH1N1) with peak activity during November-December 2009. To understand the geographic extent, risk factors, and attack rate of pH1N1 infection in China we conducted a nationwide serological survey to determine the prevalence of antibodies to pH1N1.

Methodology/Principal Findings

Stored serum samples (n = 2,379) collected during 2006-2008 were used to estimate baseline serum reactogenicity to pH1N1. In January 2010, we used a multistage-stratified random sampling method to select 50,111 subjects who met eligibility criteria and collected serum samples and administered a standardized questionnaire. Antibody response to pH1N1 was measured using haemagglutination inhibition (HI) assay and the weighted seroprevalence was calculated using the Taylor series linearization method. Multivariable logistic regression analyses were used to examine risk factors for pH1N1 seropositivity. Baseline seroprevalence of pH1N1 antibody (HI titer ≥40) was 1.2%. The weighted seroprevalence of pH1N1 among the Chinese population was 21.5%(vaccinated: 62.0%; unvaccinated: 17.1%). Among unvaccinated participants, those aged 6-15 years (32.9%) and 16-24 years (30.3%) had higher seroprevalence compared with participants aged 25–59 years (10.7%) and ≥60 years (9.9%, P<0.0001). Children in kindergarten and students had higher odds of seropositivity than children in family care (OR: 1.36 and 2.05, respectively). We estimated that 207.7 million individuals (15.9%) experienced pH1N1 infection in China.

Conclusions/Significance

The Chinese population had low pre-existing immunity to pH1N1 and experienced a relatively high attack rate in 2009 of this virus. We recommend routine control measures such as vaccination to reduce transmission and spread of seasonal and pandemic influenza viruses.     

Meta-analysis of the immunogenicity and tolerability of pandemic influenza A 2009 (H1N1) vaccines

Background

Although the 2009 (H1N1) influenza pandemic officially ended in August 2010, the virus will probably circulate in future years. Several types of H1N1 vaccines have been tested including various dosages and adjuvants, and meta-analysis is needed to identify the best formulation.

Methods

We searched MEDLINE, EMBASE, and nine clinical trial registries to April 2011, in any language for randomized clinical trials (RCTs) on healthy children, adolescents, adults and the elderly. Primary outcome was the seroconversion rate according to hemagglutinination-inhibition (HI); secondary outcomes were adverse events. For the primary outcome, we used head-to-head meta-analysis and multiple-treatments meta-analysis.

Results

Eighteen RCTs could be included in all primary analyses, for a total of 76 arms (16,725 subjects). After 2 doses, all 2009 H1N1 split/subunit inactivated vaccines were highly immunogenic and overcome CPMP seroconversion criteria. After 1 dose only, all split/subunit vaccines induced a satisfactory immunogenicity (> = 70%) in adults and adolescents, while only some formulations showed acceptable results for children and elderly (non-adjuvanted at high-doses and oil-in-water adjuvanted vaccines). Vaccines with oil-in-water adjuvants were more immunogenic than both nonadjuvanted and aluminum-adjuvanted vaccines at equal doses and their immunogenicity at doses < = 6 µg (even with as little as 1.875 µg of hemagglutinin antigen) was not significantly lower than that achieved after higher doses. Finally, the rate of serious vaccine-related adverse events was low for all 2009 H1N1 vaccines (3 cases, resolved in 10 days, out of 22826 vaccinated subjects). However, mild to moderate adverse reactions were more (and very) frequent for oil-in-water adjuvanted vaccines.

Conclusions

Several one-dose formulations might be valid for future vaccines, but 2 doses may be needed for children, especially if a low-dose non-adjuvanted vaccine is used. Given that 15 RCTs were sponsored by vaccine manufacturers, future trials sponsored by non-industry agencies and comparing vaccines using different types of adjuvants are needed.     

A review of adult mortality due to 2009 pandemic (H1N1) influenza A in California

Background

While children and young adults had the highest attack rates due to 2009 pandemic (H1N1) influenza A (2009 H1N1), studies of hospitalized cases noted high fatality in older adults. We analyzed California public health surveillance data to better characterize the populations at risk for dying due to 2009 H1N1.

Methods and Findings

A case was an adult ≥20 years who died with influenza-like symptoms and laboratory results indicative of 2009 H1N1. Demographic and clinical data were abstracted from medical records using a standardized case report form. From April 3, 2009 – August 10, 2010, 541 fatal cases ≥20 years with 2009 H1N1 were reported. Influenza fatality rates per 100,000 population were highest in persons 50–59 years (3.5; annualized rate = 2.6) and 60–69 years (2.3; annualized rate = 1.7) compared to younger and older age groups (0.4–1.9; annualized rates = 0.3–1.4). Of 486 cases hospitalized prior to death, 441 (91%) required intensive care unit (ICU) admission. ICU admission rates per 100,000 population were highest in adults 50–59 years (8.6). ICU case-fatality ratios among adults ranged from 24–42%, with the highest ratios in persons 70–79 years. A total of 425 (80%) cases had co-morbid conditions associated with severe seasonal influenza. The prevalence of most co-morbid conditions increased with increasing age, but obesity, pregnancy and obstructive sleep apnea decreased with age. Rapid testing was positive in 97 (35%) of 276 tested. Of 482 cases with available data, 384 (80%) received antiviral treatment, including 49 (15%) of 328 within 48 hours of symptom onset.

Conclusions

Adults aged 50–59 years had the highest fatality due to 2009 H1N1; older adults may have been spared due to pre-existing immunity. However, once infected and hospitalized in intensive care, case-fatality ratios were high for all adults, especially in those over 60 years. Vaccination of adults older than 50 years should be encouraged.     

Pre-admission statin use and in-hospital severity of 2009 pandemic influenza A(H1N1) disease

Background

Statins are drugs that are used to lower plasma cholesterol levels. Recently, contradictory claims have been made about possible additional effects of statins on progression of a variety of inflammatory disorders, including infections. We therefore examined the clinical course of patients admitted to hospital with 2009 pandemic influenza A(H1N1), who were or weren''t taking statins at time of admission.

Methods

A retrospective case-control study was performed using the United Kingdom Influenza Clinical Information Network (FLU-CIN) database, containing detailed information on 1,520 patients admitted to participating hospitals with confirmed 2009 pandemic influenza A(H1N1) infection between April 2009 and January 2010. We confined our analysis to those aged over 34 years. Univariate analysis was used to calculate unadjusted odds ratios (OR) and 95 percent confidence intervals (95%CI) for factors affecting progression to severe outcome (high dependency or intensive care unit level support) or death (cases); two multivariable logistic regression models were then established for age and sex, and for age, sex, obesity and “indication for statin” (e.g., heart disease or hypercholesterolaemia).

Results

We found no statistically significant association between pre-admission statin use and severity of outcome after adjustment for age and sex [adjusted OR: 0.81 (95% CI: 0.46–1.38); n = 571]. After adjustment for age, sex, obesity and indication for statin, the association between pre-admission statin use and severe outcome was not statistically significant; point estimates are compatible with a small but clinically significant protective effect of statin use [adjusted OR: 0.72 (95% CI: 0.38–1.33)].

Conclusions

In this group of patients hospitalized with pandemic influenza, a significant beneficial effect of pre-admission statin use on the in-hospital course of illness was not identified. Although the database from which these observations are derived represents the largest available suitable UK hospital cohort, a larger study would be needed to confirm whether there is any benefit in this setting.     

Genetical features of influenza virus A (H1N1) strain that caused the 2009 pandemic

Genetical features of the A(H1N1) influenza virus strain that caused the 2009 pandemic are analyzed in the review. Mutations typical for this strain, unique and similar to influenza viruses of swine, avian and seasonal types, and phenotypic (pathologic) features associated with them, that are experimentally confirmed, are described. A possibility of reassortation of avian and swine influenza viruses and possible epidemiologic consequences are discussed.     

Male predominance of pneumonia and hospitalization in pandemic influenza A (H1N1) 2009 infection

Background

Based on our clinical experience, the H-reflex amplitude asymmetry might be an earlier sign of nerve root involvement than latency in patients with S1 radiculopathy. However, no data to support this assumption are available. The purpose of this study was to review and report the electrophysiological changes in H-reflex amplitude and latency in patients with radiculopathy in order to determine if there is any evidence to support the assumption that H-reflex amplitude is an earlier sign of nerve root involvement than latency.

Results

Patients with radiculopathy showed significant amplitude asymmetry when compared with healthy controls. However, latency was not always significantly different between patients and healthy controls. These findings suggest nerve root axonal compromise that reduced reflex amplitude earlier than the latency parameter (demyelination) during the pathologic processes.

Conclusion

Contrary to current clinical thought, H-reflex amplitude asymmetry is an earlier sign/parameter of nerve root involvement in patients with radiculopathy compared with latency.     

Inside the outbreak of the 2009 influenza A (H1N1)v virus in Mexico

Background

Influenza viruses pose a threat to human health because of their potential to cause global disease. Between mid March and mid April a pandemic influenza A virus emerged in Mexico. This report details 202 cases of infection of humans with the 2009 influenza A virus (H1N1)v which occurred in Mexico City as well as the spread of the virus throughout the entire country.

Methodology and Findings

From May 1st to May 5th nasopharyngeal swabs, derived from 751 patients, were collected at 220 outpatient clinics and 28 hospitals distributed throughout Mexico City. Analysis of samples using real time RT-PCR revealed that 202 patients out of the 751 subjects (26.9%) were confirmed to be infected with the new virus. All confirmed cases of human infection with the strain influenza (H1N1)v suffered respiratory symptoms. The greatest number of confirmed cases during the outbreak of the 2009 influenza A (H1N1)v were seen in neighbourhoods on the northeast side of Mexico City including Iztapalapa, Gustavo A. Madero, Iztacalco, and Tlahuac which are the most populated areas in Mexico City. Using these data, together with data reported by the Mexican Secretariat of Health (MSH) to date, we plot the course of influenza (H1N1)v activity throughout Mexico.

Conclusions

Our data, which is backed up by MSH data, show that the greatest numbers of the 2009 influenza A (H1N1) cases were seen in the most populated areas. We speculate on conditions in Mexico which may have sparked this flu pandemic, the first in 41 years. We accept the hypothesis that high population density and a mass gathering which took in Iztapalapa contributed to the rapid spread of the disease which developed in three peaks of activity throughout the Country.     

21世纪的首次大流行：2009甲型（H1N1）流感

2009年4月初,在墨西哥和美国出现一种新型甲型（H1N1）流感病毒。该病毒通过人-人传播迅速在全球范围蔓延。该病毒拥有来自人流感病毒、禽流感病毒和猪流感病毒的基因片段,其HA基因与引发1918年大流行的流感病毒株的HA基因同源性很高。该病毒倾向于感染儿童、青少年、孕妇,以及具有心肺疾病的人。据观察,它在人群中的传播能力高于季节性流感。部分感染患者具有在季节性流感中罕见的呕吐和腹泻症状。先前的流感病毒大流行和2009年爆发的甲型H1N1流感病毒大流行表明,由于流感病毒变异速度快、容易发生基因重排,新产生的变异毒株很可能造成新的大流行,威胁人类健康。由于禽流感病毒和人流感病毒都能感染猪,猪被认为是通过基因重排生成新的大流行病毒的＂混合容器＂。     

Age distribution of cases of 2009 (H1N1) pandemic influenza in comparison with seasonal influenza

Introduction

Several aspects of the epidemiology of 2009 (H1N1) pandemic influenza have not been accurately determined. We sought to study whether the age distribution of cases differs in comparison with seasonal influenza.

Methods

We searched for official, publicly available data through the internet from different countries worldwide on the age distribution of cases of influenza during the 2009 (H1N1) pandemic influenza period and most recent seasonal influenza periods. Data had to be recorded through the same surveillance system for both compared periods.

Results

For 2009 pandemic influenza versus recent influenza seasons, in USA, visits for influenza-like illness to sentinel providers were more likely to involve the age groups of 5–24, 25–64 and 0–4 years compared with the reference group of >64 years [odds ratio (OR) (95% confidence interval (CI)): 2.43 (2.39–2.47), 1.66 (1.64–1.69), and 1.51 (1.48–1.54), respectively]. Pediatric deaths were less likely in the age groups of 2–4 and <2 years than the reference group of 5–17 years [OR (95% CI): 0.46 (0.25–0.85) and 0.49 (0.30–0.81), respectively]. In Australia, notifications for laboratory-confirmed influenza were more likely in the age groups of 10–19, 5–9, 20–44, 45–64 and 0–4 years than the reference group of >65 years [OR (95% CI): 7.19 (6.67–7.75), 5.33 (4.90–5.79), 5.04 (4.70–5.41), 3.12 (2.89–3.36) and 1.89 (1.75–2.05), respectively]. In New Zealand, consultations for influenza-like illness by sentinel providers were more likely in the age groups of <1, 1–4, 35–49, 5–19, 20–34 and 50–64 years than the reference group of >65 years [OR (95% CI): 2.38 (1.74–3.26), 1.99 (1.62–2.45), 1.57 (1.30–1.89), 1.57 (1.30–1.88), 1.40 (1.17–1.69) and 1.39 (1.14–1.70), respectively].

Conclusions

The greatest increase in influenza cases during 2009 (H1N1) pandemic influenza period, in comparison with most recent seasonal influenza periods, was seen for school-aged children, adolescents, and younger adults.