Table 2.
Seroprevalence, official total proportion infected, and their ratio overall and by characteristics of population-based controls (N = 2223).
| Seroprevalence (SP) N (%) | Official Total Proportion Infected (OTPI) % | Ratio SP/OTPI [95% CI] | |
|---|---|---|---|
| Total | 442 (20) | 7 | 2.7 [2.5–3.0] |
| Period of the sample collection | p < 0.01 | ||
| Wave 1 | 57 (14) | 4 | 3.2 [2.5–4.1] |
| Wave 2 | 136 (15) | 6 | 2.6 [2.2–3.0] |
| Wave 3 | 249 (27) | 10 | 2.7 [2.4–3.0] |
| Age | p < 0.01 | ||
| <20y | 86 (22) | 3 | 6.7 [5.5–8.0] |
| 20–39y | 129 (20) | 10 | 2.0 [1.7–2.3] |
| 40–59y | 157 (23) | 9 | 2.6 [2.2–2.9] |
| 60y+ | 70 (14) | 5 | 2.9 [2.3–3.5] |
| Sex | p = 0.99 | ||
| Men | 206 (20) | 7 | 2.7 [2.4–3.1] |
| Women | 236 (20) | 7 | 2.7 [2.4–3.0] |
| Region | p = 0.04 | ||
| Non-metropolitan region | 160 (22) | 7 | 3.2 [2.8–3.7] |
| Metropolitan region | 282 (19) | 7 | 2.5 [2.2–2.8] |
OTPI: mean official total proportion infected in the study population by the day of the sample collection (expected proportion of seropositivity based on day of sample collection, age, sex and region, according to the cumulative proportion of people infected according to the Health Ministry data). SP: crude seroprevalence in population-based controls (without correction for antibody decay). p, chi-square test between the variable and seroprevalence. Twenty percent of the blood samples from population-based controls were seropositive. The official total proportion infected for a given age, sex and canton group, OTPIa,s,c,d, was averaged over the distribution of (a,s,c,d) in controls, where d is the day of sampling the control, to produce the estimate OTPI = 7%. Other values of OTPI were obtained similarly for selected values of age, sex, canton group and wave of the epidemic.