Efficacy of the 2015-2016 Flu Vaccine

Akpan Anani, Mercer University College of Pharmacy

According to the Centers for Disease Control and Prevention (CDC), the influenza vaccine may reduce the incidence of flu illness by up to 60% in the overall population. [1] Data from the 2015-2016 flu season estimates that 5.1 million influenza illnesses were avoided in the United States by virtue of vaccination. [1] Each year’s vaccine profile is chosen to select for the influenza viruses most likely to abound in that given season. [1] But due to the evolution of surface antigens on the influenza virus (antigenic drift), annual epidemics can occur—necessitating constant surveillance of the strains in circulation and changes to the makeup of the vaccine as needed. [2]

 

Influenza Vaccine Effectiveness in the United States during the 2015–2016 Season [3]
Design A test-negative study; N= 6879
Objective To assess the effectiveness of the influenza vaccine for the 2015-2016 flu season
Study Groups Case patients (influenza +); n= 1309

Non-case patients (influenza -); n= 5570

Methods Eligible patients were those at least six months of age who presented with acute respiratory illness in different U.S. ambulatory care clinics.

Nasal and/or oropharyngeal swabs were obtained at enrollment and tested for influenza viruses using real-time, reverse-transcriptase polymerase chain reaction (RT-PCR).

Vaccination status was determined using electronic immunization records.

Duration November 2, 2015 to April 15, 2016
Primary Outcome Measure Vaccine effectiveness [measured as (1 – Odds ratio of testing positive) x 100]
Baseline Characteristics
Characteristic All participants Influenza virus-negative participants
Influenza virus – Neg.

n= 5570

Influenza virus – Pos.

n= 1309

Unvaccinated

n= 2668

Vaccinated

n= 2902

Number of patients  (%)
Age
 6 mo to 8 yr 1272 (23) 254 (19) 642 (24) 630 (22)
 9 to 17 yr 694 (12) 164 (13) 417 (16) 277 (10)
 18 to 49 yr 1957 (35) 499 (38) 1116 (42) 841 (29)
 50 to 64 yr 918 (16) 283 (22) 356 (13) 562 (19)
 ≥65 yr 729 (13) 109 (8) 137 (5) 592 (20)
 2 to 17 yr 1655 (30) 392 (30) 942 (35) 713 (25)
Race/ethnic group
 White 4233 (76) 919 (70) 1928 (72) 2305 (79)
 Black 402 (7) 157 (12) 246 (9) 156 (5)
 Other 456 (8) 132 (10) 224 (8) 232 (8)
 Hispanic 455 (8) 98 (7) 258 (10) 197 (7)
 Unknown 24 (<1) 3 (<1) 12 (<1) 12 (<1)
Vaccinated 2902 (52) 494 (38)
Results
No. of Case Patients/Total No. (%) Vaccine Effectiveness 95% CI p value
Overall 1309/6879 (19) 48 41 to 55 <0.001
Age
 6 mo to 8 yr 254/1526 (17) 51 33 to 64 <0.04
 9 to 17 yr 164/858 (19) 59 36 to 74 <0.04
 18 to 49 yr 499/2456 (20) 52 39 to 61 <0.04
 50 to 64 yr 283/1201 (24) 26 2 to 44 <0.04
 ≥65 yr 109/838 (13) 42 6 to 64 <0.04
Virus subtype
 A (H1N1) pdm09 768/6338 (12) 45 34 to 53
 A (H3N2) 72/4623 (2) 43 4 to 66
 B/Victoria 200/5770 (3) 49 30 to 64
 B/Yamagata 252/5823 (4) 57 42 to 67
Vaccine type
 Any IIV 1271/6718 (19) 51 44 to 57 <0.001
 IIV3 955/4473 (21) 41 25 to 53 <0.001
 IIV4 1072/5368 (20) 54 45 to 61 <0.001
 LAIV4 (2-49yr only) 674/2839 (24) 4 -47 to 37 0.86
Adverse Events Common Adverse Events: N/A
Serious Adverse Events: N/A
Percentage Discontinued due to Adverse Events: N/A
Study Author Conclusions While influenza vaccines reduced the risk of influenza illness in 2015-2016, the live attenuated vaccine was ineffective.

 

An inescapable inquiry for any healthcare practitioner is a question on the effectiveness of the influenza vaccine. Results of this study show that while the inactivated influenza vaccine was very effective for the 2015-2016 flu season, the live attenuated vaccine was not. Although the study was large, its observational nature means that unmeasured confounding (specifically, biased effect estimates) cannot be ruled out. Additionally, the test-negative design is a newer method of conducting studies and thus may be subject to biases not yet elucidated. Clinically, the obvious utility of this study is more knowledgeable recommendations to patients in regards to future flu vaccination selection and efficacy.

 

[1] Influenza (flu). Centers for Disease Control and Prevention website. https://www.cdc.gov/flu/index.htm. Accessed August 19, 2017.

[2] Zambon MC. Epidemiology and pathogenesis of influenza. J Antimicrob Chemother. 1999;44 Suppl B:3-9.

[3] Jackson ML, Chung JR, Jackson LA, et al. Influenza Vaccine Effectiveness in the United States during the 2015-2016 Season. N Engl J Med. 2017;377(6):534-543.

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