Influenza vaccine effectiveness against medically attended influenza illness in Beijing, China, 2014/15 season

Figures & data

Figure 1. Flow chart of subject enrollment in the test-negative design study for estimating influenza vaccine effectiveness during the 2014/15 influenza season in Beijing, China.

Table 1. Demographic characteristics of participants by influenza positive/negative status during the 2014/15 season.

Characteristics	Influenza Positive No. (Col.%)	Influenza Negative No. (Col.%)	P value
Overall	3434 (100)	5863 (100)
Sex
Male	1734 (50.5)	3146 (53.7)	0.003
Female	1700 (49.5)	2717 (46.3)
Age groups
6–35 months	167 (4.9)	713 (12.2)	<0.001
3–17 years	947 (27.6)	1778 (30.3)
18–59 years	2012 (58.6)	2978 (50.8)
≥ 60 years	308 (9.0)	394 (6.7)
Presence of at least one chronic disease
Yes	251 (7.3)	318 (5.4)	<0.001
No	3183 (92.7)	5545 (94.6)
Interval between ILI onset and specimen collection
≤ 3 days	3309 (96.4)	5592 (95.4)	0.024
4–7 days	125 (3.6)	271 (4.6)

ILI: influenza-like illness; Col.: column.

^a Pearson's χ² test was used to assess the demographic characteristic differences between influenza-positive cases and influenza-negative controls.

Table 2. Percentage of participants receiving the seasonal 2014/15 influenza vaccine among influenza-positive cases and influenza-negative controls, and the unadjusted and adjusted estimates of influenza VE in the 2014/15 season in Beijing, China.

	Influenza positive	Influenza negative	Unadjusted VE	Adjusted VE
Groups	Vaccinated No./total (Row %)	Vaccinated No./total (Row %)	VE% (95% CI)	VE% (95% CI)
Overall	149/3434 (4.3)	215/5863 (3.7)	−19 (−47 to 4)	−18 (−49 to 6)
Influenza type/subtype
Influenza A (H3N2)	95/2167 (4.4)	215/5863 (3.7)	−20 (−54 to 6)	−25 (−70 to 8)
Influenza B	54/1261 (4.3)	215/5863 (3.7)	−18 (−59 to 13)	−8 (−50 to 23)
Age groups (yrs)
6–35 months	0/167 (0.0)	4/713 (0.6)	NA	NA
3–17 years	120/947 (12.7)	174/1778 (9.8)	−34 (−71 to −4)	−22 (−59 to 6)
18–59 years	7/2012 (0.3)	6/2978 (0.2)	−73 (−415 to 42)	−60 (−415 to 50)
≥ 60 years	22/308 (7.1)	31/394 (7.9)	10 (−59 to 49)	16 (−54 to 54)

ILI: influenza-like illness; VE: vaccine effectiveness; CI: confidence interval.

* VE was calculated as 100% × (1-odds ratio [the odds of vaccination among influenza-positive cases vs. the odds of vaccination among influenza-negative controls]) using unconditional logistic regression models.

^a Participants were considered to be vaccinated if the last dose of the vaccine was given ≥ 14 d before ILI onset.

^b Unconditional logistic regression models were adjusted for sex, age group, chronic diseases, and calendar week.

^c Not available because the models did not converge due to small sample sizes.

Table 3. Percentage of participants vaccinated with the 2014/15 influenza vaccine and the 2013/14 influenza vaccine among influenza-positive cases and influenza-negative controls, with estimates of influenza VE in the 2014/15 season in Beijing, China.

	Influenza Positive	Influenza Negative	Unadjusted VE	Adjusted VE
Vaccination status	Vaccinated No. (Col.%)	Vaccinated No. (Col.%)	VE% (95% CI)	VE% (95% CI)
Overall	3434 (100)	5863 (100)
Unvaccinated	3218 (93.7)	5555 (94.7)	Reference	Reference
2014/15 vaccination only	63 (1.8)	105 (1.8)	−4 (−42 to 24)	−12 (−57 to 20)
2013/14 vaccination only	67 (2.0)	93 (1.6)	−24 (−71 to 9)	−31 (−84 to 7)
Both 2014/15 and 2013/14 vaccinations	86 (2.5)	110 (1.9)	−35 (−80 to −1)	−27 (−72 to 7)

ILI: influenza-like illness; VE: vaccine effectiveness; CI: confidence interval; Col.: column.

* VE was calculated as 100% × (1-odds ratio [the odds of vaccination among influenza-positives cases vs. the odds of vaccination among influenza-negative controls]) using unconditional logistic regression models.

^a Participants were considered to be vaccinated if the last dose of the vaccine was given ≥ 14 d before ILI onset.

^b Unconditional logistic regression models were adjusted for sex, age group, chronic diseases, and calendar week.