Effectiveness of 1 dose of influenza A (H1N1) 2009 monovalent vaccines in preventing reverse-transcription polymerase chain reaction-confirmed H1N1 infection among school-aged children in maine

Live Attenuated Cold-Adapted Influenza Vaccines

Live attenuated, cold-adapted influenza vaccines exhibit several desirable characteristics, including the induction of systemic, mucosal, and cell-mediated immunity resulting in breadth of protection, ease of administration, and yield. Seasonal live attenuated influenza vaccines (LAIVs) were developed in the United States and Russia and have been used in several countries. In the last decade, following the incorporation of the 2009 pandemic H1N1 strain, the performance of both LAIVs has been variable and the U.S.-backbone LAIV was less effective than the corresponding inactivated influenza vaccines. The cause appears to be reduced replicative fitness of some H1N1pdm09 viruses, indicating a need for careful selection of strains included in multivalent LAIV formulations. Assays are now being implemented to select optimal strains. An improved understanding of the determinants of replicative fitness of vaccine strains and of vaccine effectiveness of LAIVs is needed for public health systems to take full advantage of these valuable vaccines.

Impact of Pre-Existing Immunity to Influenza on Live-Attenuated Influenza Vaccine (LAIV) Immunogenicity

During the previous influenza seasons, between 2010 and 2016, the live attenuated influenza vaccine (LAIV) provided variable efficacy against influenza in the U.S., causing the recommendation against the use of the LAIV. In striking contrast, pre-clinical studies have repeatedly demonstrated superior efficacy of LAIV against mismatched influenza viruses, compared to inactivated influenza vaccines (IIV). This disparity in reported vaccine efficacies between pre-clinical and clinical studies may in part be explained by limitations of the animal models of influenza. In particular, the absence of pre-existing immunity in animal models has recently emerged as a potential explanation for the discrepancies between preclinical findings and human studies. This commentary focuses on the potential impact of pre-existing immunity on LAIV induced immunogenicity with an emphasis on cross-protective immunity.

Parent Intent and Willingness to Immunize Children Against Influenza in the Pediatric Emergency Department

OBJECTIVES

To determine rates of influenza immunization among children treated in a pediatric emergency department (ED) and to ascertain parent willingness for children to receive influenza vaccine (IV) in the ED.

METHODS

Interviews were conducted with parents of children 6 months or older evaluated in the ED for minor illness or injury. Demographic data, IV history, and intent and willingness to receive future IV were recorded during the summer of 2013. Participants were contacted in March 2014 to assess IV status, barriers to obtaining IV, and willingness to obtain IV in the ED. Chart review determined number of patients who were at high risk.

RESULTS

Of 457 families approached, 285 (62%) were enrolled. Two hundred forty-two (85%) intended to vaccinate; 83% reported willingness to receive IV at a future ED visit. Common reasons for not receiving IV were concerns about adverse effects (31%) and lack of time or interest (24%). Of the 224 participants (79%) reached in follow-up, 112 (50%) had received IV in the prior season. Among those who did not receive IV, 65 (66%) had intended to vaccinate, and 54 (55%) indicated they would have accepted IV in the ED. Fifty-three (54%) of unvaccinated patients at follow-up had high risk of influenza complications.

CONCLUSIONS

Our data support an IV program in the pediatric ED as a means of increasing vaccination rates, particularly among high-risk patients. Parents are often concerned about adverse effects of IV, and providers should target education in this area.

2015-2016 Vaccine Effectiveness of Live Attenuated and Inactivated Influenza Vaccines in Children in the United States

Background

In the 2015-2016 season, quadrivalent live attenuated influenza vaccine (LAIV) and both trivalent and quadrivalent inactivated influenza vaccine (IIV) were available in the United States.

Methods

This study, conducted according to a test-negative case-control design, enrolled children aged 2-17 years presenting to outpatient settings with fever and respiratory symptoms for <5 days at 8 sites across the United States between 30 November 2015 and 15 April 2016. A nasal swab was obtained for reverse-transcriptase polymerase chain reaction (RT-PCR) testing for influenza, and influenza vaccination was verified in the medical record or vaccine registry. Influenza vaccine effectiveness (VE) was estimated using a logistic regression model.

Results

Of 1012 children retained for analysis, most children (59%) were unvaccinated, 10% received LAIV, and 31% received IIV. Influenza A (predominantly antigenically similar to the A/California/7/2009 strain) was detected in 14% and influenza B (predominantly a B/Victoria lineage) in 10%. For all influenza, VE was 46% (95% confidence interval [CI], 7%-69%) for LAIV and 65% (48%-76%) for IIV. VE against influenza A(H1N1)pdm09 was 50% (95% CI, -2% to 75%) for LAIV and 71% (51%-82%) for IIV. The odds ratio for vaccine failure with RT-PCR-confirmed A(H1N1)pdm09 was 1.71 (95% CI, 0.78-3.73) in LAIV versus IIV recipients.

Conclusions

LAIV and IIV demonstrated effectiveness against any influenza among children aged 2-17 years in 2015-2016. When compared to all unvaccinated children, VE against influenza A(H1N1)pdm09 was significant for IIV but not LAIV.

Clinical Trials Registration

NCT01997450.

Perspectives from the Society for Pediatric Research: Decreased Effectiveness of the Live Attenuated Influenza Vaccine

The intranasal live attenuated influenza vaccine (LAIV), FluMist, has been widely appreciated by pediatricians, parents, and children alike for its ease of administration. However, concerns regarding lack of effectiveness in recent influenza seasons led to the CDC Advisory Committee on Immunization Practices (ACIP) recommendation to administer inactivated influenza vaccines (IIVs), and not LAIV, during the 2016-17 and 2017-18 seasons. Given that data from previous years demonstrated equivalent and even improved efficacy of LAIV compared with IIV, these recent data were surprising, raising many questions about the potential mechanisms underlying this change. This review seeks to summarize the history of LAIV studies and ACIP recommendations with a focus on the recent decrease in vaccine effectiveness (VE) and discordant results among studies performed in different countries. Decreased VE for A/H1N1pdm09 viruses represents the most consistent finding across studies, as VE has been low every season these viruses predominated since 2010-11. Potential explanations underlying diminished effectiveness include the hypothesis that prior vaccination, reduced thermostability of A/H1N1pdm09, addition of a fourth virus, or reduced replication fitness of A/H1N1pdm09 strains may have contributed to this phenomenon. Ongoing studies and potential alterations to LAIV formulations provide hope for a return of effective LAIV in future influenza seasons.

Cost-effectiveness of Strategies for Offering Influenza Vaccine in the Pediatric Emergency Department

IMPORTANCE

Influenza is a significant public health burden, causing morbidity and mortality in children, yet vaccination rates remain low. Vaccination in the pediatric emergency department (PED) setting may be beneficial but, to date, has not been proven to be cost-effective.

OBJECTIVE

To compare the cost-effectiveness of 4 strategies for PED-based influenza vaccine: offering vaccine to all patients, only to patients younger than 5 years, only to high-risk patients (all ages), or to no patients.

DESIGN, SETTING, AND PARTICIPANTS

Using commercial decision analysis software, a cost-effectiveness analysis was performed from January 1, 2016, to June 1, 2017, to compare influenza vaccine strategies at a tertiary, urban, freestanding PED with an estimated 60 000 visits per year among a hypothetical cohort of children visiting the above PED during influenza season. Sensitivity analyses estimated the effect of uncertainties across a variety of input variables (eg, influenza prevalence, vaccine price and effectiveness, and costs of complications).

MAIN OUTCOMES AND MEASURES

The primary outcomes were cost and incremental cost-effectiveness ratio in dollars per influenza case averted. Secondary outcomes included total societal costs, hospitalizations and deaths averted, and quality-adjusted life-years gained.

RESULTS

Offering influenza vaccine to all eligible patients has the lowest cost, at $114.45 (95% CI, $55.48-$245.45) per case of influenza averted. This strategy saves $33.51 (95% CI, $18-$62) per case averted compared with no vaccination, and averages 27 fewer cases of influenza per 1000 patients. Offering vaccine to all patients resulted in 0.72 days (95% CI, 0.18-1.78 days) of quality-adjusted life-years lost, whereas offering to none resulted in 0.91 days (95% CI, 0.25-2.2 days) of quality-adjusted life-years lost. In sensitivity analyses, this strategy remains robustly cost-effective across a wide range of assumptions. In addition to being the most cost-effective strategy regardless of age or risk status, routine vaccination in the PED results in a net societal monetary benefit under many circumstances. In Monte Carlo analysis, offering vaccine to all patients was superior to other strategies in at least 99.8% of cases.

CONCLUSIONS AND RELEVANCE

Although few PEDs routinely offer influenza vaccination, doing so appears to be cost-effective, with the potential to significantly reduce the economic (and patient) burden of pediatric influenza.

Live-Attenuated Influenza Vaccine Effectiveness in Children From 2009 to 2015-2016: A Systematic Review and Meta-Analysis

BACKGROUND

This systematic review and meta-analysis describes and consolidates findings from all studies that assessed the effectiveness of live-attenuated influenza vaccine (LAIV) against laboratory-confirmed influenza since the 2009 pandemic in children and young adults.

METHODS

A MEDLINE search was conducted for articles published from January 1, 2010 to November 30, 2016. All original publications reporting an effectiveness estimate of LAIV against cases of influenza confirmed by reverse-transcription polymerase chain reaction or culture were retained for analysis. Effectiveness estimates were categorized by LAIV formulation (monovalent, trivalent, and quadrivalent) and strain (any influenza strain, A(H1N1)pdm09, A(H3N2), and B strains). Consolidated estimates were obtained with a random-effects model.

RESULTS

A total of 24 publications presenting 29 observational studies were retained for meta-analysis. Live-attenuated influenza vaccine was not shown to be effective against A(H1N1)pdm09 strains as a monovalent formulation in 2009-2010 or as a trivalent formulation from 2010-2011 to 2013-2014, but consolidated sample sizes were small. It was effective as a quadrivalent formulation but less effective than inactivated influenza vaccine (IIV). Live-attenuated influenza vaccine was consistently effective against B strains and matched A(H3N2) strains but was not shown to provide significant protection against mismatched A(H3N2) strains in 2014-2015.

CONCLUSIONS

These findings confirm that effectiveness of LAIV against A(H1N1)pdm09 strains has been lower than IIV. A systematic investigation has been initiated to determine the root cause of the difference in effectiveness between pre- and postpandemic A(H1N1) vaccine strains and to identify a more consistently effective A(H1N1)pdm09 vaccine strain.

Association of vaccine handling conditions with effectiveness of live attenuated influenza vaccine against H1N1pdm09 viruses in the United States

PURPOSE

This analysis examined potential causes of the lack of vaccine effectiveness (VE) of live attenuated influenza vaccine (LAIV) against A/H1N1pdm09 viruses in the United States (US) during the 2013-2014 season. Laboratory studies have demonstrated reduced thermal stability of A/California/07/2009, the A/H1N1pdm09 strain utilized in LAIV from 2009 through 2013-2014.

METHODS

Post hoc analyses of a 2013-2014 test-negative case-control (TNCC) effectiveness study investigated associations between vaccine shipping conditions and LAIV lot effectiveness. Investigational sites provided the LAIV lot numbers administered to each LAIV recipient enrolled in the study, and the vaccine distributor used by the site for commercially purchased vaccine. Additionally, a review was conducted of 2009-2014 pediatric observational TNCC effectiveness studies of LAIV, summarizing effectiveness by type/subtype, season, and geographic location.

RESULTS

From the 2013 to 2014 TNCC study, the proportion of LAIV recipients who tested positive for H1N1pdm09 was significantly higher among children who received a lot released between August 1 and September 15, 2013, compared with a lot shipped either earlier or later (21% versus 4%; P<0.01). A linear relationship was observed between the proportion of subjects testing positive for H1N1pdm09 and outdoor temperatures during truck unloading at distributors' central locations. The review of LAIV VE studies showed that in the 2010-2011 and 2013-2014 influenza seasons, no significant effectiveness of LAIV against H1N1pdm09 was demonstrated for the trivalent or quadrivalent formulations of LAIV in the US, respectively, in contrast to significant effectiveness against A/H3N2 and B strains during 2010-2014.

CONCLUSIONS

This study showed that the lack of VE observed with LAIV in the US against H1N1pdm09 viruses was associated with exposure of some LAIV lots to temperatures above recommended storage conditions during US distribution, and is likely explained by the increased susceptibility of the A/California/7/2009 (H1N1pdm09) LAIV strain to thermal degradation.

CLINICAL TRIAL REGISTRY

NCT01997450.

Seasonal Effectiveness of Live Attenuated and Inactivated Influenza Vaccine

BACKGROUND

Few observational studies have evaluated the relative effectiveness of live attenuated (LAIV) and inactivated (IIV) influenza vaccines against medically attended laboratory-confirmed influenza.

METHODS

We analyzed US Influenza Vaccine Effectiveness Network data from participants aged 2 to 17 years during 4 seasons (2010-2011 through 2013-2014) to compare relative effectiveness of LAIV and IIV against influenza-associated illness. Vaccine receipt was confirmed via provider/electronic medical records or immunization registry. We calculated the ratio (odds) of influenza-positive to influenza-negative participants among those age-appropriately vaccinated with either LAIV or IIV for the corresponding season. We examined relative effectiveness of LAIV and IIV by using adjusted odds ratios (ORs) and 95% confidence intervals (CIs) from logistic regression.

RESULTS

Of 6819 participants aged 2 to 17 years, 2703 were age-appropriately vaccinated with LAIV (n = 637) or IIV (n = 2066). Odds of influenza were similar for LAIV and IIV recipients during 3 seasons (2010-2011 through 2012-2013). In 2013-2014, odds of influenza were significantly higher among LAIV recipients compared with IIV recipients 2 to 8 years old (OR 5.36; 95% CI, 2.37 to 12.13). Participants vaccinated with LAIV or IIV had similar odds of illness associated with influenza A/H3N2 or B. LAIV recipients had greater odds of illness due to influenza A/H1N1pdm09 in 2010-2011 and 2013-2014.

CONCLUSIONS

We observed lower effectiveness of LAIV compared with IIV against influenza A/H1N1pdm09 but not A(H3N2) or B among children and adolescents, suggesting poor performance related to the LAIV A/H1N1pdm09 viral construct.

Influenza Vaccine Effectiveness Against 2009 Pandemic Influenza A(H1N1) Virus Differed by Vaccine Type During 2013-2014 in the United States

BACKGROUND

The predominant strain during the 2013-2014 influenza season was 2009 pandemic influenza A(H1N1) virus (A[H1N1]pdm09). This vaccine-component has remained unchanged from 2009.

METHODS

The US Flu Vaccine Effectiveness Network enrolled subjects aged ≥6 months with medically attended acute respiratory illness (MAARI), including cough, with illness onset ≤7 days before enrollment. Influenza was confirmed by reverse-transcription polymerase chain reaction (RT-PCR). We determined the effectiveness of trivalent or quadrivalent inactivated influenza vaccine (IIV) among subjects ages ≥6 months and the effectiveness of quadrivalent live attenuated influenza vaccine (LAIV4) among children aged 2-17 years, using a test-negative design. The effect of prior receipt of any A(H1N1)pdm09-containing vaccine since 2009 on the effectiveness of current-season vaccine was assessed.

RESULTS

We enrolled 5999 subjects; 5637 (94%) were analyzed; 18% had RT-PCR-confirmed A(H1N1)pdm09-related MAARI. Overall, the effectiveness of vaccine against A(H1N1)pdm09-related MAARI was 54% (95% confidence interval [CI], 46%-61%). Among fully vaccinated children aged 2-17 years, the effectiveness of LAIV4 was 17% (95% CI, -39% to 51%) and the effectiveness of IIV was 60% (95% CI, 36%-74%). Subjects aged ≥9 years showed significant residual protection of any prior A(H1N1)pdm09-containing vaccine dose(s) received since 2009, as did children <9 years old considered fully vaccinated by prior season.

CONCLUSIONS

During 2013-2014, IIV was significantly effective against A(H1N1)pdm09. Lack of LAIV4 effectiveness in children highlights the importance of continued annual monitoring of effectiveness of influenza vaccines in the United States.

Efficacy and effectiveness of live attenuated influenza vaccine in school-age children

Evidence of high efficacy of live attenuated influenza vaccine (LAIV) from randomized controlled trials is strong for children 2-6 years of age, but fewer data exist for older school-age children. We reviewed the published data on efficacy and effectiveness of LAIV in children ≥5 years. QUOSA (Elsevier database) was searched for articles published from January 1990 to June 2014 that included 'FluMist', 'LAIV', 'CAIV', 'cold adapted influenza vaccine', 'live attenuated influenza vaccine', 'live attenuated cold adapted' or 'flu mist'. Studies evaluated included randomized controlled trials, effectiveness and indirect protection studies. This review demonstrates that LAIV has considerable efficacy and effectiveness in school-age children.

Relative timing of influenza disease by age group

A detailed understanding of influenza movement in communities during yearly epidemics is needed to inform improved influenza control programs. We sought to determine the relative timing of influenza presentation and symptom onset by age group and influenza strain. Prospective, laboratory-confirmed surveillance was performed over three moderate influenza seasons in emergency departments and inpatient settings of both medical centers in Winston-Salem, NC. Influenza disease presented first in school age children through community epidemics of influenza A(H1N1)pdm09 and influenza B, and first in persons 5-49 years old for influenza A(H3N2). This finding indicates that influenza prevention in persons 5-49 years of age may be particularly important in influenza epidemic control.

Clinical and immune responses to inactivated influenza A(H1N1)pdm09 vaccine in children

BACKGROUND

As the influenza A H1N1 pandemic emerged in 2009, children were found to experience high morbidity and mortality and were prioritized for vaccination. This multicenter, randomized, double-blind, age-stratified trial assessed the safety and immunogenicity of inactivated influenza A(H1N1)pdm09 vaccine in healthy children aged 6 months to 17 years.

METHODS

Children received 2 doses of approximately 15 or 30 µg hemagglutin antigen 21 days apart. Reactogenicity was assessed for 8 days after each dose, adverse events through day 42, and serious adverse events or new-onset chronic illnesses through day 201. Serum hemagglutination inhibition titers were measured on days 0 (prevaccination), 8, 21, 29 and 42.

RESULTS

A total of 583 children received the first dose and 571 received the second dose of vaccine. Vaccinations were generally well-tolerated and no related serious adverse events were observed. The 15 µg dosage elicited a seroprotective hemagglutination inhibition (≥ 1:40) in 20%, 47% and 93% of children in the 6-35 month, 3-9 year and 10-17 year age strata 21 days after dose 1 and in 78%, 82% and 98% of children 21 days after dose 2, respectively. The 30 µg vaccine dosage induced similar responses.

CONCLUSIONS

The inactivated influenza A(H1N1)pdm09 vaccine exhibited a favorable safety profile at both dosage levels. While a single 15 or 30 µg dose induced seroprotective antibody responses in most children 10-17 years of age, younger children required 2 doses, even when receiving dosages 4- to 6-fold higher than recommended. Well-tolerated vaccines are needed that induce immunity after a single dose for use in young children during influenza pandemics.

Development of live attenuated influenza vaccines against pandemic influenza strains

Avian and animal influenza viruses can sporadically transmit to humans, causing outbreaks of varying severity. In some cases, further human-to-human virus transmission does not occur, and the outbreak in humans is limited. In other cases, sustained human-to-human transmission occurs, resulting in worldwide influenza pandemics. Preparation for future pandemics is an important global public health goal. A key objective of preparedness is to gain an understanding of how to design, test, and manufacture effective vaccines that could be stockpiled for use in a pandemic. This review summarizes results of an ongoing collaboration to produce, characterize, and clinically test a library of live attenuated influenza vaccine strains (based on Ann Arbor attenuated Type A strain) containing protective antigens from influenza viruses considered to be of high pandemic potential.

Protective effects of influenza A (H1N1) pandemic 2009 vaccination against the onset of influenza-like illness and asthma exacerbation in Japanese children

BACKGROUND

Vaccination against influenza A(H1N1)pdm09 in Japan started in October 2009. Children with asthma are considered as a high-risk group and are recommended to preferentially receive the vaccine.

OBJECTIVE

To identify the clinical effects of vaccination in Japanese children with and without asthma.

METHODS

We conducted a cross-sectional, questionnaire-based survey to compare vaccination rates, vaccine effectiveness against physician-diagnosed influenza A infection (PDIA), and consecutive asthma exacerbations between children with and without asthma.

RESULTS

Of the 460 children included in this study, those with asthma had higher vaccination rates (46.5%, 67/144) than those without asthma (30.4%, 96/316). Influenza A infections were diagnosed in 28 of 163 vaccinated children (17.2%) compared to 164 of 297 unvaccinated children (55.2%, p < 0.001). Comparison of positive influenza diagnosis rates between vaccinated and unvaccinated children with and without asthma showed that unvaccinated children with asthma had an elevated odds ratio (13.235; 95% confidence interval [CI], 5.564-32.134) and that treatment for asthma exacerbations was needed in a larger proportion of unvaccinated children. Vaccine effectiveness against PDIA was 87% (95% CI, 78-93%) overall, 92% (95% CI, 81-96%) in children with asthma and 81% (95% CI, 63-91%) in children without asthma, respectively.

CONCLUSIONS

The administration of an inactivated, split-virus, non-adjuvanted monovalent A(H1N1)pdm09 vaccine during the pandemic period reduced the number of physician-diagnosed influenza A infections and asthma exacerbations in children with asthma. Therefore, we strongly recommend that high-risk children with a history of asthma receive vaccines during pandemics.

Interim estimates of 2013/14 vaccine effectiveness against influenza A(H1N1)pdm09 from Canada s sentinel surveillance network, January 2014

The 2013/14 influenza season to date in Canada has been characterised by predominant (90%) A(H1N1)pdm09 activity. Vaccine effectiveness (VE) was assessed in January 2014 by Canada's sentinel surveillance network using a test-negative case-control design. Interim adjusted-VE against medically-attended laboratory-confirmed influenza A(H1N1)pdm09 infection was 74% (95% CI: 58-83). Relative to vaccine, A(H1N1)pdm09 viruses were antigenically similar and genetically well conserved, with most showing just three mutations across the 50 amino acids comprising antigenic sites of the haemagglutinin protein.

Inactivated influenza vaccines for prevention of community-acquired pneumonia: the limits of using nonspecific outcomes in vaccine effectiveness studies

BACKGROUND

One to 4 million cases of community-acquired pneumonia (CAP) occur annually in the United States, resulting in 600,000 hospitalizations and 45,000 deaths. Influenza infection facilitates secondary bacterial infections, and influenza vaccination may prevent CAP directly by preventing influenza pneumonia or indirectly by preventing secondary bacterial CAP.

METHODS

We investigated how influenza vaccination could affect incidence of CAP using deterministic probability and stochastic simulation models. The models included likely influential factors, including vaccine effectiveness (VE) against influenza, rates of influenza in the unvaccinated, vaccination coverage, and the relative risk (RR) of pneumonia, given influenza infection. To estimate effectiveness of influenza vaccine against CAP, we assumed mean VE against influenza of 55% and vaccine coverage of 38%.

RESULTS

Given our baseline parameters, influenza vaccine had a mean effectiveness against CAP of 7% (95% confidence interval = 0-25%). Effectiveness of influenza vaccine against CAP increased as its effectiveness against influenza increased, as RR of pneumonia after influenza infection increased, and as rates of influenza among unvaccinated persons increased.

CONCLUSIONS

No matter how effective vaccine may be in preventing influenza infection, it is only modestly effective at preventing CAP. Because of the large annual burden of CAP, a vaccine that is only moderately effective in preventing influenza infection has the potential to prevent a substantial number of CAP cases. This modeling approach may be useful for planning influenza vaccine-probe studies and evaluating the effectiveness of other interventions targeted against infections that manifest in nonspecific outcomes.

Elementary school-based influenza vaccination: evaluating impact on respiratory illness absenteeism and laboratory-confirmed influenza

BACKGROUND

Studies of influenza vaccine effectiveness in schools have assessed all-cause absenteeism rather than laboratory-confirmed influenza. We conducted an observational pilot study to identify absences due to respiratory illness and laboratory-confirmed influenza in schools with and without school-based vaccination.

METHODS

A local public health agency initiated school-based influenza vaccination in two Wisconsin elementary schools during October 2010 (exposed schools); two nearby schools served as a comparison group (non-exposed schools). Absences due to fever or cough illness were monitored for 12 weeks. During the 4 weeks of peak influenza activity, parents of absent children with fever/cough illness were contacted and offered influenza testing.

RESULTS

Parental consent for sharing absenteeism data was obtained for 937 (57%) of 1,640 students. Fifty-two percent and 28%, respectively, of all students in exposed and non-exposed schools were vaccinated. Absences due to fever or cough illness were significantly lower in the exposed schools during seven of 12 surveillance weeks. Twenty-seven percent of students at exposed schools and 39% at unexposed schools had one or more days of absence due to fever/cough illness (p<0.0001). There was no significant difference in the proportion of students absent for other reasons (p = 0.23). During the 4 week period of influenza testing, respiratory samples were obtained for 68 (42%) of 163 episodes of absence due to fever or cough illness. Influenza was detected in 6 students; 3 attended exposed schools.

CONCLUSIONS

Detection of laboratory-confirmed influenza in schools was challenging due to multiple consent requirements, difficulty obtaining samples from absent children, and a mild influenza season. School-based influenza vaccination was associated with reduced absenteeism due to fever or cough illness, but not absenteeism for other reasons. Although nonspecific, absence due to fever or cough illness may be a useful surrogate endpoint in school-based studies if identification of laboratory confirmed influenza is not feasible.

Effectiveness of nonadjuvanted monovalent influenza A(H1N1)pdm09 vaccines for preventing reverse transcription polymerase chain reaction-confirmed pandemic influenza hospitalizations: case-control study of children and adults at 10 US influenza surveillance network sites

During 2009–2010, we examined 217 patients hospitalized with laboratory-confirmed pandemic influenza in 9 Influenza Hospitalization Surveillance Network sites and 413 age- and community-matched controls and found that a single dose of monovalent nonadjuvanted influenza A(H1N1)pdm09 vaccine was 50% (95% confidence interval, 13%–71%) effective in preventing hospitalization associated with A(H1N1)pdm09 virus infection.