Table. Incidence of subclinical and symptomatic chikungunya virus infection during 2 years of active surveillance in an age-stratified cohort, Cebu, Philippines, 2012–2014*.
| Cohort |
No. persons/prevalence of neutralizing antibodies at beginning of year, % |
No. cases/no. cases per 100 person-years (95% CI) |
||
|---|---|---|---|---|
| Acute symptomatic infections |
Subclinical infections |
All infections |
||
| Year 1 | ||||
| All persons with paired serum samples, by age | ||||
| 6 mo–5 y | 203/0.7 | 5/3.23 (1.23–7.08) | 10/6.46 (3.23–11.47) | 15/9.69 (5.66–15.59) |
| 6–15 y | 201/1.1 | 8/4.23 (1.99–7.98) | 24/12.68 (8.23–18.55) | 32/16.91(11/.8–23.56) |
| 16–30 y | 200/20.24 | 2/1.13 (0.23–3.63) | 20/11.32 (7.13–17.4) | 22/12.45 (8.02–18.51) |
| 31–50 y | 204/52.9 | 4/2.38 (0.79–5.65) | 21/12.48 (7.95–18.7) | 25/14.85 (9.85–21.57) |
| >50 y | 200/61.06 | 1/0.56 (0.05–2.6) | 12/6.7 (3.66–11.34) | 13/7.25 (4.06–12.05) |
| All ages | 1,008/28.0 | 20/2.3 (1.5–3.49) | 87/10.02 (8.08–12.3) | 107/12.32 (10.15–14.83) |
| Only persons with negative neutralizing antibodies at beginning of 1st year, by age | ||||
|---|---|---|---|---|
| 6 mo–5 y | 5/3.26 (1.23–7.4) | 10/6.51 (3.34–11.55) | 15/9.77 (5.71–15.7) | |
| 6–15 y | 8/4.27 (2.02–8.6) | 24/12.82 (8.43–18.76) | 32/17.09 (11.91–23.82) | |
| 16–30 y | 2/1.42 (0.28–4.56) | 20/14.22 (8.96–21.52) | 22/15.64 (10.08–23.25) | |
| 31–50 y | 4/5.02 (1.68–11.94) | 21/26.37 (16.81–39.54) | 25/31.39 (20.81–45.59) | |
| >50 y | 1/1.41(0.13–6.6) | 12/16.98 (9.27–28.75) | 13/18.39 (10.3–30.55) | |
| All ages | 20/3.17 (1.99–4.79) | 87/13.77(11.1–16.9) | 107/16.94 (13.95–20.38) | |
| Year 2 | ||||
|---|---|---|---|---|
| All persons with paired serum samples | ||||
| 6 mo–5 y | 148/8.6 | 2/1.68 (0.33–5.37) | 1/0.84 (0.08–3.91) | 3/2.51 (0.7–6.71) |
| 6–15 y | 184/18.6 | 1/0.63 (0.06–2.93) | 3/1.88 (0.52–5.03) | 4/2.51 (0.84–5.97) |
| 16–30 y | 168/35.04 | 1/0.74 (0.1–5.36) | 4/2.98 (1.0–7.08) | 5/3.72 (1.41–8.16) |
| 31–50 y | 172/70.4 | 1/0.62 (0.06–2.87) | 4/2.46 (0.82–5.85) | 5/3.08 (1.17–6.74) |
| >50 y | 182/69.7 | 1/0.61 (0.06–2.86) | 3/1.84 (0.51–4.9) | 4/2.45 (0.82–5.83) |
| All ages | 854/42.0 | 6/0.81 (0.34–1.67) | 15/2.03 (1.19–3.27) | 21/2.84 (1.81–4.26) |
| Only persons with negative neutralizing antibodies at beginning of 2nd year, by age | ||||
|---|---|---|---|---|
| 6 mo–5 y | 2/1.84 (0.37–5.89) | 1/0.92 (0.08–4.28) | 3/2.76 (0.76–7.35) | |
| 6–15 y | 1/0.77 (0.07–3.6) | 3/2.32 (0.64–6.18) | 4/3.09 (1.03–7.35) | |
| 16–30 y | 1/1.15 (0.1–5.36) | 4/4.6 (1.54–10.94) | 5/5.75 (2.18–12.6) | |
| 31–50 y | 1/2.11 (0.19–9.82) | 4/8.43 (2.82–20.03) | 5/10.53 (3.99–23.09) | |
| >50 y | 1/2.05 (0.19–9.55) | 3/6.15 (1.7–16.4) | 4/8.2 (2.74–19.49) | |
| All ages | 6/1.42 (0.59–2.93) | 15/3.56 (2.08–5.72) | 21/4.98 (3.18–7.47) | |
Bold indicates significantly different (p<0.05) from incidence observed among the corresponding age groups during the first year of surveillance. Prevalence of neutralizing antibodies at the beginning of the first year was 28% and at the beginning of the second year was 42%.