Table 1. Comparison of observational studies evaluating the effectiveness of seasonal influenza vaccination to prevent 2009 pA/H1N1 morbidity in civilian populations.
| Reference | Study Setting | Study Design | Sample Size (% of Young Adults) | Outcome | Vaccine Effectiveness Estimatea (95% CI) |
| Skowronski et al. [5] | Canada; 17 April to 22 July 2009 (first wave) | Sentinel test-negative case control; community-based GPsb | 144 cases, 536 controls (49% aged 20–49 y) | 2009 H1N1pdm RT-PCR-confirmed | −68% (−174% to −3%)b, c |
| Garcia-Garcia et al. [10] | Specialty hospital, Mexico City, Mexico; 29 March to 20 May 2009 | Matched hospital case-control | 60 cases; 180 controls (63% aged 21–60 y) | 2009 H1N1pdm RT-PCR-confirmed | 73% (34% to 89%)d |
| Echevarria-Zuna et al. [11] | Mexico, 28 April to 31 July 2009 | Prospective surveillance (case-negative controls in inpatients and outpatients) | 1,766 cases; 8,096 controls (N/A) | 2009 H1N1pdm RT-PCR-confirmed | 35% (23% to 45%) |
| Kelly et al., [12] | Victoria state, Australia; 27 April to 12 July 2009 | Sentinel test-negative case control; community-based GPs | 212 cases; 365 controls (54% aged 20–49 y) | 2009 H1N1pdm RT-PCR-confirmed | 3% (−56% to 40%)e |
| Gargiullo et al., [13] | Eight states, USA; May–June 2009 | Case-based (ie, case- cohort) | 356 cases; vacc coverage sample size = 20,689 (77% aged 18–49 y) | 2009 H1N1pdm RT-PCR-confirmed | −10% (−46% to 15%)d |
| Iuliano et al., [14] | University of Delaware outbreak, USA; 27 March to 9 May 2009 | Online retrospective survey | 7,450 respondents (90% aged 18–49 y); 677 had ILI | ILI (fever, sore throat or cough) | −10% (−40% to 10%)d |
| Lessler et al., [15] | New York city school outbreak, USA; April 2009 | Online retrospective survey | 2,225 respondents (0% over age 20 y); 694 had ILI | ILI (fever, sore throat or cough) | −5% (−20% to 9%) |
All studies were conducted during the first wave of the 2009 pandemic (April–July 2009) and most include a majority of young adults.
a
Negative estimates of vaccine effectiveness indicate that vaccination may be a risk factor for 2009 pandemic illness, while positive estimates suggest a protective effect.
b
Three other study designs are considered in this publication but the Sentinel system is the most well-established.
c
Adjusted for age, comorbidities, province, interval between symptoms onset and sample collection.
d
Adjusted for age and comorbidities.
e
Adjusted for age.
ILI, influenza like illness.