Table 3. Incidence of CHIKV infections among individuals with negative baseline CHIKV PRNT in different age groups. ^a .

Age	Subjects with negative CHIKV PRNT b , n	Symptomatic CHIKV infection, n [n/100 person-yrs (95% CI)]	Subclinical CHIKV infection, n [n/100 person-yrs (95% CI)]	Total CHIKV infection, n [n/100 person-yrs (95% CI)]
6 mos—5 yrs	147	5 [3.26 (1.23, 7.14)]	10 [6.51 (3.34, 11.55)]	15 [9.77 (5.71, 15.70)]
6–15 yrs	182	8 [4.27 (2.02, 8.06)]	24 [12.82 (8.43, 18.76)]	32 [17.09 (11.91, 23.82)]
16–30 yrs	134	2 [1.42 (0.28, 4.56)]	20 [14.22 (8.96, 21.52)]	22 [15.64 (10.08, 23.25)]
31–50 yrs	80	3 [3.81 (1.06, 10.18)]	21 [26.70 (17.02, 40.04)]	24 [30.51 (20.05, 44.64)]
>50 yrs	71	1 [1.41 (0.13, 6.60)]	12 [16.98 (9.27, 28.75)]	13 [18.39 (10.30, 30.55)]
All ages	614	19 [3.01 (1.87, 4.61)]	87 [13.79 (11.12, 16.92)]	106 [16.80 (13.83, 20.24)]

^aBased on 614 subjects with both enrollment and 12-month blood collections who had negative baseline CHIKV PRNT titer.

^bCHIKV PRNT titer at enrollment using 80% plaque reduction.

CHIKV = chikungunya virus; n = number; CI = confidence interval; PRNT = plaque reduction neutralization test.