실험결과 및 토론 간략 정리

240명의 건강한 자원자를 대상으로 신종플루 백신 접종 21일 후 항체가 검사를 해보았더니 15-µg 용량을 주사한 120명 중에서 116명에게서 항체형성 확인(96.7%)되었으며, 30-µg을 주사한 120명 중에서 112명에게서 항체 형성 확인(93.3%) 되었다.

사망자나 심각한 부작용 또는 특별한 부작용은 보고되지 않았다. 실험적으로 예방주사를 맞은 46.3%는 국소적인 열감이나 통증을 호소했으며, 45.0%는 두통 등 전신 증상을 호소했지만 이것은 모두 가볍거나 보통의 강도로 나타난 일반적인 백신 부작용이었다.

그러므로 성인의 경우 2009년 대유행 인플루엔자 백신은 15-µg을 1회 접종만 해도 21일 후 항체가 형성될 것으로 판단된다.

Published at www.nejm.org September 10, 2009 (10.1056/NEJMoa0907413)

Michael E. Greenberg, M.D., M.P.H., Michael H. Lai, B.Med.Sc., M.B., B.S., M.Med.Sc., Gunter F. Hartel, M.S., Ph.D., Christine H. Wichems, Ph.D., Charmaine Gittleson, B.Sc., M.B., B.Ch., Jillian Bennet, M.Sc., M.P.H., Gail Dawson, B.Pharm., Wilson Hu, M.D., M.B.A., Connie Leggio, B.Sc., Diane Washington, M.D., and Russell L. Basser, M.B., B.S., M.D., F.R.A.C.P.

ABSTRACT

Background A novel influenza A (H1N1) 2009 virus is responsible for the first influenza pandemic in 41 years. A safe and effective vaccine is urgently needed. A randomized, observer-blind, parallel-group trial evaluating two doses of an inactivated, split-virus 2009 H1N1 vaccine in healthy adults between the ages of 18 and 64 years is ongoing at a single site in Australia.

Methods This preliminary report evaluates the immunogenicity and safety of the vaccine 21 days after the first of two scheduled doses. A total of 240 subjects, equally divided into two age groups (<50 years and 50 years), were enrolled and underwent randomization to receive either 15 µg or 30 µg of hemagglutinin antigen by intramuscular injection. We measured antibody titers using hemagglutination-inhibition and microneutralization assays at baseline and 21 days after vaccination. The coprimary immunogenicity end points were the proportion of subjects with antibody titers of 1:40 or more on hemagglutination-inhibition assay, the proportion of subjects with either seroconversion or a significant increase in antibody titer, and the factor increase in the geometric mean titer.

Results By day 21 after vaccination, antibody titers of 1:40 or more were observed in 116 of 120 subjects (96.7%) who received the 15-µg dose and in 112 of 120 subjects (93.3%) who received the 30-µg dose. No deaths, serious adverse events, or adverse events of special interest were reported. Local discomfort (e.g., injection-site tenderness or pain) was reported by 46.3% of subjects, and systemic symptoms (e.g., headache) by 45.0% of subjects. Nearly all events were mild to moderate in intensity.

Conclusions A single 15-µg dose of 2009 H1N1 vaccine was immunogenic in adults, with mild-to-moderate vaccine-associated reactions. (ClinicalTrials.gov number, NCT00938639 [ClinicalTrials.gov] .)

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The rapid global spread of a novel influenza A (H1N1) 2009 virus (2009 H1N1) prompted the World Health Organization (WHO), on June 11, 2009, to declare the first influenza pandemic in 41 years.¹ In the Southern Hemisphere, 2009 H1N1 infection has been dominant during the current influenza season.² In the Northern Hemisphere, the incidence of 2009 H1N1 infection is likely to increase substantially during the approaching influenza season, with major public health ramifications. Early availability of safe and effective vaccines is a critical component of efforts to prevent 2009 H1N1 infection and mitigate the overall effect of the pandemic.^3,4

Shortly after the identification of 2009 H1N1, influenza vaccine manufacturers, in conjunction with public health and regulatory agencies, started developing a 2009 H1N1 vaccine.⁵ The sense of urgency was particularly notable in the Southern Hemisphere, where the timing of the pandemic coincided with the onset of winter. Ideally, clinical trials are needed to establish the safety and adverse-effect profiles of the new vaccines and to confirm the optimal dose and regimen.⁶

We undertook a clinical trial in healthy adults to examine the immunogenicity, safety, and tolerability of two different doses of a monovalent, split-virus 2009 H1N1 influenza vaccine (H1N1 vaccine). The vaccine was manufactured with the same procedures that have been used for the production of the company’s seasonal trivalent inactivated vaccine. We examined a two-dose regimen of either 15 µg or 30 µg of hemagglutinin antigen, because there was uncertainty as to whether a higher antigen content or a two-dose series might be required to produce a satisfactory immune response. We enrolled equal numbers of subjects 50 years of age or older and below the age of 50 years to explore potential age-related differences in immune response that might result from previous exposure to H1N1 viruses that were displaced from circulation by the H2N2 subtype in the 1957–1958 influenza pandemic.⁷

In the current pandemic, rapid sharing of clinical-trial findings is critical, since such data may assist in the planning of national vaccination programs. This preliminary report includes results that are available to date from our ongoing Australian study in healthy adults after the first of two scheduled vaccinations.

Methods

Study Design

This phase 2, prospective, randomized, observer-blind, parallel-group clinical trial is ongoing at a single site in Adelaide, Australia (CMAX, a division of the Institute of Drug Technology). The purpose of this study is to evaluate the immunogenicity and safety of two different doses of the H1N1 vaccine in healthy adults between the ages of 18 and 64 years in a two-dose regimen. All subjects provided written informed consent.

The randomization code was prepared by a statistician, employed by CSL Limited, with the use of SAS software (version 9.1.3) and JMP (version 8.0.1) (SAS Institute); permuted-block randomization was used. The randomization code was provided to the vaccine administrator, who was aware of study-group assignments, as a list in a sealed envelope, although all subjects and investigators were unaware of such assignments.

The study was approved by the Bellberry Human Research Ethics Committee (Adelaide, Australia) and was conducted in accordance with the principles of the Declaration of Helsinki, the standards of Good Clinical Practice (as defined by the International Conference on Harmonization), and Australian regulatory requirements. All authors contributed to the content of the manuscript, had full access to all study data, and vouch for the completeness and accuracy of the data.

Vaccine

The H1N1 vaccine, a monovalent, unadjuvanted, inactivated, split-virus vaccine, was produced by CSL Biotherapies (Parkville, Australia). The seed virus was prepared from the reassortant vaccine virus NYMC X-179A (New York Medical College, New York), derived from the A/California/7/2009 (H1N1) virus, one of the candidate reassortant vaccine viruses recommended by the WHO.^8,9 The vaccine was prepared in embryonated chicken eggs with the same standard techniques that are used for the production of seasonal trivalent inactivated vaccine¹⁰ and was presented in 10-ml multidose vials with thimerosal added as a preservative (final concentration, 0.01% weight per volume). The two doses were 15 µg of hemagglutinin antigen per 0.25-ml dose and 30 µg of hemagglutinin antigen per 0.5-ml dose.

Subjects and Study Procedures

Healthy, nonpregnant adults between the ages of 18 and 64 years were eligible for enrollment. We excluded subjects with confirmed or suspected 2009 H1N1 infection and those who had received an experimental influenza vaccine during the preceding 6 months.

A total of 240 eligible subjects underwent randomization to receive either 15 µg or 30 µg of hemagglutinin antigen in a 1:1 ratio. An equal number of subjects from 18 to 49 years of age and from 50 to 64 years were included. Each dose was administered intramuscularly into the deltoid muscle. Since the injection volume differed between the two study doses, personnel who prepared and administered the study vaccine had no further involvement in the study.

Safety Assessments

We collected solicited reports of local and systemic adverse events, using a 7-day diary card. All solicited local adverse events were considered to be related to the H1N1 vaccine, whereas the investigator assessed the causality of solicited systemic adverse events. Subjects used a standard scale to grade adverse events during the 7-day period.

Because of the novelty of the pandemic H1N1 strain, we prospectively collected data relating to the occurrence of select adverse events of special interest. These events included several neurologic (e.g., Guillain–Barré syndrome), immune-system, and other disorders. Any adverse events of special interest or serious adverse event was to be reported within 24 hours.

A safety-review committee monitored the safety of the study. Stopping rules were in place during the 7 days after vaccination but were not met, and all doses were given.

Assessment of Influenza-Like Illness

Subjects who reported having an influenza-like illness were asked to provide specimens of nasal and throat swabs for virologic testing. An influenza-like illness was defined as an oral temperature of more than 38°C (100.4°F) or a history of fever or chills and at least one influenza-like symptom.

Laboratory Assays

Anti-influenza antibody titers were measured at enrollment and 21 days after each vaccination. The immunogenicity of the H1N1 vaccine was evaluated with the use of hemagglutination-inhibition and microneutralization assays with methods that have been described previously^11,12 (for details, see the Supplementary Appendix, available with the full text of this article at NEJM.org). Virologic testing of nasal- and throat-swab specimens was performed with the use of the protocol of the Centers for Disease Control and Prevention for real-time reverse-transcriptase–polymerase-chain-reaction assay for 2009 H1N1 virus.¹³ All laboratory assays were performed by Focus Diagnostics.

Primary and Secondary End Points

The three coprimary immunogenicity end points after vaccination were chosen according to international guidelines used to evaluate influenza vaccines.^14,15 The coprimary immunogenicity end points were the proportion of subjects with antibody titers of 1:40 or more on hemagglutination-inhibition assay, the proportion of subjects with either seroconversion or a significant increase in antibody titer, and the factor increase in the geometric mean titer.

The secondary safety end points were the frequency, duration, and intensity of solicited adverse events during the 7 days after vaccination and the incidence of serious adverse events and adverse events of special interest during the study period.

Statistical Analysis

A sample size of 120 subjects per study group was chosen because it provided sufficient power to assess the primary immunogenicity end points. The primary and secondary end-point analyses were descriptive and consisted of an assessment of the lower confidence bounds of each end point for each study group. On the assumption of a population seroconversion rate of 53%, the study had a power of at least 80% with 120 subjects per group to show the seroconversion rate to be significantly more than 40%. For categorical variables, statistical summaries included counts and percentages relative to the appropriate population. The safety population included all subjects who received a dose of H1N1 vaccine. The population that could be evaluated included all subjects in the safety population who provided serum samples at baseline and after vaccination. The 95% confidence intervals, which were calculated on the basis of the binomial distribution, are provided for descriptive statistics.

Results

Study Subjects

From July 22 to July 26, 2009, we enrolled 240 subjects, who underwent randomization (Table 1 and Figure 1). All subjects received a dose of H1N1 vaccine and were included in the safety population. All subjects provided a blood sample before and after vaccination and were included in the population that could be evaluated. All subjects returned the 7-day diary; there were no withdrawals from the study. Of the 240 subjects, 45.0% reported having received a 2009 Southern Hemisphere seasonal trivalent inactivated vaccine. The proportion of subjects who received the 2009 seasonal vaccine did not differ between the age groups (P=0.24 by Fisher’s exact test).

View this table: [in this window] [in a new window]   Table 1. Demographic Characteristics of the Subjects.

 

View larger version (28K): [in this window] [in a new window]   Figure 1. Enrollment and Outcomes.

 
Immunogenicity

At baseline, 76 of 240 subjects (31.7%) had antibody titers of 1:40 or more on hemagglutination-inhibition assay (Table 2 and Figure 2A and 2C), with no significant differences between either age groups (P=0.21) or dose groups (P=0.68). Similarly, there were no significant differences in baseline geometric mean titers (GMTs) between age groups or dose groups (Table 3). Of note, baseline titers of 1:40 or more on hemagglutination-inhibition assay were observed in 48 of 108 subjects who had received the 2009 seasonal vaccine (44.4%; 95% confidence interval [CI], 35.4 to 53.8), as compared with 28 of 132 subjects who had not received the seasonal vaccine (21.2%; 95% CI, 15.1 to 28.9; P<0.001 by Fisher's exact test).

View this table: [in this window] [in a new window]   Table 2. Immune Response after One Dose of the H1N1 Vaccine, as Measured on Hemagglutination-Inhibition (HI) Assay.

 

View larger version (40K): [in this window] [in a new window]   Figure 2. Reverse Cumulative Distribution Curves of Antibody Titers in Serum before and 21 Days after a Single Dose of H1N1 Vaccine, According to the Type of Assay. Shown are levels of antibody titer against the 2009 H1N1 virus on hemagglutination-inhibition (HI) assay before vaccination (Panel A) and after vaccination (Panel B) and in the two age groups in the study (18 to 49 years and 50 to 64 years) (Panels C and D). Also shown are levels of antibody titer against the 2009 H1N1 virus on microneutralization (MN) assay before vaccination (Panel E) and after vaccination (Panel F) and in the two age groups (Panels G and H).

 

View this table: [in this window] [in a new window]   Table 3. Geometric Mean Titers and Factor Increases in the Geometric Mean Titer after One Dose of the H1N1 Vaccine, as Measured on Hemagglutination-Inhibition Assay and Microneutralization Assay.

 
A single 15-µg or 30-µg dose of the H1N1 vaccine produced a robust immune response in a majority of subjects (Table 2 and Figure 2). Post-vaccination titers of 1:40 or more on hemagglutination-inhibition assay were observed in 96.7% (95% CI, 91.7 to 98.7) of recipients of the 15-µg dose and in 93.3% (95% CI, 87.4 to 96.6) of the recipients of the 30-µg dose (Table 2 and Figure 2). Seroconversion or a significant increase in titer on hemagglutination-inhibition assay occurred in 74.2% of subjects, and the effect was similar between the two study groups (Table 2).

After vaccination, there was a substantial rise in GMTs, with no significant differences in factor increases between the two groups (Table 3). However, we observed age-related differences. Subjects who were 50 years of age or older had a numerically lower factor increase in the GMT than those under the age of 50 years. This age-related effect was reflected in all measures of immunogenicity.

In general, the pattern of antibody responses, as measured by the microneutralization assay, was similar to those observed with the hemagglutination-inhibition assay (Table 3 and Figure 2E through 2H). Baseline microneutralization GMTs in the younger age group were significantly higher than those in the older age group (P<0.001). Postvaccination microneutralization GMTs were also significantly higher in the younger age group than in the older age group, regardless of dose (P<0.001).

We performed an additional analysis examining the effect of baseline serostatus on the immune response to H1N1 vaccination. Subjects who were seronegative at baseline (with a hemagglutination-inhibition or microneutralization titer of <1:10) had lower postvaccination GMTs than those with baseline titers of 1:10 or more. However, subjects who were seronegative at baseline had significantly higher factor increases in the GMT (P<0.001 for both hemagglutination-inhibition and microneutralization assays) (Table 3 in the Supplementary Appendix). The proportion of subjects who were seronegative at baseline and who achieved seroconversion exceeded 86% on the hemagglutination-inhibition assay and 70% on the microneutralization assay. Among subjects with a baseline titer of 1:10 or more, the proportion of those achieving seroconversion exceeded 60% on the hemagglutination-inhibition assay and 70% on the microneutralization assay.

Adverse Events

No deaths, serious adverse events, or adverse events of special interest were reported. Stopping rules were not triggered, and no subjects withdrew from the study. Since the study is ongoing and individual study-group assignments remain blinded, data regarding solicited adverse events are presented as aggregate totals of both study-dose groups. Data regarding unsolicited adverse events are being collected but are unavailable for this preliminary report.

Solicited local adverse events were reported by 46.3% (95% CI, 40.1 to 52.6) of subjects (Table 4). The most commonly reported events were injection-site tenderness (36.7% of subjects) and pain (21.7% of subjects). Solicited local adverse events were graded as mild by 105 of 111 subjects who reported having such an event (94.6%), with no severe local adverse events reported.

View this table: [in this window] [in a new window]   Table 4. Proportion of 240 Subjects Who Reported Having a Solicited Local or Systemic Adverse Event within 7 Days after Receiving One Dose of the H1N1 Vaccine.

 
Solicited systemic adverse events were reported by 45.0% (95% CI, 38.8 to 51.3) of subjects (Table 4). The most commonly reported events were headache, malaise, and myalgia. Solicited systemic adverse events that were considered to be related to the H1N1 vaccine were reported by 30.4% of subjects. Of the subjects who had a solicited systemic adverse event, the majority reported events that were mild to moderate in intensity. Two subjects reported adverse events that were graded as severe: vaccine-related myalgia, malaise, and nausea that resolved within 5 days with standard treatment in one subject and non-vaccine-related nausea from day 6 through day 10 after vaccination in the other.

Three subjects had an influenza-like illness, one of whom tested positive for 2009 H1N1 on day 8 after vaccination. The remaining two subjects tested negative for 2009 H1N1.

Discussion

A single 15-µg dose of unadjuvanted 2009 H1N1 vaccine resulted in titers of 1:40 or more on hemagglutination-inhibition assay in 96.7% of adult subjects, despite the prevailing assumption that two doses of vaccine would be required. These results will help to inform pandemic planning, especially in light of widespread concern about vaccine availability because of low manufacturing yields.¹⁶ The high level of immune protection afforded by a single 15-µg dose should improve the coverage and logistics of mass H1N1 vaccination programs.

The robust immune response to the H1N1 vaccine after a single dose was unanticipated. Much of the current global pandemic planning is predicated on previous experience that two doses of vaccine are required to elicit a protective immune response in populations that are immunologically naive to a new influenza strain.^{17,18,19,20,21}

The initiation of the study coincided with the peak of the first pandemic wave in Australia. The weekly age-standardized H1N1 notification rate in South Australia, the state in which the study site is located, was higher than the national average at that time (113.6 per 100,000 population in South Australia, and 81.8 per 100,000 population in Australia).²² However, we do not believe that intercurrent infection significantly contributed to the postvaccination response, since we monitored all subjects for influenza-like illness, and only one subject tested positive for 2009 H1N1 during the 21 days after vaccination.

The proportion of subjects with titers of 1:40 or more on hemagglutination-inhibition assay at baseline was higher than expected. Among subjects who were 50 years of age or older, this finding could be attributed to the presence of preexisting antibodies from exposure to H1N1 viruses circulating before 1957.²³ It was surprising, however, to see similar baseline antibody titers in the younger age group. A number of factors could have contributed to the observed titers in both age groups at baseline. It is probable that there was some degree of previous 2009 H1N1 infection in the study population, despite stringent exclusion criteria. Cross-reactive antibodies to 2009 H1N1 may also have played a role. In this issue of the Journal, a study by Hancock et al. that analyzed stored-serum samples from trials of seasonal trivalent inactivated vaccine predating the current pandemic showed the presence of cross-reactive antibodies to 2009 H1N1 in adults.²⁴ The same study showed that vaccination with the seasonal vaccine resulted in a doubling in titers of these cross-reactive antibodies. The latter finding is particularly relevant, given that 45% of the subjects in our study had received the 2009 seasonal vaccine.

Even in subjects with no measurable antibodies at baseline, a single dose of vaccine elicited a robust immune response. The question remains: Why did these subjects have such a brisk response? The 2009 H1N1 pandemic differs from previous pandemics in that although the virus is antigenically very distant from recently circulating H1N1 viruses, it is still of the same H1N1 subtype.²⁵ Cross-protection that was afforded by exposure to antigenically drifted strains of the same influenza subtype has been described.¹⁹ In addition, the 2009 H1N1 virus shares three gene sequences with the recently circulating seasonal H1N1 virus and three sequences with the current seasonal H3N2 virus.²³ Perhaps there is more immunotypic similarity between the 2009 H1N1 virus and recent seasonal strains than has been recognized previously.

The side-effect profile of the H1N1 vaccine, particularly the frequency and severity of solicited adverse events, is consistent with our previous experience with seasonal influenza vaccines in adults.¹⁰ The full safety profile of H1N1 vaccine has not yet been elucidated. Population-based postlicensure surveillance will be required for all H1N1 vaccines, especially to assess rare outcomes, such as the Guillain–Barré syndrome.

Several important questions remain unanswered in this trial. First, since we studied healthy adults, trials need to be conducted in other populations that may have different responses to the vaccine, such as the elderly, children, and those with impaired immunity. Second, given the robust immune response to a 15-µg dose, lower antigen doses should be explored. Third, although our study is being carried out in one locality in Australia during winter in the Southern Hemisphere, our findings need to be borne out by studies in locations where the epidemiology of the pandemic may be different. Finally, estimates of the true effect of the vaccine when used in mass immunization programs will come from vaccine-effectiveness studies.

Supported by CSL with funding from the Department of Health and Aging of the Australian government.

All authors report being employees of CSL, and Dr. Greenberg, Dr. Lai, Dr. Hartel, Dr. Gittleson, Ms. Bennet, Ms. Dawson, Dr. Washington, and Dr. Basser report having an equity interest in the company. No other potential conflict of interest relevant to this article was reported.

We thank the subjects for their critical role in this study, the staff of CSL, and other staff participants, including the following: Dr. Sepehr Shakib and the staff at CMAX, a division of IDT Australia; the Clinical Trials Department at Focus Diagnostics in Cypress, CA; Quintiles of Australia; and Medidata of New York.

Source Information

From Clinical Research and Development, CSL, Parkville, VIC, Australia.

This article (10.1056/NEJMoa0907413) was published on September 10, 2009, at NEJM.org.

Address reprint requests to Dr. Greenberg at Vaccines Clinical Research and Development, CSL, 45 Poplar Rd., Parkville, VIC 3052, Australia, or at michael.greenberg@csl.com.au

.

References

1. DG statement following the meeting of the Emergency Committee. Geneva: World Health Organization, 2009. (Accessed September 9, 2009, at http://www.who.int/csr/disease/swineflu/4th_meeting_ihr/en/index.html.) 
   
2. Pandemic (H1N1) 2009 — update 63. Geneva: World Health Organization, 2009. (Accessed September 9, 2009, at http://www.who.int/csr/don/2009_08_28/en/index.html.)
   
3. Fiore AE, Shay DK, Broder K, et al. Prevention and control of seasonal influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009. MMWR Recomm Rep 2009;58:1-52. [Erratum, MMWR Recomm Rep 2009;58:896-7.] [Medline]
   
4. President’s Council of Advisors on Science and Technology. Report to the president on U.S. preparations for 2009-H1N1 influenza. Washington, DC: White House, August 7, 2009. (Accessed September 8, 2009, at http://www.whitehouse.gov/assets/documents/PCAST_H1N1_Report.pdf.)
   
5. Kuehn BM. H1N1 vaccine. JAMA 2009;302:375-375. [Free Full Text]
   
6. Mathematical modelling of the pandemic H1N1 2009. Wkly Epidemiol Rec 2009;84:341-348. [Medline]
   
7. Henderson DA, Courtney B, Inglesby TV, Toner E, Neuzzo JB. Public health and medical responses to the 1957-58 influenza pandemic. Biosecur Bioterror 2009 August 5 (Epub ahead of print).
   
8. World Health Organization. Availability of a candidate reassortant vaccine virus for the novel influenza A (H1N1) vaccine development. June 2009. (Accessed September 9, 2009 at http://www.who.int/csr/resources/publications/swineflu/ivr153_20090608_en.pdf.)
   
9. Idem. Summary of available candidate vaccine viruses for development of pandemic (H1N1) 2009 virus vaccines. July 2009. (Accessed September 9, 2009 at http://www.who.int/csr/resources/publications/swineflu/summary_candidate_vaccine.pdf.)
   
10. Talbot HK, Keitel W, Cate TR, et al. Immunogenicity, safety and consistency of new trivalent inactivated influenza vaccine. Vaccine 2008;26:4057-4061. [CrossRef][Web of Science][Medline]
    
11. Kendal AP, Pereira MS, Skehel JJ, eds. Concepts and procedures for laboratory-based influenza surveillance. Atlanta: Centers for Disease Control, 1982.
    
12. Rowe T, Abernathy RA, Hu-Primmer J, et al. Detection of antibody to avian influenza A (H5N1) virus in human serum by using a combination of serologic assays. J Clin Microbiol 1999;37:937-943. [Free Full Text]
    
13. CDC protocol of realtime RTPCR for influenza A (H1N1). (CDC reference no. I-007-05.) Geneva: World Health Organization, 2009. (Accessed September 9, 2009, at http://www.who.int/csr/resources/publications/swineflu/CDCRealtimeRTPCR_SwineH1Assay-2009_20090430.pdf.)
    
14. Committee for Proprietary Medicinal Products. Note for Guidance on Harmonisation of Requirements for Influenza Vaccines. London: European Medicines Agency, 1996. (Publication no. CPMP/BWP/214/96.) (Accessed September 9, 2009, at http://www.emea.europa.eu/pdfs/human/bwp/021496en.pdf.)
    
15. Department of Health and Human Services, Food and Drug Administration, Center for Biologics Evaluation and Research. Guidance for industry: clinical data needed to support the licensure of pandemic influenza vaccines. May 2007. (Accessed September 9, 2009, at http://www.fda.gov/downloads/BiologicsBloodVaccines/GuidanceComplianceRegulatoryInformation/Guidances/Vaccines/ucm091985.pdf.)
    
16. Neergaard L. Factory logjam could delay some swine flu shots. New York: Associated Press, August 18, 2009. (Accessed September 9, 2009, at http://news.aol.com/article/factory-logjam-could-delay-some-swine/448143.)
    
17. Englund JA, Walter EB, Gbadebo A, Monto AS, Zhu Y, Neuzil KM. Immunization with trivalent inactivated influenza vaccine in partially immunized toddlers. Pediatrics 2006;118:e579-e585. [Free Full Text]
    
18. Neuzil KM, Jackson LA, Nelson J, et al. Immunogenicity and reactogenicity of 1 versus 2 doses of trivalent inactivated influenza vaccine in vaccine-naive 5-8-year-old children. J Infect Dis 2006;194:1032-1039. [CrossRef][Web of Science][Medline]
    
19. Parkman PD, Hopps HE, Rastogi SC, Meyer HM Jr. Summary of clinical trials of influenza virus vaccines in adults. J Infect Dis 1977;136:Suppl:S722-S730. [Web of Science][Medline]
    
20. Sencer DJ, Millar JD. Reflections on the 1976 swine flu vaccination program. Emerg Infect Dis 2006;12:29-33. [Web of Science][Medline]
    
21. Treanor JJ, Campbell JD, Zangwill KM, Rowe T, Wolff M. Safety and immunogenicity of an inactivated subvirion influenza A (H5N1) vaccine. N Engl J Med 2006;354:1343-1351. [Free Full Text]
    
22. Australian Department of Health and Ageing. Australian influenza report 2009 — current report — 8 to 14 August 2009. (Accessed September 9, 2009, at http://www.health.gov.au/internet/main/publishing.nsf/content/cda-surveil-ozflu-flucurr.htm.)
    
23. Zimmer SM, Burke DS. Historical perspective — emergence of influenza A (H1N1) viruses. N Engl J Med 2009;361:279-285. [Free Full Text]
    
24. Hancock K, Veguilla V, Lu X, et al. Cross-reactive antibody responses to the 2009 pandemic H1N1 influenza virus. N Engl J Med 2009;361. DOI: 10.1056/NEJMoa0906453.
    
25. Garten RJ, Davis CT, Russell CA, et al. Antigenic and genetic characteristics of swine-origin 2009 A(H1N1) influenza viruses circulating in humans. Science 2009;325:197-201. [Free Full Text]