Immunogenicity of a trivalent influenza vaccine and persistence of induced immunity in adults aged ≥60 years in Taizhou City, Zhejiang Province, China, during the 2018–2019 season

Figures & data

Figure 1. Flow chart of study participants and their follow-up schedule.

Table 1. Participants characteristics at baseline (n = 422).

Variable	Total Number (%)	Vaccinated N (%)	Unvaccinated N (%)	χ^2	P value
Age
Mean (range)	67.8(60–88)
Age group
60-years	274 (64.9)	194 (68.3)	80 (58.0)	6.8	0.034
70-years	121 (28.7)	77 (27.1)	44 (31.9)
80-years	27 (6.4)	13 (4.6)	14 (10.1)
Gender
Female	224 (53.1)	152 (53.5)	72 (53.1)	0.1	0.795
Male	198 (46.9)	132 (46.5)	66 (46.9)
Chronic illness			4
Hypertension	181 (42.9)	106 (37.3)	75 (54.3)	11.0	0.001
Diabetes	45 (10.7)	25 (8.8)	20 (14.5)	3.2	0.076
Chronic bronchitis	9 (2.1)	7 (2.5)	2 (1.4)	-	0.724
Coronary heart disease	5 (1.2)	4 (1.4)	1 (0.7)	0.4	0.543
Chronic emphysema	2 (0.5)	2 (0.7)	0 (0.0)	-	1.000

P-values result from a comparison between the two groups using two-sided χ² tests for categorical data.

Table 2. Percentage of group with seroconversion^a at each of two timepoints.

		30 days % (seropositive/total)				180 days % (seropositive/total)
Virus type	Group	Seroconversion rates	95%CI	χ^2	P value	seroconversion rates	95%CI	χ^2	P value
AH1N1	Vaccinated	72.5 (206/284)	63.7–77.9%	29.0	<.001	69.4 (197/284)	64.0–74.8%	0.7	0.391
	Unvaccinated	45.7 (63/138)	37.2–54.1%			65.2 (90/138)	57.2–73.3%
AH3N2	Vaccinated	71.1 (202/284)	65.8–76.4%	125.6	<.001	71.1 (202/284)	65.8–76.4%	2.3	0.126
	Unvaccinated	13.0 (18/138)	7.4–18.7%			63.8 (88/138)	55.6–71.9%
B/Victoria	Vaccinated	47.2 (134/284)	41.3–53.0%	51.3	<.001	28.5 (81/284)	23.2–33.8%	31.1	<.001
	Unvaccinated	11.6 (16/138)	6.2–17.0%			4.3 (6/138)	0.9–7.8%

P-values result from a comparison between the two groups using two-sided chi-squared tests for categorical data.

*Using an adjusted chi-squared test. **using a Fisher’s exact test. CI = confidence interval.

Seroconversion was defined as a pre-immunization HI antibody titer of < 10 to a post-immunization one of ≥ 40 or a pre-immunization HI antibody titer of ≥ 10 and exhibited a ≥ 4 -fold increase.

Table 3. Percentage of group with seroprotection^a at each of three timepoints.

		Baseline % (seropositive/total)				30 days % (seropositive/total)				180 days % (seropositive/total)
Virus type	Group	Seroprotection Rates	95%CI	x^2	P value	Seroprotection rates	95%CI	x^2	P value	Seroprotection rates	95%CI	x^2	P value
AH1N1	Vaccinated	88.4 (251/284)	84.6–92.1%	0.2	0.651	98.9 (281/284)	97.7–100.1%	0.4	0.552*	99.6 (283/284)	99.0–100.3%	-	1.000**
	Unvaccinated	89.9 (124/138)	84.8–95.0%			100 (138/138)	100–100%			100 (138/138)	100–100%
AH3N2	Vaccinated	73.2 (208/281)	68.1–78.4%	4.5	0.034	96.8 (275/284)	94.8–98.9%	22.3	<.001	97.9 (278/284)	96.2–99.6%	9.9	0.002
	Unvaccinated	82.6 (114/138)	76.2–89.0%			84.1 (116/138)	77.9–90.2%			91.3 (126/138)	86.5–96.1%
B/Victoria	Vaccinated	47.9 (136/284)	42.0–53.7%	0.7	0.394	95.1 (270/284)	92.5–97.6%	122.1	<.001	68.3 (194/284)	62.9–73.8%	110.5	<.001
	Unvaccinated	43.5 (60/138)	35.1–51.9%			49.3 (68/138)	40.8–57.7%			13.8 (19/138)	7.9–19.6%

P-values result from a comparison between the two groups using two-sided chi-squared tests for categorical data. *using an adjusted chi-square test. **using a Fisher’s exact test. CI = confidence interval.

Seroprotection was defined as a HI titer ≥40.

Figure 2. Time course of HI antibody GMTs in vaccinated and unvaccinated participants. Influenza hemagglutination inhibiting (HI) antibody geometric mean titers (GMTs) between the vaccinated and unvaccinated groups by strain at day 0, day 30, and day 180 post-vaccination. AH1N1 vac: GMT of HI antibody to AH1N1 in the vaccinated group; AH1N1 unvac: GMT of HI antibody to AH1N1 in the unvaccinated group; AH3N2 vac: GMT of HI antibody to AH3N2 in the vaccinated group; AH3N2 unvac: GMT of HI antibody to AH3N2 in the unvaccinated group; BV vac: GMT of HI antibody to B/Victoria in the vaccinated group; BV unvac: GMT of HI antibody to B/Victoria in the unvaccinated group.

Figure 3. Influenza-Specific hemagglutination inhibiting antibody geometric mean titers (GMTs) between two vaccinated and unvaccinated groups by strain at day 0, day 30, and day 180 post-vaccination. GMT of HI antibody against AH1N1 (a), AH3N2 (b), or B/Victoria (BV) (c). GMT is shown above each bar. Error bars indicate 95% CIs. Unvaccinated participants did not receive the study vaccine (n = 138). Vaccinated participants received one dose of the study vaccine (n = 284). P-values result from a comparison between the two groups (vaccinated group versus unvaccinated group).

Table 4. Geometric mean titers (GMTs) of HI antibodies and geometric mean ratios among participants in the two groups by strain (N = 422).

		GMT (95%CI)
Antigen	Group	Day 0	Day 30	Day 180	Ratio^a	Ratio^b
AH1N1	Vaccinated	66.6(60.1–73.9)	360.7(325.2–400.0)	324.7(301.2–350.1)	5.4	0.9
	Unvaccinated	66.1(58.2–75.0)	185.1(162.1–211.3)	268.4(244.2–295.1)	2.8	1.5
AH3N2	Vaccinated	48.4(43.9–53.4)	263.2(231.7–299.0)	255.6(229.3–285.0)	5.4	1.0
	Unvaccinated	56.3(48.7–65.0)	66.4(56.5–78.1)	210.9(171.3–259.7)	1.2	3.2
B	Vaccinated	27.0(24.6–29.6)	73.8(68.2–79.9)	44.6(40.4–49.4)	2.7	0.6
	Unvaccinated	23.0(20.4–26.0)	28.7(25.5–32.3)	14.0(12.5–15.7)	1.2	0.5

Ratio^a: represents the ratio of the GMT at Day 30 post-vaccination to the GMT at Day 0 post-vaccination of HI antibody for each strain.

Ratio^b: represents the ratio of the GMT at Day 180 post-vaccination to the GMT at Day 30 post-vaccination of HI antibody for each strain.